Source file src/runtime/mgc.go
Documentation: runtime
1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Garbage collector (GC). 6 // 7 // The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple 8 // GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is 9 // non-generational and non-compacting. Allocation is done using size segregated per P allocation 10 // areas to minimize fragmentation while eliminating locks in the common case. 11 // 12 // The algorithm decomposes into several steps. 13 // This is a high level description of the algorithm being used. For an overview of GC a good 14 // place to start is Richard Jones' gchandbook.org. 15 // 16 // The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see 17 // Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978. 18 // On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978), 19 // 966-975. 20 // For journal quality proofs that these steps are complete, correct, and terminate see 21 // Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world. 22 // Concurrency and Computation: Practice and Experience 15(3-5), 2003. 23 // 24 // 1. GC performs sweep termination. 25 // 26 // a. Stop the world. This causes all Ps to reach a GC safe-point. 27 // 28 // b. Sweep any unswept spans. There will only be unswept spans if 29 // this GC cycle was forced before the expected time. 30 // 31 // 2. GC performs the mark phase. 32 // 33 // a. Prepare for the mark phase by setting gcphase to _GCmark 34 // (from _GCoff), enabling the write barrier, enabling mutator 35 // assists, and enqueueing root mark jobs. No objects may be 36 // scanned until all Ps have enabled the write barrier, which is 37 // accomplished using STW. 38 // 39 // b. Start the world. From this point, GC work is done by mark 40 // workers started by the scheduler and by assists performed as 41 // part of allocation. The write barrier shades both the 42 // overwritten pointer and the new pointer value for any pointer 43 // writes (see mbarrier.go for details). Newly allocated objects 44 // are immediately marked black. 45 // 46 // c. GC performs root marking jobs. This includes scanning all 47 // stacks, shading all globals, and shading any heap pointers in 48 // off-heap runtime data structures. Scanning a stack stops a 49 // goroutine, shades any pointers found on its stack, and then 50 // resumes the goroutine. 51 // 52 // d. GC drains the work queue of grey objects, scanning each grey 53 // object to black and shading all pointers found in the object 54 // (which in turn may add those pointers to the work queue). 55 // 56 // e. Because GC work is spread across local caches, GC uses a 57 // distributed termination algorithm to detect when there are no 58 // more root marking jobs or grey objects (see gcMarkDone). At this 59 // point, GC transitions to mark termination. 60 // 61 // 3. GC performs mark termination. 62 // 63 // a. Stop the world. 64 // 65 // b. Set gcphase to _GCmarktermination, and disable workers and 66 // assists. 67 // 68 // c. Perform housekeeping like flushing mcaches. 69 // 70 // 4. GC performs the sweep phase. 71 // 72 // a. Prepare for the sweep phase by setting gcphase to _GCoff, 73 // setting up sweep state and disabling the write barrier. 74 // 75 // b. Start the world. From this point on, newly allocated objects 76 // are white, and allocating sweeps spans before use if necessary. 77 // 78 // c. GC does concurrent sweeping in the background and in response 79 // to allocation. See description below. 80 // 81 // 5. When sufficient allocation has taken place, replay the sequence 82 // starting with 1 above. See discussion of GC rate below. 83 84 // Concurrent sweep. 85 // 86 // The sweep phase proceeds concurrently with normal program execution. 87 // The heap is swept span-by-span both lazily (when a goroutine needs another span) 88 // and concurrently in a background goroutine (this helps programs that are not CPU bound). 89 // At the end of STW mark termination all spans are marked as "needs sweeping". 90 // 91 // The background sweeper goroutine simply sweeps spans one-by-one. 92 // 93 // To avoid requesting more OS memory while there are unswept spans, when a 94 // goroutine needs another span, it first attempts to reclaim that much memory 95 // by sweeping. When a goroutine needs to allocate a new small-object span, it 96 // sweeps small-object spans for the same object size until it frees at least 97 // one object. When a goroutine needs to allocate large-object span from heap, 98 // it sweeps spans until it frees at least that many pages into heap. There is 99 // one case where this may not suffice: if a goroutine sweeps and frees two 100 // nonadjacent one-page spans to the heap, it will allocate a new two-page 101 // span, but there can still be other one-page unswept spans which could be 102 // combined into a two-page span. 103 // 104 // It's critical to ensure that no operations proceed on unswept spans (that would corrupt 105 // mark bits in GC bitmap). During GC all mcaches are flushed into the central cache, 106 // so they are empty. When a goroutine grabs a new span into mcache, it sweeps it. 107 // When a goroutine explicitly frees an object or sets a finalizer, it ensures that 108 // the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish). 109 // The finalizer goroutine is kicked off only when all spans are swept. 110 // When the next GC starts, it sweeps all not-yet-swept spans (if any). 111 112 // GC rate. 113 // Next GC is after we've allocated an extra amount of memory proportional to 114 // the amount already in use. The proportion is controlled by GOGC environment variable 115 // (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M 116 // (this mark is tracked in gcController.heapGoal variable). This keeps the GC cost in 117 // linear proportion to the allocation cost. Adjusting GOGC just changes the linear constant 118 // (and also the amount of extra memory used). 119 120 // Oblets 121 // 122 // In order to prevent long pauses while scanning large objects and to 123 // improve parallelism, the garbage collector breaks up scan jobs for 124 // objects larger than maxObletBytes into "oblets" of at most 125 // maxObletBytes. When scanning encounters the beginning of a large 126 // object, it scans only the first oblet and enqueues the remaining 127 // oblets as new scan jobs. 128 129 package runtime 130 131 import ( 132 "internal/cpu" 133 "runtime/internal/atomic" 134 "unsafe" 135 ) 136 137 const ( 138 _DebugGC = 0 139 _ConcurrentSweep = true 140 _FinBlockSize = 4 * 1024 141 142 // debugScanConservative enables debug logging for stack 143 // frames that are scanned conservatively. 144 debugScanConservative = false 145 146 // sweepMinHeapDistance is a lower bound on the heap distance 147 // (in bytes) reserved for concurrent sweeping between GC 148 // cycles. 149 sweepMinHeapDistance = 1024 * 1024 150 ) 151 152 func gcinit() { 153 if unsafe.Sizeof(workbuf{}) != _WorkbufSize { 154 throw("size of Workbuf is suboptimal") 155 } 156 // No sweep on the first cycle. 157 mheap_.sweepDrained = 1 158 159 // Initialize GC pacer state. 160 // Use the environment variable GOGC for the initial gcPercent value. 161 gcController.init(readGOGC()) 162 163 work.startSema = 1 164 work.markDoneSema = 1 165 lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters) 166 lockInit(&work.assistQueue.lock, lockRankAssistQueue) 167 lockInit(&work.wbufSpans.lock, lockRankWbufSpans) 168 } 169 170 // Temporary in order to enable register ABI work. 171 // TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts. 172 var gcenable_setup chan int 173 174 // gcenable is called after the bulk of the runtime initialization, 175 // just before we're about to start letting user code run. 176 // It kicks off the background sweeper goroutine, the background 177 // scavenger goroutine, and enables GC. 178 func gcenable() { 179 // Kick off sweeping and scavenging. 180 gcenable_setup = make(chan int, 2) 181 go bgsweep() 182 go bgscavenge() 183 <-gcenable_setup 184 <-gcenable_setup 185 gcenable_setup = nil 186 memstats.enablegc = true // now that runtime is initialized, GC is okay 187 } 188 189 // Garbage collector phase. 190 // Indicates to write barrier and synchronization task to perform. 191 var gcphase uint32 192 193 // The compiler knows about this variable. 194 // If you change it, you must change builtin/runtime.go, too. 195 // If you change the first four bytes, you must also change the write 196 // barrier insertion code. 197 var writeBarrier struct { 198 enabled bool // compiler emits a check of this before calling write barrier 199 pad [3]byte // compiler uses 32-bit load for "enabled" field 200 needed bool // whether we need a write barrier for current GC phase 201 cgo bool // whether we need a write barrier for a cgo check 202 alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load 203 } 204 205 // gcBlackenEnabled is 1 if mutator assists and background mark 206 // workers are allowed to blacken objects. This must only be set when 207 // gcphase == _GCmark. 208 var gcBlackenEnabled uint32 209 210 const ( 211 _GCoff = iota // GC not running; sweeping in background, write barrier disabled 212 _GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED 213 _GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED 214 ) 215 216 //go:nosplit 217 func setGCPhase(x uint32) { 218 atomic.Store(&gcphase, x) 219 writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination 220 writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo 221 } 222 223 // gcMarkWorkerMode represents the mode that a concurrent mark worker 224 // should operate in. 225 // 226 // Concurrent marking happens through four different mechanisms. One 227 // is mutator assists, which happen in response to allocations and are 228 // not scheduled. The other three are variations in the per-P mark 229 // workers and are distinguished by gcMarkWorkerMode. 230 type gcMarkWorkerMode int 231 232 const ( 233 // gcMarkWorkerNotWorker indicates that the next scheduled G is not 234 // starting work and the mode should be ignored. 235 gcMarkWorkerNotWorker gcMarkWorkerMode = iota 236 237 // gcMarkWorkerDedicatedMode indicates that the P of a mark 238 // worker is dedicated to running that mark worker. The mark 239 // worker should run without preemption. 240 gcMarkWorkerDedicatedMode 241 242 // gcMarkWorkerFractionalMode indicates that a P is currently 243 // running the "fractional" mark worker. The fractional worker 244 // is necessary when GOMAXPROCS*gcBackgroundUtilization is not 245 // an integer and using only dedicated workers would result in 246 // utilization too far from the target of gcBackgroundUtilization. 247 // The fractional worker should run until it is preempted and 248 // will be scheduled to pick up the fractional part of 249 // GOMAXPROCS*gcBackgroundUtilization. 250 gcMarkWorkerFractionalMode 251 252 // gcMarkWorkerIdleMode indicates that a P is running the mark 253 // worker because it has nothing else to do. The idle worker 254 // should run until it is preempted and account its time 255 // against gcController.idleMarkTime. 256 gcMarkWorkerIdleMode 257 ) 258 259 // gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes 260 // to use in execution traces. 261 var gcMarkWorkerModeStrings = [...]string{ 262 "Not worker", 263 "GC (dedicated)", 264 "GC (fractional)", 265 "GC (idle)", 266 } 267 268 // pollFractionalWorkerExit reports whether a fractional mark worker 269 // should self-preempt. It assumes it is called from the fractional 270 // worker. 271 func pollFractionalWorkerExit() bool { 272 // This should be kept in sync with the fractional worker 273 // scheduler logic in findRunnableGCWorker. 274 now := nanotime() 275 delta := now - gcController.markStartTime 276 if delta <= 0 { 277 return true 278 } 279 p := getg().m.p.ptr() 280 selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime) 281 // Add some slack to the utilization goal so that the 282 // fractional worker isn't behind again the instant it exits. 283 return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal 284 } 285 286 var work struct { 287 full lfstack // lock-free list of full blocks workbuf 288 empty lfstack // lock-free list of empty blocks workbuf 289 pad0 cpu.CacheLinePad // prevents false-sharing between full/empty and nproc/nwait 290 291 wbufSpans struct { 292 lock mutex 293 // free is a list of spans dedicated to workbufs, but 294 // that don't currently contain any workbufs. 295 free mSpanList 296 // busy is a list of all spans containing workbufs on 297 // one of the workbuf lists. 298 busy mSpanList 299 } 300 301 // Restore 64-bit alignment on 32-bit. 302 _ uint32 303 304 // bytesMarked is the number of bytes marked this cycle. This 305 // includes bytes blackened in scanned objects, noscan objects 306 // that go straight to black, and permagrey objects scanned by 307 // markroot during the concurrent scan phase. This is updated 308 // atomically during the cycle. Updates may be batched 309 // arbitrarily, since the value is only read at the end of the 310 // cycle. 311 // 312 // Because of benign races during marking, this number may not 313 // be the exact number of marked bytes, but it should be very 314 // close. 315 // 316 // Put this field here because it needs 64-bit atomic access 317 // (and thus 8-byte alignment even on 32-bit architectures). 318 bytesMarked uint64 319 320 markrootNext uint32 // next markroot job 321 markrootJobs uint32 // number of markroot jobs 322 323 nproc uint32 324 tstart int64 325 nwait uint32 326 327 // Number of roots of various root types. Set by gcMarkRootPrepare. 328 nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int 329 330 // Base indexes of each root type. Set by gcMarkRootPrepare. 331 baseData, baseBSS, baseSpans, baseStacks, baseEnd uint32 332 333 // Each type of GC state transition is protected by a lock. 334 // Since multiple threads can simultaneously detect the state 335 // transition condition, any thread that detects a transition 336 // condition must acquire the appropriate transition lock, 337 // re-check the transition condition and return if it no 338 // longer holds or perform the transition if it does. 339 // Likewise, any transition must invalidate the transition 340 // condition before releasing the lock. This ensures that each 341 // transition is performed by exactly one thread and threads 342 // that need the transition to happen block until it has 343 // happened. 344 // 345 // startSema protects the transition from "off" to mark or 346 // mark termination. 347 startSema uint32 348 // markDoneSema protects transitions from mark to mark termination. 349 markDoneSema uint32 350 351 bgMarkReady note // signal background mark worker has started 352 bgMarkDone uint32 // cas to 1 when at a background mark completion point 353 // Background mark completion signaling 354 355 // mode is the concurrency mode of the current GC cycle. 356 mode gcMode 357 358 // userForced indicates the current GC cycle was forced by an 359 // explicit user call. 360 userForced bool 361 362 // totaltime is the CPU nanoseconds spent in GC since the 363 // program started if debug.gctrace > 0. 364 totaltime int64 365 366 // initialHeapLive is the value of gcController.heapLive at the 367 // beginning of this GC cycle. 368 initialHeapLive uint64 369 370 // assistQueue is a queue of assists that are blocked because 371 // there was neither enough credit to steal or enough work to 372 // do. 373 assistQueue struct { 374 lock mutex 375 q gQueue 376 } 377 378 // sweepWaiters is a list of blocked goroutines to wake when 379 // we transition from mark termination to sweep. 380 sweepWaiters struct { 381 lock mutex 382 list gList 383 } 384 385 // cycles is the number of completed GC cycles, where a GC 386 // cycle is sweep termination, mark, mark termination, and 387 // sweep. This differs from memstats.numgc, which is 388 // incremented at mark termination. 389 cycles uint32 390 391 // Timing/utilization stats for this cycle. 392 stwprocs, maxprocs int32 393 tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start 394 395 pauseNS int64 // total STW time this cycle 396 pauseStart int64 // nanotime() of last STW 397 398 // debug.gctrace heap sizes for this cycle. 399 heap0, heap1, heap2, heapGoal uint64 400 } 401 402 // GC runs a garbage collection and blocks the caller until the 403 // garbage collection is complete. It may also block the entire 404 // program. 405 func GC() { 406 // We consider a cycle to be: sweep termination, mark, mark 407 // termination, and sweep. This function shouldn't return 408 // until a full cycle has been completed, from beginning to 409 // end. Hence, we always want to finish up the current cycle 410 // and start a new one. That means: 411 // 412 // 1. In sweep termination, mark, or mark termination of cycle 413 // N, wait until mark termination N completes and transitions 414 // to sweep N. 415 // 416 // 2. In sweep N, help with sweep N. 417 // 418 // At this point we can begin a full cycle N+1. 419 // 420 // 3. Trigger cycle N+1 by starting sweep termination N+1. 421 // 422 // 4. Wait for mark termination N+1 to complete. 423 // 424 // 5. Help with sweep N+1 until it's done. 425 // 426 // This all has to be written to deal with the fact that the 427 // GC may move ahead on its own. For example, when we block 428 // until mark termination N, we may wake up in cycle N+2. 429 430 // Wait until the current sweep termination, mark, and mark 431 // termination complete. 432 n := atomic.Load(&work.cycles) 433 gcWaitOnMark(n) 434 435 // We're now in sweep N or later. Trigger GC cycle N+1, which 436 // will first finish sweep N if necessary and then enter sweep 437 // termination N+1. 438 gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1}) 439 440 // Wait for mark termination N+1 to complete. 441 gcWaitOnMark(n + 1) 442 443 // Finish sweep N+1 before returning. We do this both to 444 // complete the cycle and because runtime.GC() is often used 445 // as part of tests and benchmarks to get the system into a 446 // relatively stable and isolated state. 447 for atomic.Load(&work.cycles) == n+1 && sweepone() != ^uintptr(0) { 448 sweep.nbgsweep++ 449 Gosched() 450 } 451 452 // Callers may assume that the heap profile reflects the 453 // just-completed cycle when this returns (historically this 454 // happened because this was a STW GC), but right now the 455 // profile still reflects mark termination N, not N+1. 456 // 457 // As soon as all of the sweep frees from cycle N+1 are done, 458 // we can go ahead and publish the heap profile. 459 // 460 // First, wait for sweeping to finish. (We know there are no 461 // more spans on the sweep queue, but we may be concurrently 462 // sweeping spans, so we have to wait.) 463 for atomic.Load(&work.cycles) == n+1 && !isSweepDone() { 464 Gosched() 465 } 466 467 // Now we're really done with sweeping, so we can publish the 468 // stable heap profile. Only do this if we haven't already hit 469 // another mark termination. 470 mp := acquirem() 471 cycle := atomic.Load(&work.cycles) 472 if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) { 473 mProf_PostSweep() 474 } 475 releasem(mp) 476 } 477 478 // gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has 479 // already completed this mark phase, it returns immediately. 480 func gcWaitOnMark(n uint32) { 481 for { 482 // Disable phase transitions. 483 lock(&work.sweepWaiters.lock) 484 nMarks := atomic.Load(&work.cycles) 485 if gcphase != _GCmark { 486 // We've already completed this cycle's mark. 487 nMarks++ 488 } 489 if nMarks > n { 490 // We're done. 491 unlock(&work.sweepWaiters.lock) 492 return 493 } 494 495 // Wait until sweep termination, mark, and mark 496 // termination of cycle N complete. 497 work.sweepWaiters.list.push(getg()) 498 goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1) 499 } 500 } 501 502 // gcMode indicates how concurrent a GC cycle should be. 503 type gcMode int 504 505 const ( 506 gcBackgroundMode gcMode = iota // concurrent GC and sweep 507 gcForceMode // stop-the-world GC now, concurrent sweep 508 gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user) 509 ) 510 511 // A gcTrigger is a predicate for starting a GC cycle. Specifically, 512 // it is an exit condition for the _GCoff phase. 513 type gcTrigger struct { 514 kind gcTriggerKind 515 now int64 // gcTriggerTime: current time 516 n uint32 // gcTriggerCycle: cycle number to start 517 } 518 519 type gcTriggerKind int 520 521 const ( 522 // gcTriggerHeap indicates that a cycle should be started when 523 // the heap size reaches the trigger heap size computed by the 524 // controller. 525 gcTriggerHeap gcTriggerKind = iota 526 527 // gcTriggerTime indicates that a cycle should be started when 528 // it's been more than forcegcperiod nanoseconds since the 529 // previous GC cycle. 530 gcTriggerTime 531 532 // gcTriggerCycle indicates that a cycle should be started if 533 // we have not yet started cycle number gcTrigger.n (relative 534 // to work.cycles). 535 gcTriggerCycle 536 ) 537 538 // test reports whether the trigger condition is satisfied, meaning 539 // that the exit condition for the _GCoff phase has been met. The exit 540 // condition should be tested when allocating. 541 func (t gcTrigger) test() bool { 542 if !memstats.enablegc || panicking != 0 || gcphase != _GCoff { 543 return false 544 } 545 switch t.kind { 546 case gcTriggerHeap: 547 // Non-atomic access to gcController.heapLive for performance. If 548 // we are going to trigger on this, this thread just 549 // atomically wrote gcController.heapLive anyway and we'll see our 550 // own write. 551 return gcController.heapLive >= gcController.trigger 552 case gcTriggerTime: 553 if gcController.gcPercent < 0 { 554 return false 555 } 556 lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime)) 557 return lastgc != 0 && t.now-lastgc > forcegcperiod 558 case gcTriggerCycle: 559 // t.n > work.cycles, but accounting for wraparound. 560 return int32(t.n-work.cycles) > 0 561 } 562 return true 563 } 564 565 // gcStart starts the GC. It transitions from _GCoff to _GCmark (if 566 // debug.gcstoptheworld == 0) or performs all of GC (if 567 // debug.gcstoptheworld != 0). 568 // 569 // This may return without performing this transition in some cases, 570 // such as when called on a system stack or with locks held. 571 func gcStart(trigger gcTrigger) { 572 // Since this is called from malloc and malloc is called in 573 // the guts of a number of libraries that might be holding 574 // locks, don't attempt to start GC in non-preemptible or 575 // potentially unstable situations. 576 mp := acquirem() 577 if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" { 578 releasem(mp) 579 return 580 } 581 releasem(mp) 582 mp = nil 583 584 // Pick up the remaining unswept/not being swept spans concurrently 585 // 586 // This shouldn't happen if we're being invoked in background 587 // mode since proportional sweep should have just finished 588 // sweeping everything, but rounding errors, etc, may leave a 589 // few spans unswept. In forced mode, this is necessary since 590 // GC can be forced at any point in the sweeping cycle. 591 // 592 // We check the transition condition continuously here in case 593 // this G gets delayed in to the next GC cycle. 594 for trigger.test() && sweepone() != ^uintptr(0) { 595 sweep.nbgsweep++ 596 } 597 598 // Perform GC initialization and the sweep termination 599 // transition. 600 semacquire(&work.startSema) 601 // Re-check transition condition under transition lock. 602 if !trigger.test() { 603 semrelease(&work.startSema) 604 return 605 } 606 607 // For stats, check if this GC was forced by the user. 608 work.userForced = trigger.kind == gcTriggerCycle 609 610 // In gcstoptheworld debug mode, upgrade the mode accordingly. 611 // We do this after re-checking the transition condition so 612 // that multiple goroutines that detect the heap trigger don't 613 // start multiple STW GCs. 614 mode := gcBackgroundMode 615 if debug.gcstoptheworld == 1 { 616 mode = gcForceMode 617 } else if debug.gcstoptheworld == 2 { 618 mode = gcForceBlockMode 619 } 620 621 // Ok, we're doing it! Stop everybody else 622 semacquire(&gcsema) 623 semacquire(&worldsema) 624 625 if trace.enabled { 626 traceGCStart() 627 } 628 629 // Check that all Ps have finished deferred mcache flushes. 630 for _, p := range allp { 631 if fg := atomic.Load(&p.mcache.flushGen); fg != mheap_.sweepgen { 632 println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen) 633 throw("p mcache not flushed") 634 } 635 } 636 637 gcBgMarkStartWorkers() 638 639 systemstack(gcResetMarkState) 640 641 work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs 642 if work.stwprocs > ncpu { 643 // This is used to compute CPU time of the STW phases, 644 // so it can't be more than ncpu, even if GOMAXPROCS is. 645 work.stwprocs = ncpu 646 } 647 work.heap0 = atomic.Load64(&gcController.heapLive) 648 work.pauseNS = 0 649 work.mode = mode 650 651 now := nanotime() 652 work.tSweepTerm = now 653 work.pauseStart = now 654 if trace.enabled { 655 traceGCSTWStart(1) 656 } 657 systemstack(stopTheWorldWithSema) 658 // Finish sweep before we start concurrent scan. 659 systemstack(func() { 660 finishsweep_m() 661 }) 662 663 // clearpools before we start the GC. If we wait they memory will not be 664 // reclaimed until the next GC cycle. 665 clearpools() 666 667 work.cycles++ 668 669 gcController.startCycle() 670 work.heapGoal = gcController.heapGoal 671 672 // In STW mode, disable scheduling of user Gs. This may also 673 // disable scheduling of this goroutine, so it may block as 674 // soon as we start the world again. 675 if mode != gcBackgroundMode { 676 schedEnableUser(false) 677 } 678 679 // Enter concurrent mark phase and enable 680 // write barriers. 681 // 682 // Because the world is stopped, all Ps will 683 // observe that write barriers are enabled by 684 // the time we start the world and begin 685 // scanning. 686 // 687 // Write barriers must be enabled before assists are 688 // enabled because they must be enabled before 689 // any non-leaf heap objects are marked. Since 690 // allocations are blocked until assists can 691 // happen, we want enable assists as early as 692 // possible. 693 setGCPhase(_GCmark) 694 695 gcBgMarkPrepare() // Must happen before assist enable. 696 gcMarkRootPrepare() 697 698 // Mark all active tinyalloc blocks. Since we're 699 // allocating from these, they need to be black like 700 // other allocations. The alternative is to blacken 701 // the tiny block on every allocation from it, which 702 // would slow down the tiny allocator. 703 gcMarkTinyAllocs() 704 705 // At this point all Ps have enabled the write 706 // barrier, thus maintaining the no white to 707 // black invariant. Enable mutator assists to 708 // put back-pressure on fast allocating 709 // mutators. 710 atomic.Store(&gcBlackenEnabled, 1) 711 712 // Assists and workers can start the moment we start 713 // the world. 714 gcController.markStartTime = now 715 716 // In STW mode, we could block the instant systemstack 717 // returns, so make sure we're not preemptible. 718 mp = acquirem() 719 720 // Concurrent mark. 721 systemstack(func() { 722 now = startTheWorldWithSema(trace.enabled) 723 work.pauseNS += now - work.pauseStart 724 work.tMark = now 725 memstats.gcPauseDist.record(now - work.pauseStart) 726 }) 727 728 // Release the world sema before Gosched() in STW mode 729 // because we will need to reacquire it later but before 730 // this goroutine becomes runnable again, and we could 731 // self-deadlock otherwise. 732 semrelease(&worldsema) 733 releasem(mp) 734 735 // Make sure we block instead of returning to user code 736 // in STW mode. 737 if mode != gcBackgroundMode { 738 Gosched() 739 } 740 741 semrelease(&work.startSema) 742 } 743 744 // gcMarkDoneFlushed counts the number of P's with flushed work. 745 // 746 // Ideally this would be a captured local in gcMarkDone, but forEachP 747 // escapes its callback closure, so it can't capture anything. 748 // 749 // This is protected by markDoneSema. 750 var gcMarkDoneFlushed uint32 751 752 // gcMarkDone transitions the GC from mark to mark termination if all 753 // reachable objects have been marked (that is, there are no grey 754 // objects and can be no more in the future). Otherwise, it flushes 755 // all local work to the global queues where it can be discovered by 756 // other workers. 757 // 758 // This should be called when all local mark work has been drained and 759 // there are no remaining workers. Specifically, when 760 // 761 // work.nwait == work.nproc && !gcMarkWorkAvailable(p) 762 // 763 // The calling context must be preemptible. 764 // 765 // Flushing local work is important because idle Ps may have local 766 // work queued. This is the only way to make that work visible and 767 // drive GC to completion. 768 // 769 // It is explicitly okay to have write barriers in this function. If 770 // it does transition to mark termination, then all reachable objects 771 // have been marked, so the write barrier cannot shade any more 772 // objects. 773 func gcMarkDone() { 774 // Ensure only one thread is running the ragged barrier at a 775 // time. 776 semacquire(&work.markDoneSema) 777 778 top: 779 // Re-check transition condition under transition lock. 780 // 781 // It's critical that this checks the global work queues are 782 // empty before performing the ragged barrier. Otherwise, 783 // there could be global work that a P could take after the P 784 // has passed the ragged barrier. 785 if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) { 786 semrelease(&work.markDoneSema) 787 return 788 } 789 790 // forEachP needs worldsema to execute, and we'll need it to 791 // stop the world later, so acquire worldsema now. 792 semacquire(&worldsema) 793 794 // Flush all local buffers and collect flushedWork flags. 795 gcMarkDoneFlushed = 0 796 systemstack(func() { 797 gp := getg().m.curg 798 // Mark the user stack as preemptible so that it may be scanned. 799 // Otherwise, our attempt to force all P's to a safepoint could 800 // result in a deadlock as we attempt to preempt a worker that's 801 // trying to preempt us (e.g. for a stack scan). 802 casgstatus(gp, _Grunning, _Gwaiting) 803 forEachP(func(_p_ *p) { 804 // Flush the write barrier buffer, since this may add 805 // work to the gcWork. 806 wbBufFlush1(_p_) 807 808 // Flush the gcWork, since this may create global work 809 // and set the flushedWork flag. 810 // 811 // TODO(austin): Break up these workbufs to 812 // better distribute work. 813 _p_.gcw.dispose() 814 // Collect the flushedWork flag. 815 if _p_.gcw.flushedWork { 816 atomic.Xadd(&gcMarkDoneFlushed, 1) 817 _p_.gcw.flushedWork = false 818 } 819 }) 820 casgstatus(gp, _Gwaiting, _Grunning) 821 }) 822 823 if gcMarkDoneFlushed != 0 { 824 // More grey objects were discovered since the 825 // previous termination check, so there may be more 826 // work to do. Keep going. It's possible the 827 // transition condition became true again during the 828 // ragged barrier, so re-check it. 829 semrelease(&worldsema) 830 goto top 831 } 832 833 // There was no global work, no local work, and no Ps 834 // communicated work since we took markDoneSema. Therefore 835 // there are no grey objects and no more objects can be 836 // shaded. Transition to mark termination. 837 now := nanotime() 838 work.tMarkTerm = now 839 work.pauseStart = now 840 getg().m.preemptoff = "gcing" 841 if trace.enabled { 842 traceGCSTWStart(0) 843 } 844 systemstack(stopTheWorldWithSema) 845 // The gcphase is _GCmark, it will transition to _GCmarktermination 846 // below. The important thing is that the wb remains active until 847 // all marking is complete. This includes writes made by the GC. 848 849 // There is sometimes work left over when we enter mark termination due 850 // to write barriers performed after the completion barrier above. 851 // Detect this and resume concurrent mark. This is obviously 852 // unfortunate. 853 // 854 // See issue #27993 for details. 855 // 856 // Switch to the system stack to call wbBufFlush1, though in this case 857 // it doesn't matter because we're non-preemptible anyway. 858 restart := false 859 systemstack(func() { 860 for _, p := range allp { 861 wbBufFlush1(p) 862 if !p.gcw.empty() { 863 restart = true 864 break 865 } 866 } 867 }) 868 if restart { 869 getg().m.preemptoff = "" 870 systemstack(func() { 871 now := startTheWorldWithSema(true) 872 work.pauseNS += now - work.pauseStart 873 memstats.gcPauseDist.record(now - work.pauseStart) 874 }) 875 semrelease(&worldsema) 876 goto top 877 } 878 879 // Disable assists and background workers. We must do 880 // this before waking blocked assists. 881 atomic.Store(&gcBlackenEnabled, 0) 882 883 // Wake all blocked assists. These will run when we 884 // start the world again. 885 gcWakeAllAssists() 886 887 // Likewise, release the transition lock. Blocked 888 // workers and assists will run when we start the 889 // world again. 890 semrelease(&work.markDoneSema) 891 892 // In STW mode, re-enable user goroutines. These will be 893 // queued to run after we start the world. 894 schedEnableUser(true) 895 896 // endCycle depends on all gcWork cache stats being flushed. 897 // The termination algorithm above ensured that up to 898 // allocations since the ragged barrier. 899 nextTriggerRatio := gcController.endCycle(work.userForced) 900 901 // Perform mark termination. This will restart the world. 902 gcMarkTermination(nextTriggerRatio) 903 } 904 905 // World must be stopped and mark assists and background workers must be 906 // disabled. 907 func gcMarkTermination(nextTriggerRatio float64) { 908 // Start marktermination (write barrier remains enabled for now). 909 setGCPhase(_GCmarktermination) 910 911 work.heap1 = gcController.heapLive 912 startTime := nanotime() 913 914 mp := acquirem() 915 mp.preemptoff = "gcing" 916 _g_ := getg() 917 _g_.m.traceback = 2 918 gp := _g_.m.curg 919 casgstatus(gp, _Grunning, _Gwaiting) 920 gp.waitreason = waitReasonGarbageCollection 921 922 // Run gc on the g0 stack. We do this so that the g stack 923 // we're currently running on will no longer change. Cuts 924 // the root set down a bit (g0 stacks are not scanned, and 925 // we don't need to scan gc's internal state). We also 926 // need to switch to g0 so we can shrink the stack. 927 systemstack(func() { 928 gcMark(startTime) 929 // Must return immediately. 930 // The outer function's stack may have moved 931 // during gcMark (it shrinks stacks, including the 932 // outer function's stack), so we must not refer 933 // to any of its variables. Return back to the 934 // non-system stack to pick up the new addresses 935 // before continuing. 936 }) 937 938 systemstack(func() { 939 work.heap2 = work.bytesMarked 940 if debug.gccheckmark > 0 { 941 // Run a full non-parallel, stop-the-world 942 // mark using checkmark bits, to check that we 943 // didn't forget to mark anything during the 944 // concurrent mark process. 945 startCheckmarks() 946 gcResetMarkState() 947 gcw := &getg().m.p.ptr().gcw 948 gcDrain(gcw, 0) 949 wbBufFlush1(getg().m.p.ptr()) 950 gcw.dispose() 951 endCheckmarks() 952 } 953 954 // marking is complete so we can turn the write barrier off 955 setGCPhase(_GCoff) 956 gcSweep(work.mode) 957 }) 958 959 _g_.m.traceback = 0 960 casgstatus(gp, _Gwaiting, _Grunning) 961 962 if trace.enabled { 963 traceGCDone() 964 } 965 966 // all done 967 mp.preemptoff = "" 968 969 if gcphase != _GCoff { 970 throw("gc done but gcphase != _GCoff") 971 } 972 973 // Record heapGoal and heap_inuse for scavenger. 974 gcController.lastHeapGoal = gcController.heapGoal 975 memstats.last_heap_inuse = memstats.heap_inuse 976 977 // Update GC trigger and pacing for the next cycle. 978 gcController.commit(nextTriggerRatio) 979 980 // Update timing memstats 981 now := nanotime() 982 sec, nsec, _ := time_now() 983 unixNow := sec*1e9 + int64(nsec) 984 work.pauseNS += now - work.pauseStart 985 work.tEnd = now 986 memstats.gcPauseDist.record(now - work.pauseStart) 987 atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user 988 atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us 989 memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS) 990 memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow) 991 memstats.pause_total_ns += uint64(work.pauseNS) 992 993 // Update work.totaltime. 994 sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm) 995 // We report idle marking time below, but omit it from the 996 // overall utilization here since it's "free". 997 markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime 998 markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm) 999 cycleCpu := sweepTermCpu + markCpu + markTermCpu 1000 work.totaltime += cycleCpu 1001 1002 // Compute overall GC CPU utilization. 1003 totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs) 1004 memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu) 1005 1006 // Reset sweep state. 1007 sweep.nbgsweep = 0 1008 sweep.npausesweep = 0 1009 1010 if work.userForced { 1011 memstats.numforcedgc++ 1012 } 1013 1014 // Bump GC cycle count and wake goroutines waiting on sweep. 1015 lock(&work.sweepWaiters.lock) 1016 memstats.numgc++ 1017 injectglist(&work.sweepWaiters.list) 1018 unlock(&work.sweepWaiters.lock) 1019 1020 // Finish the current heap profiling cycle and start a new 1021 // heap profiling cycle. We do this before starting the world 1022 // so events don't leak into the wrong cycle. 1023 mProf_NextCycle() 1024 1025 // There may be stale spans in mcaches that need to be swept. 1026 // Those aren't tracked in any sweep lists, so we need to 1027 // count them against sweep completion until we ensure all 1028 // those spans have been forced out. 1029 sl := newSweepLocker() 1030 sl.blockCompletion() 1031 1032 systemstack(func() { startTheWorldWithSema(true) }) 1033 1034 // Flush the heap profile so we can start a new cycle next GC. 1035 // This is relatively expensive, so we don't do it with the 1036 // world stopped. 1037 mProf_Flush() 1038 1039 // Prepare workbufs for freeing by the sweeper. We do this 1040 // asynchronously because it can take non-trivial time. 1041 prepareFreeWorkbufs() 1042 1043 // Free stack spans. This must be done between GC cycles. 1044 systemstack(freeStackSpans) 1045 1046 // Ensure all mcaches are flushed. Each P will flush its own 1047 // mcache before allocating, but idle Ps may not. Since this 1048 // is necessary to sweep all spans, we need to ensure all 1049 // mcaches are flushed before we start the next GC cycle. 1050 systemstack(func() { 1051 forEachP(func(_p_ *p) { 1052 _p_.mcache.prepareForSweep() 1053 }) 1054 }) 1055 // Now that we've swept stale spans in mcaches, they don't 1056 // count against unswept spans. 1057 sl.dispose() 1058 1059 // Print gctrace before dropping worldsema. As soon as we drop 1060 // worldsema another cycle could start and smash the stats 1061 // we're trying to print. 1062 if debug.gctrace > 0 { 1063 util := int(memstats.gc_cpu_fraction * 100) 1064 1065 var sbuf [24]byte 1066 printlock() 1067 print("gc ", memstats.numgc, 1068 " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ", 1069 util, "%: ") 1070 prev := work.tSweepTerm 1071 for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} { 1072 if i != 0 { 1073 print("+") 1074 } 1075 print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev)))) 1076 prev = ns 1077 } 1078 print(" ms clock, ") 1079 for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} { 1080 if i == 2 || i == 3 { 1081 // Separate mark time components with /. 1082 print("/") 1083 } else if i != 0 { 1084 print("+") 1085 } 1086 print(string(fmtNSAsMS(sbuf[:], uint64(ns)))) 1087 } 1088 print(" ms cpu, ", 1089 work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ", 1090 work.heapGoal>>20, " MB goal, ", 1091 work.maxprocs, " P") 1092 if work.userForced { 1093 print(" (forced)") 1094 } 1095 print("\n") 1096 printunlock() 1097 } 1098 1099 semrelease(&worldsema) 1100 semrelease(&gcsema) 1101 // Careful: another GC cycle may start now. 1102 1103 releasem(mp) 1104 mp = nil 1105 1106 // now that gc is done, kick off finalizer thread if needed 1107 if !concurrentSweep { 1108 // give the queued finalizers, if any, a chance to run 1109 Gosched() 1110 } 1111 } 1112 1113 // gcBgMarkStartWorkers prepares background mark worker goroutines. These 1114 // goroutines will not run until the mark phase, but they must be started while 1115 // the work is not stopped and from a regular G stack. The caller must hold 1116 // worldsema. 1117 func gcBgMarkStartWorkers() { 1118 // Background marking is performed by per-P G's. Ensure that each P has 1119 // a background GC G. 1120 // 1121 // Worker Gs don't exit if gomaxprocs is reduced. If it is raised 1122 // again, we can reuse the old workers; no need to create new workers. 1123 for gcBgMarkWorkerCount < gomaxprocs { 1124 go gcBgMarkWorker() 1125 1126 notetsleepg(&work.bgMarkReady, -1) 1127 noteclear(&work.bgMarkReady) 1128 // The worker is now guaranteed to be added to the pool before 1129 // its P's next findRunnableGCWorker. 1130 1131 gcBgMarkWorkerCount++ 1132 } 1133 } 1134 1135 // gcBgMarkPrepare sets up state for background marking. 1136 // Mutator assists must not yet be enabled. 1137 func gcBgMarkPrepare() { 1138 // Background marking will stop when the work queues are empty 1139 // and there are no more workers (note that, since this is 1140 // concurrent, this may be a transient state, but mark 1141 // termination will clean it up). Between background workers 1142 // and assists, we don't really know how many workers there 1143 // will be, so we pretend to have an arbitrarily large number 1144 // of workers, almost all of which are "waiting". While a 1145 // worker is working it decrements nwait. If nproc == nwait, 1146 // there are no workers. 1147 work.nproc = ^uint32(0) 1148 work.nwait = ^uint32(0) 1149 } 1150 1151 // gcBgMarkWorker is an entry in the gcBgMarkWorkerPool. It points to a single 1152 // gcBgMarkWorker goroutine. 1153 type gcBgMarkWorkerNode struct { 1154 // Unused workers are managed in a lock-free stack. This field must be first. 1155 node lfnode 1156 1157 // The g of this worker. 1158 gp guintptr 1159 1160 // Release this m on park. This is used to communicate with the unlock 1161 // function, which cannot access the G's stack. It is unused outside of 1162 // gcBgMarkWorker(). 1163 m muintptr 1164 } 1165 1166 func gcBgMarkWorker() { 1167 gp := getg() 1168 1169 // We pass node to a gopark unlock function, so it can't be on 1170 // the stack (see gopark). Prevent deadlock from recursively 1171 // starting GC by disabling preemption. 1172 gp.m.preemptoff = "GC worker init" 1173 node := new(gcBgMarkWorkerNode) 1174 gp.m.preemptoff = "" 1175 1176 node.gp.set(gp) 1177 1178 node.m.set(acquirem()) 1179 notewakeup(&work.bgMarkReady) 1180 // After this point, the background mark worker is generally scheduled 1181 // cooperatively by gcController.findRunnableGCWorker. While performing 1182 // work on the P, preemption is disabled because we are working on 1183 // P-local work buffers. When the preempt flag is set, this puts itself 1184 // into _Gwaiting to be woken up by gcController.findRunnableGCWorker 1185 // at the appropriate time. 1186 // 1187 // When preemption is enabled (e.g., while in gcMarkDone), this worker 1188 // may be preempted and schedule as a _Grunnable G from a runq. That is 1189 // fine; it will eventually gopark again for further scheduling via 1190 // findRunnableGCWorker. 1191 // 1192 // Since we disable preemption before notifying bgMarkReady, we 1193 // guarantee that this G will be in the worker pool for the next 1194 // findRunnableGCWorker. This isn't strictly necessary, but it reduces 1195 // latency between _GCmark starting and the workers starting. 1196 1197 for { 1198 // Go to sleep until woken by 1199 // gcController.findRunnableGCWorker. 1200 gopark(func(g *g, nodep unsafe.Pointer) bool { 1201 node := (*gcBgMarkWorkerNode)(nodep) 1202 1203 if mp := node.m.ptr(); mp != nil { 1204 // The worker G is no longer running; release 1205 // the M. 1206 // 1207 // N.B. it is _safe_ to release the M as soon 1208 // as we are no longer performing P-local mark 1209 // work. 1210 // 1211 // However, since we cooperatively stop work 1212 // when gp.preempt is set, if we releasem in 1213 // the loop then the following call to gopark 1214 // would immediately preempt the G. This is 1215 // also safe, but inefficient: the G must 1216 // schedule again only to enter gopark and park 1217 // again. Thus, we defer the release until 1218 // after parking the G. 1219 releasem(mp) 1220 } 1221 1222 // Release this G to the pool. 1223 gcBgMarkWorkerPool.push(&node.node) 1224 // Note that at this point, the G may immediately be 1225 // rescheduled and may be running. 1226 return true 1227 }, unsafe.Pointer(node), waitReasonGCWorkerIdle, traceEvGoBlock, 0) 1228 1229 // Preemption must not occur here, or another G might see 1230 // p.gcMarkWorkerMode. 1231 1232 // Disable preemption so we can use the gcw. If the 1233 // scheduler wants to preempt us, we'll stop draining, 1234 // dispose the gcw, and then preempt. 1235 node.m.set(acquirem()) 1236 pp := gp.m.p.ptr() // P can't change with preemption disabled. 1237 1238 if gcBlackenEnabled == 0 { 1239 println("worker mode", pp.gcMarkWorkerMode) 1240 throw("gcBgMarkWorker: blackening not enabled") 1241 } 1242 1243 if pp.gcMarkWorkerMode == gcMarkWorkerNotWorker { 1244 throw("gcBgMarkWorker: mode not set") 1245 } 1246 1247 startTime := nanotime() 1248 pp.gcMarkWorkerStartTime = startTime 1249 1250 decnwait := atomic.Xadd(&work.nwait, -1) 1251 if decnwait == work.nproc { 1252 println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc) 1253 throw("work.nwait was > work.nproc") 1254 } 1255 1256 systemstack(func() { 1257 // Mark our goroutine preemptible so its stack 1258 // can be scanned. This lets two mark workers 1259 // scan each other (otherwise, they would 1260 // deadlock). We must not modify anything on 1261 // the G stack. However, stack shrinking is 1262 // disabled for mark workers, so it is safe to 1263 // read from the G stack. 1264 casgstatus(gp, _Grunning, _Gwaiting) 1265 switch pp.gcMarkWorkerMode { 1266 default: 1267 throw("gcBgMarkWorker: unexpected gcMarkWorkerMode") 1268 case gcMarkWorkerDedicatedMode: 1269 gcDrain(&pp.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit) 1270 if gp.preempt { 1271 // We were preempted. This is 1272 // a useful signal to kick 1273 // everything out of the run 1274 // queue so it can run 1275 // somewhere else. 1276 if drainQ, n := runqdrain(pp); n > 0 { 1277 lock(&sched.lock) 1278 globrunqputbatch(&drainQ, int32(n)) 1279 unlock(&sched.lock) 1280 } 1281 } 1282 // Go back to draining, this time 1283 // without preemption. 1284 gcDrain(&pp.gcw, gcDrainFlushBgCredit) 1285 case gcMarkWorkerFractionalMode: 1286 gcDrain(&pp.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit) 1287 case gcMarkWorkerIdleMode: 1288 gcDrain(&pp.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit) 1289 } 1290 casgstatus(gp, _Gwaiting, _Grunning) 1291 }) 1292 1293 // Account for time. 1294 duration := nanotime() - startTime 1295 switch pp.gcMarkWorkerMode { 1296 case gcMarkWorkerDedicatedMode: 1297 atomic.Xaddint64(&gcController.dedicatedMarkTime, duration) 1298 atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1) 1299 case gcMarkWorkerFractionalMode: 1300 atomic.Xaddint64(&gcController.fractionalMarkTime, duration) 1301 atomic.Xaddint64(&pp.gcFractionalMarkTime, duration) 1302 case gcMarkWorkerIdleMode: 1303 atomic.Xaddint64(&gcController.idleMarkTime, duration) 1304 } 1305 1306 // Was this the last worker and did we run out 1307 // of work? 1308 incnwait := atomic.Xadd(&work.nwait, +1) 1309 if incnwait > work.nproc { 1310 println("runtime: p.gcMarkWorkerMode=", pp.gcMarkWorkerMode, 1311 "work.nwait=", incnwait, "work.nproc=", work.nproc) 1312 throw("work.nwait > work.nproc") 1313 } 1314 1315 // We'll releasem after this point and thus this P may run 1316 // something else. We must clear the worker mode to avoid 1317 // attributing the mode to a different (non-worker) G in 1318 // traceGoStart. 1319 pp.gcMarkWorkerMode = gcMarkWorkerNotWorker 1320 1321 // If this worker reached a background mark completion 1322 // point, signal the main GC goroutine. 1323 if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { 1324 // We don't need the P-local buffers here, allow 1325 // preemption becuse we may schedule like a regular 1326 // goroutine in gcMarkDone (block on locks, etc). 1327 releasem(node.m.ptr()) 1328 node.m.set(nil) 1329 1330 gcMarkDone() 1331 } 1332 } 1333 } 1334 1335 // gcMarkWorkAvailable reports whether executing a mark worker 1336 // on p is potentially useful. p may be nil, in which case it only 1337 // checks the global sources of work. 1338 func gcMarkWorkAvailable(p *p) bool { 1339 if p != nil && !p.gcw.empty() { 1340 return true 1341 } 1342 if !work.full.empty() { 1343 return true // global work available 1344 } 1345 if work.markrootNext < work.markrootJobs { 1346 return true // root scan work available 1347 } 1348 return false 1349 } 1350 1351 // gcMark runs the mark (or, for concurrent GC, mark termination) 1352 // All gcWork caches must be empty. 1353 // STW is in effect at this point. 1354 func gcMark(startTime int64) { 1355 if debug.allocfreetrace > 0 { 1356 tracegc() 1357 } 1358 1359 if gcphase != _GCmarktermination { 1360 throw("in gcMark expecting to see gcphase as _GCmarktermination") 1361 } 1362 work.tstart = startTime 1363 1364 // Check that there's no marking work remaining. 1365 if work.full != 0 || work.markrootNext < work.markrootJobs { 1366 print("runtime: full=", hex(work.full), " next=", work.markrootNext, " jobs=", work.markrootJobs, " nDataRoots=", work.nDataRoots, " nBSSRoots=", work.nBSSRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n") 1367 panic("non-empty mark queue after concurrent mark") 1368 } 1369 1370 if debug.gccheckmark > 0 { 1371 // This is expensive when there's a large number of 1372 // Gs, so only do it if checkmark is also enabled. 1373 gcMarkRootCheck() 1374 } 1375 if work.full != 0 { 1376 throw("work.full != 0") 1377 } 1378 1379 // Clear out buffers and double-check that all gcWork caches 1380 // are empty. This should be ensured by gcMarkDone before we 1381 // enter mark termination. 1382 // 1383 // TODO: We could clear out buffers just before mark if this 1384 // has a non-negligible impact on STW time. 1385 for _, p := range allp { 1386 // The write barrier may have buffered pointers since 1387 // the gcMarkDone barrier. However, since the barrier 1388 // ensured all reachable objects were marked, all of 1389 // these must be pointers to black objects. Hence we 1390 // can just discard the write barrier buffer. 1391 if debug.gccheckmark > 0 { 1392 // For debugging, flush the buffer and make 1393 // sure it really was all marked. 1394 wbBufFlush1(p) 1395 } else { 1396 p.wbBuf.reset() 1397 } 1398 1399 gcw := &p.gcw 1400 if !gcw.empty() { 1401 printlock() 1402 print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork) 1403 if gcw.wbuf1 == nil { 1404 print(" wbuf1=<nil>") 1405 } else { 1406 print(" wbuf1.n=", gcw.wbuf1.nobj) 1407 } 1408 if gcw.wbuf2 == nil { 1409 print(" wbuf2=<nil>") 1410 } else { 1411 print(" wbuf2.n=", gcw.wbuf2.nobj) 1412 } 1413 print("\n") 1414 throw("P has cached GC work at end of mark termination") 1415 } 1416 // There may still be cached empty buffers, which we 1417 // need to flush since we're going to free them. Also, 1418 // there may be non-zero stats because we allocated 1419 // black after the gcMarkDone barrier. 1420 gcw.dispose() 1421 } 1422 1423 // Update the marked heap stat. 1424 gcController.heapMarked = work.bytesMarked 1425 1426 // Flush scanAlloc from each mcache since we're about to modify 1427 // heapScan directly. If we were to flush this later, then scanAlloc 1428 // might have incorrect information. 1429 for _, p := range allp { 1430 c := p.mcache 1431 if c == nil { 1432 continue 1433 } 1434 gcController.heapScan += uint64(c.scanAlloc) 1435 c.scanAlloc = 0 1436 } 1437 1438 // Update other GC heap size stats. This must happen after 1439 // cachestats (which flushes local statistics to these) and 1440 // flushallmcaches (which modifies gcController.heapLive). 1441 gcController.heapLive = work.bytesMarked 1442 gcController.heapScan = uint64(gcController.scanWork) 1443 1444 if trace.enabled { 1445 traceHeapAlloc() 1446 } 1447 } 1448 1449 // gcSweep must be called on the system stack because it acquires the heap 1450 // lock. See mheap for details. 1451 // 1452 // The world must be stopped. 1453 // 1454 //go:systemstack 1455 func gcSweep(mode gcMode) { 1456 assertWorldStopped() 1457 1458 if gcphase != _GCoff { 1459 throw("gcSweep being done but phase is not GCoff") 1460 } 1461 1462 lock(&mheap_.lock) 1463 mheap_.sweepgen += 2 1464 mheap_.sweepDrained = 0 1465 mheap_.pagesSwept = 0 1466 mheap_.sweepArenas = mheap_.allArenas 1467 mheap_.reclaimIndex = 0 1468 mheap_.reclaimCredit = 0 1469 unlock(&mheap_.lock) 1470 1471 sweep.centralIndex.clear() 1472 1473 if !_ConcurrentSweep || mode == gcForceBlockMode { 1474 // Special case synchronous sweep. 1475 // Record that no proportional sweeping has to happen. 1476 lock(&mheap_.lock) 1477 mheap_.sweepPagesPerByte = 0 1478 unlock(&mheap_.lock) 1479 // Sweep all spans eagerly. 1480 for sweepone() != ^uintptr(0) { 1481 sweep.npausesweep++ 1482 } 1483 // Free workbufs eagerly. 1484 prepareFreeWorkbufs() 1485 for freeSomeWbufs(false) { 1486 } 1487 // All "free" events for this mark/sweep cycle have 1488 // now happened, so we can make this profile cycle 1489 // available immediately. 1490 mProf_NextCycle() 1491 mProf_Flush() 1492 return 1493 } 1494 1495 // Background sweep. 1496 lock(&sweep.lock) 1497 if sweep.parked { 1498 sweep.parked = false 1499 ready(sweep.g, 0, true) 1500 } 1501 unlock(&sweep.lock) 1502 } 1503 1504 // gcResetMarkState resets global state prior to marking (concurrent 1505 // or STW) and resets the stack scan state of all Gs. 1506 // 1507 // This is safe to do without the world stopped because any Gs created 1508 // during or after this will start out in the reset state. 1509 // 1510 // gcResetMarkState must be called on the system stack because it acquires 1511 // the heap lock. See mheap for details. 1512 // 1513 //go:systemstack 1514 func gcResetMarkState() { 1515 // This may be called during a concurrent phase, so lock to make sure 1516 // allgs doesn't change. 1517 forEachG(func(gp *g) { 1518 gp.gcscandone = false // set to true in gcphasework 1519 gp.gcAssistBytes = 0 1520 }) 1521 1522 // Clear page marks. This is just 1MB per 64GB of heap, so the 1523 // time here is pretty trivial. 1524 lock(&mheap_.lock) 1525 arenas := mheap_.allArenas 1526 unlock(&mheap_.lock) 1527 for _, ai := range arenas { 1528 ha := mheap_.arenas[ai.l1()][ai.l2()] 1529 for i := range ha.pageMarks { 1530 ha.pageMarks[i] = 0 1531 } 1532 } 1533 1534 work.bytesMarked = 0 1535 work.initialHeapLive = atomic.Load64(&gcController.heapLive) 1536 } 1537 1538 // Hooks for other packages 1539 1540 var poolcleanup func() 1541 1542 //go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup 1543 func sync_runtime_registerPoolCleanup(f func()) { 1544 poolcleanup = f 1545 } 1546 1547 func clearpools() { 1548 // clear sync.Pools 1549 if poolcleanup != nil { 1550 poolcleanup() 1551 } 1552 1553 // Clear central sudog cache. 1554 // Leave per-P caches alone, they have strictly bounded size. 1555 // Disconnect cached list before dropping it on the floor, 1556 // so that a dangling ref to one entry does not pin all of them. 1557 lock(&sched.sudoglock) 1558 var sg, sgnext *sudog 1559 for sg = sched.sudogcache; sg != nil; sg = sgnext { 1560 sgnext = sg.next 1561 sg.next = nil 1562 } 1563 sched.sudogcache = nil 1564 unlock(&sched.sudoglock) 1565 1566 // Clear central defer pools. 1567 // Leave per-P pools alone, they have strictly bounded size. 1568 lock(&sched.deferlock) 1569 for i := range sched.deferpool { 1570 // disconnect cached list before dropping it on the floor, 1571 // so that a dangling ref to one entry does not pin all of them. 1572 var d, dlink *_defer 1573 for d = sched.deferpool[i]; d != nil; d = dlink { 1574 dlink = d.link 1575 d.link = nil 1576 } 1577 sched.deferpool[i] = nil 1578 } 1579 unlock(&sched.deferlock) 1580 } 1581 1582 // Timing 1583 1584 // itoaDiv formats val/(10**dec) into buf. 1585 func itoaDiv(buf []byte, val uint64, dec int) []byte { 1586 i := len(buf) - 1 1587 idec := i - dec 1588 for val >= 10 || i >= idec { 1589 buf[i] = byte(val%10 + '0') 1590 i-- 1591 if i == idec { 1592 buf[i] = '.' 1593 i-- 1594 } 1595 val /= 10 1596 } 1597 buf[i] = byte(val + '0') 1598 return buf[i:] 1599 } 1600 1601 // fmtNSAsMS nicely formats ns nanoseconds as milliseconds. 1602 func fmtNSAsMS(buf []byte, ns uint64) []byte { 1603 if ns >= 10e6 { 1604 // Format as whole milliseconds. 1605 return itoaDiv(buf, ns/1e6, 0) 1606 } 1607 // Format two digits of precision, with at most three decimal places. 1608 x := ns / 1e3 1609 if x == 0 { 1610 buf[0] = '0' 1611 return buf[:1] 1612 } 1613 dec := 3 1614 for x >= 100 { 1615 x /= 10 1616 dec-- 1617 } 1618 return itoaDiv(buf, x, dec) 1619 } 1620 1621 // Helpers for testing GC. 1622 1623 // gcTestMoveStackOnNextCall causes the stack to be moved on a call 1624 // immediately following the call to this. It may not work correctly 1625 // if any other work appears after this call (such as returning). 1626 // Typically the following call should be marked go:noinline so it 1627 // performs a stack check. 1628 // 1629 // In rare cases this may not cause the stack to move, specifically if 1630 // there's a preemption between this call and the next. 1631 func gcTestMoveStackOnNextCall() { 1632 gp := getg() 1633 gp.stackguard0 = stackForceMove 1634 } 1635 1636 // gcTestIsReachable performs a GC and returns a bit set where bit i 1637 // is set if ptrs[i] is reachable. 1638 func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) { 1639 // This takes the pointers as unsafe.Pointers in order to keep 1640 // them live long enough for us to attach specials. After 1641 // that, we drop our references to them. 1642 1643 if len(ptrs) > 64 { 1644 panic("too many pointers for uint64 mask") 1645 } 1646 1647 // Block GC while we attach specials and drop our references 1648 // to ptrs. Otherwise, if a GC is in progress, it could mark 1649 // them reachable via this function before we have a chance to 1650 // drop them. 1651 semacquire(&gcsema) 1652 1653 // Create reachability specials for ptrs. 1654 specials := make([]*specialReachable, len(ptrs)) 1655 for i, p := range ptrs { 1656 lock(&mheap_.speciallock) 1657 s := (*specialReachable)(mheap_.specialReachableAlloc.alloc()) 1658 unlock(&mheap_.speciallock) 1659 s.special.kind = _KindSpecialReachable 1660 if !addspecial(p, &s.special) { 1661 throw("already have a reachable special (duplicate pointer?)") 1662 } 1663 specials[i] = s 1664 // Make sure we don't retain ptrs. 1665 ptrs[i] = nil 1666 } 1667 1668 semrelease(&gcsema) 1669 1670 // Force a full GC and sweep. 1671 GC() 1672 1673 // Process specials. 1674 for i, s := range specials { 1675 if !s.done { 1676 printlock() 1677 println("runtime: object", i, "was not swept") 1678 throw("IsReachable failed") 1679 } 1680 if s.reachable { 1681 mask |= 1 << i 1682 } 1683 lock(&mheap_.speciallock) 1684 mheap_.specialReachableAlloc.free(unsafe.Pointer(s)) 1685 unlock(&mheap_.speciallock) 1686 } 1687 1688 return mask 1689 } 1690 1691 // gcTestPointerClass returns the category of what p points to, one of: 1692 // "heap", "stack", "data", "bss", "other". This is useful for checking 1693 // that a test is doing what it's intended to do. 1694 // 1695 // This is nosplit simply to avoid extra pointer shuffling that may 1696 // complicate a test. 1697 // 1698 //go:nosplit 1699 func gcTestPointerClass(p unsafe.Pointer) string { 1700 p2 := uintptr(noescape(p)) 1701 gp := getg() 1702 if gp.stack.lo <= p2 && p2 < gp.stack.hi { 1703 return "stack" 1704 } 1705 if base, _, _ := findObject(p2, 0, 0); base != 0 { 1706 return "heap" 1707 } 1708 for _, datap := range activeModules() { 1709 if datap.data <= p2 && p2 < datap.edata || datap.noptrdata <= p2 && p2 < datap.enoptrdata { 1710 return "data" 1711 } 1712 if datap.bss <= p2 && p2 < datap.ebss || datap.noptrbss <= p2 && p2 <= datap.enoptrbss { 1713 return "bss" 1714 } 1715 } 1716 KeepAlive(p) 1717 return "other" 1718 } 1719