Source file src/runtime/mgcsweep.go
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: sweeping 6 7 // The sweeper consists of two different algorithms: 8 // 9 // * The object reclaimer finds and frees unmarked slots in spans. It 10 // can free a whole span if none of the objects are marked, but that 11 // isn't its goal. This can be driven either synchronously by 12 // mcentral.cacheSpan for mcentral spans, or asynchronously by 13 // sweepone, which looks at all the mcentral lists. 14 // 15 // * The span reclaimer looks for spans that contain no marked objects 16 // and frees whole spans. This is a separate algorithm because 17 // freeing whole spans is the hardest task for the object reclaimer, 18 // but is critical when allocating new spans. The entry point for 19 // this is mheap_.reclaim and it's driven by a sequential scan of 20 // the page marks bitmap in the heap arenas. 21 // 22 // Both algorithms ultimately call mspan.sweep, which sweeps a single 23 // heap span. 24 25 package runtime 26 27 import ( 28 "runtime/internal/atomic" 29 "unsafe" 30 ) 31 32 var sweep sweepdata 33 34 // State of background sweep. 35 type sweepdata struct { 36 lock mutex 37 g *g 38 parked bool 39 started bool 40 41 nbgsweep uint32 42 npausesweep uint32 43 44 // active tracks outstanding sweepers and the sweep 45 // termination condition. 46 active activeSweep 47 48 // centralIndex is the current unswept span class. 49 // It represents an index into the mcentral span 50 // sets. Accessed and updated via its load and 51 // update methods. Not protected by a lock. 52 // 53 // Reset at mark termination. 54 // Used by mheap.nextSpanForSweep. 55 centralIndex sweepClass 56 } 57 58 // sweepClass is a spanClass and one bit to represent whether we're currently 59 // sweeping partial or full spans. 60 type sweepClass uint32 61 62 const ( 63 numSweepClasses = numSpanClasses * 2 64 sweepClassDone sweepClass = sweepClass(^uint32(0)) 65 ) 66 67 func (s *sweepClass) load() sweepClass { 68 return sweepClass(atomic.Load((*uint32)(s))) 69 } 70 71 func (s *sweepClass) update(sNew sweepClass) { 72 // Only update *s if its current value is less than sNew, 73 // since *s increases monotonically. 74 sOld := s.load() 75 for sOld < sNew && !atomic.Cas((*uint32)(s), uint32(sOld), uint32(sNew)) { 76 sOld = s.load() 77 } 78 // TODO(mknyszek): This isn't the only place we have 79 // an atomic monotonically increasing counter. It would 80 // be nice to have an "atomic max" which is just implemented 81 // as the above on most architectures. Some architectures 82 // like RISC-V however have native support for an atomic max. 83 } 84 85 func (s *sweepClass) clear() { 86 atomic.Store((*uint32)(s), 0) 87 } 88 89 // split returns the underlying span class as well as 90 // whether we're interested in the full or partial 91 // unswept lists for that class, indicated as a boolean 92 // (true means "full"). 93 func (s sweepClass) split() (spc spanClass, full bool) { 94 return spanClass(s >> 1), s&1 == 0 95 } 96 97 // nextSpanForSweep finds and pops the next span for sweeping from the 98 // central sweep buffers. It returns ownership of the span to the caller. 99 // Returns nil if no such span exists. 100 func (h *mheap) nextSpanForSweep() *mspan { 101 sg := h.sweepgen 102 for sc := sweep.centralIndex.load(); sc < numSweepClasses; sc++ { 103 spc, full := sc.split() 104 c := &h.central[spc].mcentral 105 var s *mspan 106 if full { 107 s = c.fullUnswept(sg).pop() 108 } else { 109 s = c.partialUnswept(sg).pop() 110 } 111 if s != nil { 112 // Write down that we found something so future sweepers 113 // can start from here. 114 sweep.centralIndex.update(sc) 115 return s 116 } 117 } 118 // Write down that we found nothing. 119 sweep.centralIndex.update(sweepClassDone) 120 return nil 121 } 122 123 const sweepDrainedMask = 1 << 31 124 125 // activeSweep is a type that captures whether sweeping 126 // is done, and whether there are any outstanding sweepers. 127 // 128 // Every potential sweeper must call begin() before they look 129 // for work, and end() after they've finished sweeping. 130 type activeSweep struct { 131 // state is divided into two parts. 132 // 133 // The top bit (masked by sweepDrainedMask) is a boolean 134 // value indicating whether all the sweep work has been 135 // drained from the queue. 136 // 137 // The rest of the bits are a counter, indicating the 138 // number of outstanding concurrent sweepers. 139 state atomic.Uint32 140 } 141 142 // begin registers a new sweeper. Returns a sweepLocker 143 // for acquiring spans for sweeping. Any outstanding sweeper blocks 144 // sweep termination. 145 // 146 // If the sweepLocker is invalid, the caller can be sure that all 147 // outstanding sweep work has been drained, so there is nothing left 148 // to sweep. Note that there may be sweepers currently running, so 149 // this does not indicate that all sweeping has completed. 150 // 151 // Even if the sweepLocker is invalid, its sweepGen is always valid. 152 func (a *activeSweep) begin() sweepLocker { 153 for { 154 state := a.state.Load() 155 if state&sweepDrainedMask != 0 { 156 return sweepLocker{mheap_.sweepgen, false} 157 } 158 if a.state.CompareAndSwap(state, state+1) { 159 return sweepLocker{mheap_.sweepgen, true} 160 } 161 } 162 } 163 164 // end deregisters a sweeper. Must be called once for each time 165 // begin is called if the sweepLocker is valid. 166 func (a *activeSweep) end(sl sweepLocker) { 167 if sl.sweepGen != mheap_.sweepgen { 168 throw("sweeper left outstanding across sweep generations") 169 } 170 for { 171 state := a.state.Load() 172 if (state&^sweepDrainedMask)-1 >= sweepDrainedMask { 173 throw("mismatched begin/end of activeSweep") 174 } 175 if a.state.CompareAndSwap(state, state-1) { 176 if state != sweepDrainedMask { 177 return 178 } 179 if debug.gcpacertrace > 0 { 180 print("pacer: sweep done at heap size ", gcController.heapLive>>20, "MB; allocated ", (gcController.heapLive-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept.Load(), " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n") 181 } 182 return 183 } 184 } 185 } 186 187 // markDrained marks the active sweep cycle as having drained 188 // all remaining work. This is safe to be called concurrently 189 // with all other methods of activeSweep, though may race. 190 // 191 // Returns true if this call was the one that actually performed 192 // the mark. 193 func (a *activeSweep) markDrained() bool { 194 for { 195 state := a.state.Load() 196 if state&sweepDrainedMask != 0 { 197 return false 198 } 199 if a.state.CompareAndSwap(state, state|sweepDrainedMask) { 200 return true 201 } 202 } 203 } 204 205 // sweepers returns the current number of active sweepers. 206 func (a *activeSweep) sweepers() uint32 { 207 return a.state.Load() &^ sweepDrainedMask 208 } 209 210 // isDone returns true if all sweep work has been drained and no more 211 // outstanding sweepers exist. That is, when the sweep phase is 212 // completely done. 213 func (a *activeSweep) isDone() bool { 214 return a.state.Load() == sweepDrainedMask 215 } 216 217 // reset sets up the activeSweep for the next sweep cycle. 218 // 219 // The world must be stopped. 220 func (a *activeSweep) reset() { 221 assertWorldStopped() 222 a.state.Store(0) 223 } 224 225 // finishsweep_m ensures that all spans are swept. 226 // 227 // The world must be stopped. This ensures there are no sweeps in 228 // progress. 229 // 230 //go:nowritebarrier 231 func finishsweep_m() { 232 assertWorldStopped() 233 234 // Sweeping must be complete before marking commences, so 235 // sweep any unswept spans. If this is a concurrent GC, there 236 // shouldn't be any spans left to sweep, so this should finish 237 // instantly. If GC was forced before the concurrent sweep 238 // finished, there may be spans to sweep. 239 for sweepone() != ^uintptr(0) { 240 sweep.npausesweep++ 241 } 242 243 // Make sure there aren't any outstanding sweepers left. 244 // At this point, with the world stopped, it means one of two 245 // things. Either we were able to preempt a sweeper, or that 246 // a sweeper didn't call sweep.active.end when it should have. 247 // Both cases indicate a bug, so throw. 248 if sweep.active.sweepers() != 0 { 249 throw("active sweepers found at start of mark phase") 250 } 251 252 // Reset all the unswept buffers, which should be empty. 253 // Do this in sweep termination as opposed to mark termination 254 // so that we can catch unswept spans and reclaim blocks as 255 // soon as possible. 256 sg := mheap_.sweepgen 257 for i := range mheap_.central { 258 c := &mheap_.central[i].mcentral 259 c.partialUnswept(sg).reset() 260 c.fullUnswept(sg).reset() 261 } 262 263 // Sweeping is done, so if the scavenger isn't already awake, 264 // wake it up. There's definitely work for it to do at this 265 // point. 266 scavenger.wake() 267 268 nextMarkBitArenaEpoch() 269 } 270 271 func bgsweep(c chan int) { 272 sweep.g = getg() 273 274 lockInit(&sweep.lock, lockRankSweep) 275 lock(&sweep.lock) 276 sweep.parked = true 277 c <- 1 278 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1) 279 280 for { 281 for sweepone() != ^uintptr(0) { 282 sweep.nbgsweep++ 283 Gosched() 284 } 285 for freeSomeWbufs(true) { 286 Gosched() 287 } 288 lock(&sweep.lock) 289 if !isSweepDone() { 290 // This can happen if a GC runs between 291 // gosweepone returning ^0 above 292 // and the lock being acquired. 293 unlock(&sweep.lock) 294 continue 295 } 296 sweep.parked = true 297 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1) 298 } 299 } 300 301 // sweepLocker acquires sweep ownership of spans. 302 type sweepLocker struct { 303 // sweepGen is the sweep generation of the heap. 304 sweepGen uint32 305 valid bool 306 } 307 308 // sweepLocked represents sweep ownership of a span. 309 type sweepLocked struct { 310 *mspan 311 } 312 313 // tryAcquire attempts to acquire sweep ownership of span s. If it 314 // successfully acquires ownership, it blocks sweep completion. 315 func (l *sweepLocker) tryAcquire(s *mspan) (sweepLocked, bool) { 316 if !l.valid { 317 throw("use of invalid sweepLocker") 318 } 319 // Check before attempting to CAS. 320 if atomic.Load(&s.sweepgen) != l.sweepGen-2 { 321 return sweepLocked{}, false 322 } 323 // Attempt to acquire sweep ownership of s. 324 if !atomic.Cas(&s.sweepgen, l.sweepGen-2, l.sweepGen-1) { 325 return sweepLocked{}, false 326 } 327 return sweepLocked{s}, true 328 } 329 330 // sweepone sweeps some unswept heap span and returns the number of pages returned 331 // to the heap, or ^uintptr(0) if there was nothing to sweep. 332 func sweepone() uintptr { 333 gp := getg() 334 335 // Increment locks to ensure that the goroutine is not preempted 336 // in the middle of sweep thus leaving the span in an inconsistent state for next GC 337 gp.m.locks++ 338 339 // TODO(austin): sweepone is almost always called in a loop; 340 // lift the sweepLocker into its callers. 341 sl := sweep.active.begin() 342 if !sl.valid { 343 gp.m.locks-- 344 return ^uintptr(0) 345 } 346 347 // Find a span to sweep. 348 npages := ^uintptr(0) 349 var noMoreWork bool 350 for { 351 s := mheap_.nextSpanForSweep() 352 if s == nil { 353 noMoreWork = sweep.active.markDrained() 354 break 355 } 356 if state := s.state.get(); state != mSpanInUse { 357 // This can happen if direct sweeping already 358 // swept this span, but in that case the sweep 359 // generation should always be up-to-date. 360 if !(s.sweepgen == sl.sweepGen || s.sweepgen == sl.sweepGen+3) { 361 print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sl.sweepGen, "\n") 362 throw("non in-use span in unswept list") 363 } 364 continue 365 } 366 if s, ok := sl.tryAcquire(s); ok { 367 // Sweep the span we found. 368 npages = s.npages 369 if s.sweep(false) { 370 // Whole span was freed. Count it toward the 371 // page reclaimer credit since these pages can 372 // now be used for span allocation. 373 mheap_.reclaimCredit.Add(npages) 374 } else { 375 // Span is still in-use, so this returned no 376 // pages to the heap and the span needs to 377 // move to the swept in-use list. 378 npages = 0 379 } 380 break 381 } 382 } 383 sweep.active.end(sl) 384 385 if noMoreWork { 386 // The sweep list is empty. There may still be 387 // concurrent sweeps running, but we're at least very 388 // close to done sweeping. 389 390 // Move the scavenge gen forward (signaling 391 // that there's new work to do) and wake the scavenger. 392 // 393 // The scavenger is signaled by the last sweeper because once 394 // sweeping is done, we will definitely have useful work for 395 // the scavenger to do, since the scavenger only runs over the 396 // heap once per GC cycle. This update is not done during sweep 397 // termination because in some cases there may be a long delay 398 // between sweep done and sweep termination (e.g. not enough 399 // allocations to trigger a GC) which would be nice to fill in 400 // with scavenging work. 401 if debug.scavtrace > 0 { 402 systemstack(func() { 403 lock(&mheap_.lock) 404 released := atomic.Loaduintptr(&mheap_.pages.scav.released) 405 printScavTrace(released, false) 406 atomic.Storeuintptr(&mheap_.pages.scav.released, 0) 407 unlock(&mheap_.lock) 408 }) 409 } 410 scavenger.ready() 411 } 412 413 gp.m.locks-- 414 return npages 415 } 416 417 // isSweepDone reports whether all spans are swept. 418 // 419 // Note that this condition may transition from false to true at any 420 // time as the sweeper runs. It may transition from true to false if a 421 // GC runs; to prevent that the caller must be non-preemptible or must 422 // somehow block GC progress. 423 func isSweepDone() bool { 424 return sweep.active.isDone() 425 } 426 427 // Returns only when span s has been swept. 428 // 429 //go:nowritebarrier 430 func (s *mspan) ensureSwept() { 431 // Caller must disable preemption. 432 // Otherwise when this function returns the span can become unswept again 433 // (if GC is triggered on another goroutine). 434 _g_ := getg() 435 if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 { 436 throw("mspan.ensureSwept: m is not locked") 437 } 438 439 // If this operation fails, then that means that there are 440 // no more spans to be swept. In this case, either s has already 441 // been swept, or is about to be acquired for sweeping and swept. 442 sl := sweep.active.begin() 443 if sl.valid { 444 // The caller must be sure that the span is a mSpanInUse span. 445 if s, ok := sl.tryAcquire(s); ok { 446 s.sweep(false) 447 sweep.active.end(sl) 448 return 449 } 450 sweep.active.end(sl) 451 } 452 453 // Unfortunately we can't sweep the span ourselves. Somebody else 454 // got to it first. We don't have efficient means to wait, but that's 455 // OK, it will be swept fairly soon. 456 for { 457 spangen := atomic.Load(&s.sweepgen) 458 if spangen == sl.sweepGen || spangen == sl.sweepGen+3 { 459 break 460 } 461 osyield() 462 } 463 } 464 465 // Sweep frees or collects finalizers for blocks not marked in the mark phase. 466 // It clears the mark bits in preparation for the next GC round. 467 // Returns true if the span was returned to heap. 468 // If preserve=true, don't return it to heap nor relink in mcentral lists; 469 // caller takes care of it. 470 func (sl *sweepLocked) sweep(preserve bool) bool { 471 // It's critical that we enter this function with preemption disabled, 472 // GC must not start while we are in the middle of this function. 473 _g_ := getg() 474 if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 { 475 throw("mspan.sweep: m is not locked") 476 } 477 478 s := sl.mspan 479 if !preserve { 480 // We'll release ownership of this span. Nil it out to 481 // prevent the caller from accidentally using it. 482 sl.mspan = nil 483 } 484 485 sweepgen := mheap_.sweepgen 486 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 { 487 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n") 488 throw("mspan.sweep: bad span state") 489 } 490 491 if trace.enabled { 492 traceGCSweepSpan(s.npages * _PageSize) 493 } 494 495 mheap_.pagesSwept.Add(int64(s.npages)) 496 497 spc := s.spanclass 498 size := s.elemsize 499 500 // The allocBits indicate which unmarked objects don't need to be 501 // processed since they were free at the end of the last GC cycle 502 // and were not allocated since then. 503 // If the allocBits index is >= s.freeindex and the bit 504 // is not marked then the object remains unallocated 505 // since the last GC. 506 // This situation is analogous to being on a freelist. 507 508 // Unlink & free special records for any objects we're about to free. 509 // Two complications here: 510 // 1. An object can have both finalizer and profile special records. 511 // In such case we need to queue finalizer for execution, 512 // mark the object as live and preserve the profile special. 513 // 2. A tiny object can have several finalizers setup for different offsets. 514 // If such object is not marked, we need to queue all finalizers at once. 515 // Both 1 and 2 are possible at the same time. 516 hadSpecials := s.specials != nil 517 siter := newSpecialsIter(s) 518 for siter.valid() { 519 // A finalizer can be set for an inner byte of an object, find object beginning. 520 objIndex := uintptr(siter.s.offset) / size 521 p := s.base() + objIndex*size 522 mbits := s.markBitsForIndex(objIndex) 523 if !mbits.isMarked() { 524 // This object is not marked and has at least one special record. 525 // Pass 1: see if it has at least one finalizer. 526 hasFin := false 527 endOffset := p - s.base() + size 528 for tmp := siter.s; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next { 529 if tmp.kind == _KindSpecialFinalizer { 530 // Stop freeing of object if it has a finalizer. 531 mbits.setMarkedNonAtomic() 532 hasFin = true 533 break 534 } 535 } 536 // Pass 2: queue all finalizers _or_ handle profile record. 537 for siter.valid() && uintptr(siter.s.offset) < endOffset { 538 // Find the exact byte for which the special was setup 539 // (as opposed to object beginning). 540 special := siter.s 541 p := s.base() + uintptr(special.offset) 542 if special.kind == _KindSpecialFinalizer || !hasFin { 543 siter.unlinkAndNext() 544 freeSpecial(special, unsafe.Pointer(p), size) 545 } else { 546 // The object has finalizers, so we're keeping it alive. 547 // All other specials only apply when an object is freed, 548 // so just keep the special record. 549 siter.next() 550 } 551 } 552 } else { 553 // object is still live 554 if siter.s.kind == _KindSpecialReachable { 555 special := siter.unlinkAndNext() 556 (*specialReachable)(unsafe.Pointer(special)).reachable = true 557 freeSpecial(special, unsafe.Pointer(p), size) 558 } else { 559 // keep special record 560 siter.next() 561 } 562 } 563 } 564 if hadSpecials && s.specials == nil { 565 spanHasNoSpecials(s) 566 } 567 568 if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled || asanenabled { 569 // Find all newly freed objects. This doesn't have to 570 // efficient; allocfreetrace has massive overhead. 571 mbits := s.markBitsForBase() 572 abits := s.allocBitsForIndex(0) 573 for i := uintptr(0); i < s.nelems; i++ { 574 if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) { 575 x := s.base() + i*s.elemsize 576 if debug.allocfreetrace != 0 { 577 tracefree(unsafe.Pointer(x), size) 578 } 579 if debug.clobberfree != 0 { 580 clobberfree(unsafe.Pointer(x), size) 581 } 582 if raceenabled { 583 racefree(unsafe.Pointer(x), size) 584 } 585 if msanenabled { 586 msanfree(unsafe.Pointer(x), size) 587 } 588 if asanenabled { 589 asanpoison(unsafe.Pointer(x), size) 590 } 591 } 592 mbits.advance() 593 abits.advance() 594 } 595 } 596 597 // Check for zombie objects. 598 if s.freeindex < s.nelems { 599 // Everything < freeindex is allocated and hence 600 // cannot be zombies. 601 // 602 // Check the first bitmap byte, where we have to be 603 // careful with freeindex. 604 obj := s.freeindex 605 if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 { 606 s.reportZombies() 607 } 608 // Check remaining bytes. 609 for i := obj/8 + 1; i < divRoundUp(s.nelems, 8); i++ { 610 if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 { 611 s.reportZombies() 612 } 613 } 614 } 615 616 // Count the number of free objects in this span. 617 nalloc := uint16(s.countAlloc()) 618 nfreed := s.allocCount - nalloc 619 if nalloc > s.allocCount { 620 // The zombie check above should have caught this in 621 // more detail. 622 print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n") 623 throw("sweep increased allocation count") 624 } 625 626 s.allocCount = nalloc 627 s.freeindex = 0 // reset allocation index to start of span. 628 if trace.enabled { 629 getg().m.p.ptr().traceReclaimed += uintptr(nfreed) * s.elemsize 630 } 631 632 // gcmarkBits becomes the allocBits. 633 // get a fresh cleared gcmarkBits in preparation for next GC 634 s.allocBits = s.gcmarkBits 635 s.gcmarkBits = newMarkBits(s.nelems) 636 637 // Initialize alloc bits cache. 638 s.refillAllocCache(0) 639 640 // The span must be in our exclusive ownership until we update sweepgen, 641 // check for potential races. 642 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 { 643 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n") 644 throw("mspan.sweep: bad span state after sweep") 645 } 646 if s.sweepgen == sweepgen+1 || s.sweepgen == sweepgen+3 { 647 throw("swept cached span") 648 } 649 650 // We need to set s.sweepgen = h.sweepgen only when all blocks are swept, 651 // because of the potential for a concurrent free/SetFinalizer. 652 // 653 // But we need to set it before we make the span available for allocation 654 // (return it to heap or mcentral), because allocation code assumes that a 655 // span is already swept if available for allocation. 656 // 657 // Serialization point. 658 // At this point the mark bits are cleared and allocation ready 659 // to go so release the span. 660 atomic.Store(&s.sweepgen, sweepgen) 661 662 if spc.sizeclass() != 0 { 663 // Handle spans for small objects. 664 if nfreed > 0 { 665 // Only mark the span as needing zeroing if we've freed any 666 // objects, because a fresh span that had been allocated into, 667 // wasn't totally filled, but then swept, still has all of its 668 // free slots zeroed. 669 s.needzero = 1 670 stats := memstats.heapStats.acquire() 671 atomic.Xadd64(&stats.smallFreeCount[spc.sizeclass()], int64(nfreed)) 672 memstats.heapStats.release() 673 674 // Count the frees in the inconsistent, internal stats. 675 gcController.totalFree.Add(int64(nfreed) * int64(s.elemsize)) 676 } 677 if !preserve { 678 // The caller may not have removed this span from whatever 679 // unswept set its on but taken ownership of the span for 680 // sweeping by updating sweepgen. If this span still is in 681 // an unswept set, then the mcentral will pop it off the 682 // set, check its sweepgen, and ignore it. 683 if nalloc == 0 { 684 // Free totally free span directly back to the heap. 685 mheap_.freeSpan(s) 686 return true 687 } 688 // Return span back to the right mcentral list. 689 if uintptr(nalloc) == s.nelems { 690 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s) 691 } else { 692 mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s) 693 } 694 } 695 } else if !preserve { 696 // Handle spans for large objects. 697 if nfreed != 0 { 698 // Free large object span to heap. 699 700 // NOTE(rsc,dvyukov): The original implementation of efence 701 // in CL 22060046 used sysFree instead of sysFault, so that 702 // the operating system would eventually give the memory 703 // back to us again, so that an efence program could run 704 // longer without running out of memory. Unfortunately, 705 // calling sysFree here without any kind of adjustment of the 706 // heap data structures means that when the memory does 707 // come back to us, we have the wrong metadata for it, either in 708 // the mspan structures or in the garbage collection bitmap. 709 // Using sysFault here means that the program will run out of 710 // memory fairly quickly in efence mode, but at least it won't 711 // have mysterious crashes due to confused memory reuse. 712 // It should be possible to switch back to sysFree if we also 713 // implement and then call some kind of mheap.deleteSpan. 714 if debug.efence > 0 { 715 s.limit = 0 // prevent mlookup from finding this span 716 sysFault(unsafe.Pointer(s.base()), size) 717 } else { 718 mheap_.freeSpan(s) 719 } 720 721 // Count the free in the consistent, external stats. 722 stats := memstats.heapStats.acquire() 723 atomic.Xadd64(&stats.largeFreeCount, 1) 724 atomic.Xadd64(&stats.largeFree, int64(size)) 725 memstats.heapStats.release() 726 727 // Count the free in the inconsistent, internal stats. 728 gcController.totalFree.Add(int64(size)) 729 730 return true 731 } 732 733 // Add a large span directly onto the full+swept list. 734 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s) 735 } 736 return false 737 } 738 739 // reportZombies reports any marked but free objects in s and throws. 740 // 741 // This generally means one of the following: 742 // 743 // 1. User code converted a pointer to a uintptr and then back 744 // unsafely, and a GC ran while the uintptr was the only reference to 745 // an object. 746 // 747 // 2. User code (or a compiler bug) constructed a bad pointer that 748 // points to a free slot, often a past-the-end pointer. 749 // 750 // 3. The GC two cycles ago missed a pointer and freed a live object, 751 // but it was still live in the last cycle, so this GC cycle found a 752 // pointer to that object and marked it. 753 func (s *mspan) reportZombies() { 754 printlock() 755 print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n") 756 mbits := s.markBitsForBase() 757 abits := s.allocBitsForIndex(0) 758 for i := uintptr(0); i < s.nelems; i++ { 759 addr := s.base() + i*s.elemsize 760 print(hex(addr)) 761 alloc := i < s.freeindex || abits.isMarked() 762 if alloc { 763 print(" alloc") 764 } else { 765 print(" free ") 766 } 767 if mbits.isMarked() { 768 print(" marked ") 769 } else { 770 print(" unmarked") 771 } 772 zombie := mbits.isMarked() && !alloc 773 if zombie { 774 print(" zombie") 775 } 776 print("\n") 777 if zombie { 778 length := s.elemsize 779 if length > 1024 { 780 length = 1024 781 } 782 hexdumpWords(addr, addr+length, nil) 783 } 784 mbits.advance() 785 abits.advance() 786 } 787 throw("found pointer to free object") 788 } 789 790 // deductSweepCredit deducts sweep credit for allocating a span of 791 // size spanBytes. This must be performed *before* the span is 792 // allocated to ensure the system has enough credit. If necessary, it 793 // performs sweeping to prevent going in to debt. If the caller will 794 // also sweep pages (e.g., for a large allocation), it can pass a 795 // non-zero callerSweepPages to leave that many pages unswept. 796 // 797 // deductSweepCredit makes a worst-case assumption that all spanBytes 798 // bytes of the ultimately allocated span will be available for object 799 // allocation. 800 // 801 // deductSweepCredit is the core of the "proportional sweep" system. 802 // It uses statistics gathered by the garbage collector to perform 803 // enough sweeping so that all pages are swept during the concurrent 804 // sweep phase between GC cycles. 805 // 806 // mheap_ must NOT be locked. 807 func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) { 808 if mheap_.sweepPagesPerByte == 0 { 809 // Proportional sweep is done or disabled. 810 return 811 } 812 813 if trace.enabled { 814 traceGCSweepStart() 815 } 816 817 retry: 818 sweptBasis := mheap_.pagesSweptBasis.Load() 819 820 // Fix debt if necessary. 821 newHeapLive := uintptr(atomic.Load64(&gcController.heapLive)-mheap_.sweepHeapLiveBasis) + spanBytes 822 pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages) 823 for pagesTarget > int64(mheap_.pagesSwept.Load()-sweptBasis) { 824 if sweepone() == ^uintptr(0) { 825 mheap_.sweepPagesPerByte = 0 826 break 827 } 828 if mheap_.pagesSweptBasis.Load() != sweptBasis { 829 // Sweep pacing changed. Recompute debt. 830 goto retry 831 } 832 } 833 834 if trace.enabled { 835 traceGCSweepDone() 836 } 837 } 838 839 // clobberfree sets the memory content at x to bad content, for debugging 840 // purposes. 841 func clobberfree(x unsafe.Pointer, size uintptr) { 842 // size (span.elemsize) is always a multiple of 4. 843 for i := uintptr(0); i < size; i += 4 { 844 *(*uint32)(add(x, i)) = 0xdeadbeef 845 } 846 } 847 848 // gcPaceSweeper updates the sweeper's pacing parameters. 849 // 850 // Must be called whenever the GC's pacing is updated. 851 // 852 // The world must be stopped, or mheap_.lock must be held. 853 func gcPaceSweeper(trigger uint64) { 854 assertWorldStoppedOrLockHeld(&mheap_.lock) 855 856 // Update sweep pacing. 857 if isSweepDone() { 858 mheap_.sweepPagesPerByte = 0 859 } else { 860 // Concurrent sweep needs to sweep all of the in-use 861 // pages by the time the allocated heap reaches the GC 862 // trigger. Compute the ratio of in-use pages to sweep 863 // per byte allocated, accounting for the fact that 864 // some might already be swept. 865 heapLiveBasis := atomic.Load64(&gcController.heapLive) 866 heapDistance := int64(trigger) - int64(heapLiveBasis) 867 // Add a little margin so rounding errors and 868 // concurrent sweep are less likely to leave pages 869 // unswept when GC starts. 870 heapDistance -= 1024 * 1024 871 if heapDistance < _PageSize { 872 // Avoid setting the sweep ratio extremely high 873 heapDistance = _PageSize 874 } 875 pagesSwept := mheap_.pagesSwept.Load() 876 pagesInUse := mheap_.pagesInUse.Load() 877 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept) 878 if sweepDistancePages <= 0 { 879 mheap_.sweepPagesPerByte = 0 880 } else { 881 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance) 882 mheap_.sweepHeapLiveBasis = heapLiveBasis 883 // Write pagesSweptBasis last, since this 884 // signals concurrent sweeps to recompute 885 // their debt. 886 mheap_.pagesSweptBasis.Store(pagesSwept) 887 } 888 } 889 } 890