GC 和用户线程并发运行，在 GC 的概念中常见的 mutator 这个词是 Dijkstra 发明的，mutator 本意是改变者，变异子，在 GC 中将用户程序称为 mutator，是因为用户代码会不断地改变对象之间的引用关系。所以我们也看到有人说 mutator 是 fancy word for the 'real program')
//
// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
// GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
// non-generational and non-compacting. Allocation is done using size segregated per P allocation
// areas to minimize fragmentation while eliminating locks in the common case.
//
// The algorithm decomposes into several steps.
// This is a high level description of the algorithm being used. For an overview of GC a good
// place to start is Richard Jones' gchandbook.org.
//
// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978),
// 966-975.
// For journal quality proofs that these steps are complete, correct, and terminate see
// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
// Concurrency and Computation: Practice and Experience 15(3-5), 2003.
//
// 1. GC performs sweep termination.
//
// a. Stop the world. This causes all Ps to reach a GC safe-point.
//
// b. Sweep any unswept spans. There will only be unswept spans if
// this GC cycle was forced before the expected time.
//
// 2. GC performs the mark phase.
//
// a. Prepare for the mark phase by setting gcphase to _GCmark
// (from _GCoff), enabling the write barrier, enabling mutator
// assists, and enqueueing root mark jobs. No objects may be
// scanned until all Ps have enabled the write barrier, which is
// accomplished using STW.
//
// b. Start the world. From this point, GC work is done by mark
// workers started by the scheduler and by assists performed as
// part of allocation. The write barrier shades both the
// overwritten pointer and the new pointer value for any pointer
// writes (see mbarrier.go for details). Newly allocated objects
// are immediately marked black.
//
// c. GC performs root marking jobs. This includes scanning all
// stacks, shading all globals, and shading any heap pointers in
// off-heap runtime data structures. Scanning a stack stops a
// goroutine, shades any pointers found on its stack, and then
// resumes the goroutine.
//
// d. GC drains the work queue of grey objects, scanning each grey
// object to black and shading all pointers found in the object
// (which in turn may add those pointers to the work queue).
//
// e. Because GC work is spread across local caches, GC uses a
// distributed termination algorithm to detect when there are no
// more root marking jobs or grey objects (see gcMarkDone). At this
// point, GC transitions to mark termination.
//
// 3. GC performs mark termination.
//
// a. Stop the world.
//
// b. Set gcphase to _GCmarktermination, and disable workers and
// assists.
//
// c. Perform housekeeping like flushing mcaches.
//
// 4. GC performs the sweep phase.
//
// a. Prepare for the sweep phase by setting gcphase to _GCoff,
// setting up sweep state and disabling the write barrier.
//
// b. Start the world. From this point on, newly allocated objects
// are white, and allocating sweeps spans before use if necessary.
//
// c. GC does concurrent sweeping in the background and in response
// to allocation. See description below.
//
// 5. When sufficient allocation has taken place, replay the sequence
// starting with 1 above. See discussion of GC rate below.
// Concurrent sweep.
//
// The sweep phase proceeds concurrently with normal program execution.
// The heap is swept span-by-span both lazily (when a goroutine needs another span)
// and concurrently in a background goroutine (this helps programs that are not CPU bound).
// At the end of STW mark termination all spans are marked as "needs sweeping".
//
// The background sweeper goroutine simply sweeps spans one-by-one.
//
// To avoid requesting more OS memory while there are unswept spans, when a
// goroutine needs another span, it first attempts to reclaim that much memory
// by sweeping. When a goroutine needs to allocate a new small-object span, it
// sweeps small-object spans for the same object size until it frees at least
// one object. When a goroutine needs to allocate large-object span from heap,
// it sweeps spans until it frees at least that many pages into heap. There is
// one case where this may not suffice: if a goroutine sweeps and frees two
// nonadjacent one-page spans to the heap, it will allocate a new two-page
// span, but there can still be other one-page unswept spans which could be
// combined into a two-page span.
//
// It's critical to ensure that no operations proceed on unswept spans (that would corrupt
// mark bits in GC bitmap). During GC all mcaches are flushed into the central cache,
// so they are empty. When a goroutine grabs a new span into mcache, it sweeps it.
// When a goroutine explicitly frees an object or sets a finalizer, it ensures that
// the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish).
// The finalizer goroutine is kicked off only when all spans are swept.
// When the next GC starts, it sweeps all not-yet-swept spans (if any).
// GC rate.
// Next GC is after we've allocated an extra amount of memory proportional to
// the amount already in use. The proportion is controlled by GOGC environment variable
// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
// (this mark is tracked in next_gc variable). This keeps the GC cost in linear
// proportion to the allocation cost. Adjusting GOGC just changes the linear constant
// (and also the amount of extra memory used).
// Oblets
//
// In order to prevent long pauses while scanning large objects and to
// improve parallelism, the garbage collector breaks up scan jobs for
// objects larger than maxObletBytes into "oblets" of at most
// maxObletBytes. When scanning encounters the beginning of a large
// object, it scans only the first oblet and enqueues the remaining
// oblets as new scan jobs.
┌─────────────────┐
┌───────────────────────────────────────────────────────│ forcegchelper │
│ ├─────────────────┤
├───────────────────────────────────────────────────────│ GC │
│ ├─────────────────┤
├───────────────────────────────────────────────────────│ mallocgc │
│ └─────────────────┘
▼
┌──────────────────┐
┌─────────────────────────────────────┐ │ gcStart │
│ │ ├───┬──────────────┴────────────────────┐
▼ │ │ 1 │ semacquire(&work.startSema) │
┌───────────────┐ │ ├───┼───────────────────────────────────┤ ┌────────────────┐
│ startCycle │ │ │ 2 │ semacquire(&worldsema) │ │ schedule() │
├───┬───────────┴───────────────────────┐ │ ├───┼───────────────────────────────────┤ ┌─────────────────────────┐ ├────────────────┴──────────────────────┐
│ 1 │ init gcControllerState │ │ │ 3 │ gcBgMarkStartWorkers() │───────────▶│ gcBgMarkStartWorkers() │ │ ..... │
├───┼───────────────────────────────────┤ │ ├───┼───────────────────────────────────┤ ├───┬─────────────────────┴─────────────┐ ├───┬───────────────────────────────────┤
│ 2 │ calc nextgc │ │ │ 4 │ systemstack(gcResetMarkState) │ │ 1 │ loop until allp │ │ 1 │ gcBlackenEnabled != 0 │
├───┼───────────────────────────────────┤ │ ├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤
│ 3 │ calc c.dedicatedMarkWorkersNeeded │ │ │ 5 │ systemstack(stopTheWorldWithSema) │ │ 2 │ go gcBgMarkWorker(p) │ │ 2 │ findRunnableGCWorker() │
├───┼───────────────────────────────────┤ │ ├───┼───────────────────────────────────┤ └───┴───────────────────────────────────┘ └───┴───────────────────────────────────┘
│ 4 │ clear per-P state │ │ │ 6 │ systemstack(finishsweep_m()) │ │ ▲
├───┼───────────────────────────────────┤ │ ├───┼───────────────────────────────────┤ │ │
│ 5 │ c.revise() │ │ │ 7 │ clearpools() │ │ │
└───┴───────────────────────────────────┘ │ ├───┼───────────────────────────────────┤ │ │
└────────────────────────────│ 8 │ gcController.startCycle() │ │ │
├───┼───────────────────────────────────┤ │ │
│10 │ work.heapGoal = memstats.next_gc │ ▼ │
├───┼───────────────────────────────────┴───────────────┐┌───────────────────────────────────┐ │
│11 │if mode != gcBackgroundMode{schedEnableUser(false)}││ go gcBgMarkWorker(p) │ │
├───┼───────────────────────────────────┬───────────────┘├───┬───────────────────────────────┴──────────┐ ┌───────────┴──────┐
│11 │ setGCPhase(_GCmark) │ │ 1 │ gopark() │◀─────────────────────────────┤ wakeup │
├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤ └──────────────────┘
│12 │ gcBgMarkPrepare() │ │ 2 │ casgstatus(gp, _Grunning, _Gwaiting) │
├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤
┌────────────────────────────────────────────────────────────────────│13 │ gcMarkRootPrepare() │ │ 3 │ gcDrain() │───────────────────────┐
│ ├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤ │ ┌──────────────────┐
│ │14 │ gcMarkTinyAllocs() │ │ 4 │ casgstatus(gp, _Gwaiting, _Grunning) │ └───────────────────────▶│ gcDrain() │
│ ├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤ ├──────────┬───────┴────────────────────┐
│ │15 │atomic.Store(&gcBlackenEnabled, 1) │ │ 5 │ if all worker done │ │ loop │ markroot(gcw, job) │
│ ├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤ ├──────────┼────────────────────────────┤
│ │16 │systemstack(startTheWorldWithSema()│ │ 6 │ _p_.gcBgMarkWorker.set(nil) │ │ loop2 │ gcw.balance() │
│ ├───┼───────────────────────────────────┤ ├───┼──────────────────────────────────────────┤ ├──────────┼────────────────────────────┤
│ │17 │ semrelease(&work.startSema) │ │ 7 │ releasem(park.m.ptr()) │ │ loop2 │ gcw.tryGetFast() │
│ └───┴───────────────────────────────────┘ ├───┼──────────────────────────────────────────┤ ├──────────┼────────────────────────────┤
│ │ 8 │ gcMarkDone() │ │ loop2 │ gcw.tryGet() │
│ └───┴──────────────────────────────────────────┘ ├──────────┼────────────────────────────┤
│ │ │ loop2 │ scanobject(b, gcw) │
│ │ ├──────────┼────────────────────────────┤
│ │ │ loop2 │ flush credit to global │
│ │ ├──────────┼────────────────────────────┤
│ │ │ done │ gcFlushBgCredit │
│ │ └──────────┴────────────────────────────┘
│ │
│ │
│ ▼
│ ┌────────────────┐
│ │ gcMarkDone() │
│ ├───┬────────────┴──────────────────────────────────┐
▼ │ 1 │ semacquire(&work.markDoneSema) │
┌──────────────────────┐ ├───┼───────────────────────────────────────────────┤
│ gcMarkRootPrepare() │ │ 2 │ gcMarkDoneFlushed = 0 │
├──────────────────────┴────────────┐ ├───┼───────────────────────────────────────────────┤
│ set work.nDataRoots │ │ 3 │ casgstatus(gp, _Grunning, _Gwaiting) │
├───────────────────────────────────┤ ├───┼──────────┬──────────────────┬─────────────────┴┬──────────────────┬────────────────────────────────────┐
│ set work.nBSSRoots │ │ 4 │ forEachP │ wbBufFlush1(_p_) │_p_.gcw.dispose() │_p_.gcw.dispose() │ atomic.Xadd(&gcMarkDoneFlushed, 1) │
├───────────────────────────────────┤ ├───┼──────────┴──────────────────┴─────────────────┬┴──────────────────┴────────────────────────────────────┘
│ set work.nSpanRoots │ │ 5 │ casgstatus(gp, _Gwaiting, _Grunning) │
├───────────────────────────────────┤ ├───┼───────────────────────────────────────────────┤
│ set work.nStackRoots │ │ 6 │ getg().m.preemptoff = "gcing" │
├───────────────────────────────────┤ ├───┼───────────────────────────────────────────────┤
│ set work.markrootJobs │ │ 7 │ systemstack(stopTheWorldWithSema) │
└───────────────────────────────────┘ ┌───────────────────────────────────┐ ├───┼───────────────────────────────────────────────┤
│ │ │ 8 │ atomic.Store(&gcBlackenEnabled, 0) │
▼ │ ├───┼───────────────────────────────────────────────┤
┌───────────────────┐ │ │ 9 │ gcWakeAllAssists() │
│ gcMarkTermination │ │ ├───┼───────────────────────────────────────────────┤
├───┬───────────────┴───────────────────┐ │ │10 │ semrelease(&work.markDoneSema) │
│ 1 │ gcBlackenEnabled -> 0 │ │ ├───┼───────────────────────────────────────────────┤
├───┼───────────────────────────────────┤ │ │11 │ nextTriggerRatio := gcController.endCycle() │ ┌───────────────────┐
│ 2 │ gcphase -> _GCmarktermination │ │ ├───┼───────────────────────────────────────────────┤ │ runtime.main │
├───┼───────────────────────────────────┤ └─────────────────────────│12 │ gcMarkTermination(nextTriggerRatio) │ ├───────────────────┴──────┐
│ 3 │ mp.preemptoff = "gcing" │ └───┴───────────────────────────────────────────────┘ │ ...... │
├───┼───────────────────────────────────┤ ├──────────────────────────┤
│ 4 │ g : running -> waiting │ │ gcenable() │
├───┼───────────────────────────────────┤ └──────────────────────────┘
│ 5 │ systemstack(gcMark) │ │
├───┼───────────────────────────────────┤ │
│ 6 │ gcMark() │ │
┌─────────────┐ ├───┼───────────────────────────────────┤ │
│ gcMark() │◀──────────────────────────────────────────────────────────│ 7 │ gcphase -> _GCoff │ │
├─────────────┴──────────────────────────────┐ ├───┼───────────────────────────────────┤ ▼
│ loop for allp │ │ 8 │ gcSweep() │──────┐ ┌─────────────────────────┐
┌────┼────────────────────────────────────────────┤ ├───┼───────────────────────────────────┤ │ │ go bgsweep() │
│for │ p.wbBuf.reset() │ │ 9 │ g : waiting -> running │ │ ├───┬─────────────────────┴─────────────────────────┐
├────┼────────────────────────────────────────────┤ ├───┼───────────────────────────────────┤ │ │ 1 │ sweep.g = getg() │
│for │ gcw.dispose() │ │10 │ mp.preemptoff = "" │ │ ├───┼───────────────────────────────────────────────┤
└────┼────────────────────────────────────────────┤ ├───┼───────────────────────────────────┤ │ │ 2 │ sweep.parked = true │
│ cachestats() │ │11 │gcSetTriggerRatio(nextTriggerRatio)│ │ ├───┼───────────────────────────────────────────────┤
├────────────────────────────────────────────┤ ├───┼───────────────────────────────────┤ │ ┌─────────────────┐ │ 3 │ goparkunlock(&sweep.lock, "GC sweep wait", 1) │
│ work.markrootDone = true │ │12 │ acc stats .... │ └─────▶│ gcSweep │ ├───┼───────────────────────────────────────────────┤
├────────────────────────────────────────────┤ ├───┼───────────────────────────────────┤ ├───┬─────────────┴─────────────────────┐ ┌───────────▶│for│ sweep work │
│ cache memory stats │ │13 │ lock(&work.sweepWaiters.lock) │ │ 1 │ mheap_.sweepdone -> 0 │ │ ├───┼───────────────────────────────────────────────┤
└────────────────────────────────────────────┘ ├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤ │ │for│ sweep.parked = true │
│14 │injectglist(&work.sweepWaiters.list│ │ 2 │ mheap_.pagesSwept -> 0 │ │ ├───┼───────────────────────────────────────────────┤
├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤ ┌──────┴─────┐ │for│ goparkunlock(&sweep.lock, "GC sweep wait", 1) │
│15 │ unlock(&work.sweepWaiters.lock) │ │ 3 │ sweep.parked -> false │ │ wakeup │ └───┴───────────────────────────────────────────────┘
├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤ ┌──────────────────┐ └──────┬─────┘
│16 │ systemstack startTheWorldWithSema │ │ 4 │ ready(sweep.g) │───────────────▶│ ready(sweep.g) │ │
├───┼───────────────────────────────────┤ └───┴───────────────────────────────────┘ ├───┬──────────────┴────────────────────┐ │
│17 │ prepareFreeWorkbufs() │ │ 1 │ sweep.g : Gwaiting -> Grunnable │ │
├───┼───────────────────────────────────┤ ├───┼───────────────────────────────────┤ │
│18 │ systemstack(freeStackSpans) │ │ 2 │ runqput(sweep.g) │ │
├───┼───────────────────────────────────┴┐ ├───┼───────────────────────────────────┤ │
│ │systemstack(func() { │ │ 3 │ wakep() │◀───────────┘
│ │ forEachP(func(_p_ *p) { │ └───┴───────────────────────────────────┘
│...│ _p_.mcache.prepareForSweep()│
│ │ }) │
│ │}) │
├───┼───────────────────────────────────┬┘
│19 │ semrelease(&worldsema) │
└───┴───────────────────────────────────┘
// Initialized from $GOGC. GOGC=off means no GC.
var gcpercent int32
func gcinit() {
if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
throw("size of Workbuf is suboptimal")
}
// No sweep on the first cycle.
mheap_.sweepdone = 1
// Set a reasonable initial GC trigger.
memstats.triggerRatio = 7 / 8.0
// Fake a heap_marked value so it looks like a trigger at
// heapminimum is the appropriate growth from heap_marked.
// This will go into computing the initial GC goal.
memstats.heap_marked = uint64(float64(heapminimum) / (1 + memstats.triggerRatio))
// Set gcpercent from the environment. This will also compute
// and set the GC trigger and goal.
_ = setGCPercent(readgogc())
work.startSema = 1
work.markDoneSema = 1
}
func readgogc() int32 {
p := gogetenv("GOGC")
if p == "off" {
return -1
}
if n, ok := atoi32(p); ok {
return n
}
return 100
}
// gcenable is called after the bulk of the runtime initialization,
// just before we're about to start letting user code run.
// It kicks off the background sweeper goroutine and enables GC.
func gcenable() {
c := make(chan int, 1)
go bgsweep(c)
<-c
memstats.enablegc = true // now that runtime is initialized, GC is okay
}
//go:linkname setGCPercent runtime/debug.setGCPercent
func setGCPercent(in int32) (out int32) {
lock(&mheap_.lock)
out = gcpercent
if in < 0 {
in = -1
}
gcpercent = in
heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100
// Update pacing in response to gcpercent change.
gcSetTriggerRatio(memstats.triggerRatio)
unlock(&mheap_.lock)
// If we just disabled GC, wait for any concurrent GC to
// finish so we always return with no GC running.
if in < 0 {
// Disable phase transitions.
lock(&work.sweepWaiters.lock)
if gcphase == _GCmark {
// GC is active. Wait until we reach sweeping.
gp := getg()
gp.schedlink = work.sweepWaiters.head
work.sweepWaiters.head.set(gp)
goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
} else {
// GC isn't active.
unlock(&work.sweepWaiters.lock)
}
}
return out
}整个 gc 流程的入口是 gcStart，gcStart 的调用方为:
graph LR
mallocgc --> shouldhelpgc
shouldhelpgc --> |no|return
shouldhelpgc --> |yes|gc_condition_satisfied
gc_condition_satisfied --> |no|return
gc_condition_satisfied --> |yes|gcStart
runtime.GC --> gcStart
init --> forcegchelper
forcegchelper --> gcStart
其实就是 mallocgc，forcegchelper，runtime.GC 这三个入口。
- mallocgc，分配堆内存时触发，会检查当前是否满足触发 gc 的条件，如果触发，那么进入 gcStart。
- forcegchelper，在 forcegchelper 中会把 forcegc.g 这个全局对象的运行 g 挂起。sysmon 会调用 test 检查上次触发 gc 的时间到当前时间是否已经经过了 forcegcperiod 长的时间，如果已经超过，那么就会将 forcegc.g 注入到 globrunq。这样会在该 g 被调度到的时候触发 gc。
- runtime.GC 是由用户主动触发的，相当于强制触发 GC。
// A gcTrigger is a predicate for starting a GC cycle. Specifically,
// it is an exit condition for the _GCoff phase.
type gcTrigger struct {
kind gcTriggerKind
now int64 // gcTriggerTime: current time
n uint32 // gcTriggerCycle: cycle number to start
}
type gcTriggerKind int
const (
// 表示应该无条件地开始 GC，不管外部任何参数
// 即使 GOGC 设置为 off，或者当前已经在进行 GC 进行中
gcTriggerAlways gcTriggerKind = iota
// 该枚举值表示 GC 会在 heap 大小达到 controller 计算出的阈值之后开始
gcTriggerHeap
// 表示从上一次 GC 之后，经过了 forcegcperiod 纳秒
// 基于时间触发本次 GC
gcTriggerTime
// gcTriggerCycle indicates that a cycle should be started if
// we have not yet started cycle number gcTrigger.n (relative
// to work.cycles).
gcTriggerCycle
) if shouldhelpgc {
if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
gcStart(gcBackgroundMode, t)
}
}在 mallocgc 中进行 gc 可以防止内存分配过快，导致 GC 回收不过来。
// We're now in sweep N or later. Trigger GC cycle N+1, which
// will first finish sweep N if necessary and then enter sweep
// termination N+1.
gcStart(gcBackgroundMode, gcTrigger{kind: gcTriggerCycle, n: n + 1}) // Time-triggered, fully concurrent.
gcStart(gcBackgroundMode, gcTrigger{kind: gcTriggerTime, now: nanotime()})这里和前面三条不一样的是，这种情况下如果 trigger.test 返回 true，会使用 forcegchelper 所在的 g 来执行 gcStart，具体做法就是上面提到的把 forcegc.g 注入到全局 runq;
// check if we need to force a GC
if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && atomic.Load(&forcegc.idle) != 0 {
lock(&forcegc.lock)
forcegc.idle = 0
forcegc.g.schedlink = 0
injectglist(forcegc.g)
unlock(&forcegc.lock)
}// test returns true if the trigger condition is satisfied, meaning
// that the exit condition for the _GCoff phase has been met. The exit
// condition should be tested when allocating.
func (t gcTrigger) test() bool {
if !memstats.enablegc || panicking != 0 {
return false
}
if t.kind == gcTriggerAlways {
return true
}
if gcphase != _GCoff {
return false
}
switch t.kind {
case gcTriggerHeap:
// Non-atomic access to heap_live for performance. If
// we are going to trigger on this, this thread just
// atomically wrote heap_live anyway and we'll see our
// own write.
return memstats.heap_live >= memstats.gc_trigger
case gcTriggerTime:
if gcpercent < 0 {
return false
}
lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
return lastgc != 0 && t.now-lastgc > forcegcperiod
case gcTriggerCycle:
// t.n > work.cycles, but accounting for wraparound.
return int32(t.n-work.cycles) > 0
}
return true
}// 该函数会将 GC 状态从 _GCoff 切换到 _GCmark(if !mode.stwMark)
// 或者 _GCmarktermination(if mode.stwMark)
// 开始执行 sweep termination 或者 GC 初始化
//
// 函数可能会在一些情况下未进行状态变更就返回
// 比如在 system stack 中被调用，或者 locks 被别人持有
func gcStart(mode gcMode, trigger gcTrigger) {
// Since this is called from malloc and malloc is called in
// the guts of a number of libraries that might be holding
// locks, don't attempt to start GC in non-preemptible or
// potentially unstable situations.
mp := acquirem()
if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
releasem(mp)
return
}
releasem(mp)
mp = nil
// Pick up the remaining unswept/not being swept spans concurrently
//
// This shouldn't happen if we're being invoked in background
// mode since proportional sweep should have just finished
// sweeping everything, but rounding errors, etc, may leave a
// few spans unswept. In forced mode, this is necessary since
// GC can be forced at any point in the sweeping cycle.
//
// We check the transition condition continuously here in case
// this G gets delayed in to the next GC cycle.
for trigger.test() && gosweepone() != ^uintptr(0) {
sweep.nbgsweep++
}
// 进行 GC 初始化和 sweep termination
semacquire(&work.startSema)
// 在 transition lock 下再检查一次 transition 条件
if !trigger.test() {
semrelease(&work.startSema)
return
}
// For stats, check if this GC was forced by the user.
work.userForced = trigger.kind == gcTriggerAlways || trigger.kind == gcTriggerCycle
// In gcstoptheworld debug mode, upgrade the mode accordingly.
// We do this after re-checking the transition condition so
// that multiple goroutines that detect the heap trigger don't
// start multiple STW GCs.
if mode == gcBackgroundMode {
if debug.gcstoptheworld == 1 {
mode = gcForceMode
} else if debug.gcstoptheworld == 2 {
mode = gcForceBlockMode
}
}
// 获取全局锁，让别人都停下 stw
semacquire(&worldsema)
// 在 background 模式下
// 启动所有标记 worker
if mode == gcBackgroundMode {
gcBgMarkStartWorkers()
}
gcResetMarkState()
work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
if work.stwprocs > ncpu {
// This is used to compute CPU time of the STW phases,
// so it can't be more than ncpu, even if GOMAXPROCS is.
work.stwprocs = ncpu
}
work.heap0 = atomic.Load64(&memstats.heap_live)
work.pauseNS = 0
work.mode = mode
now := nanotime()
work.tSweepTerm = now
work.pauseStart = now
systemstack(stopTheWorldWithSema)
// Finish sweep before we start concurrent scan.
systemstack(func() {
finishsweep_m()
})
// 清除全局的 :
// 1. sudogcache(sudog 数据结构的链表)
// 2. deferpool(defer struct 的链表的数组)
// 3. sync.Pool
clearpools()
work.cycles++
if mode == gcBackgroundMode { // 尽量多地提高并发度
gcController.startCycle()
work.heapGoal = memstats.next_gc
// 进入并发标记阶段，并让 write barriers 开始生效
//
// Because the world is stopped, all Ps will
// observe that write barriers are enabled by
// the time we start the world and begin
// scanning.
//
// Write barriers must be enabled before assists are
// enabled because they must be enabled before
// any non-leaf heap objects are marked. Since
// allocations are blocked until assists can
// happen, we want enable assists as early as
// possible.
setGCPhase(_GCmark)
gcBgMarkPrepare() // Must happen before assist enable.
gcMarkRootPrepare()
// Mark all active tinyalloc blocks. Since we're
// allocating from these, they need to be black like
// other allocations. The alternative is to blacken
// the tiny block on every allocation from it, which
// would slow down the tiny allocator.
gcMarkTinyAllocs()
// At this point all Ps have enabled the write
// barrier, thus maintaining the no white to
// black invariant. Enable mutator assists to
// put back-pressure on fast allocating
// mutators.
atomic.Store(&gcBlackenEnabled, 1)
// 协助标记的 g 和 worker 在 start the world 之后就可以开始工作了
gcController.markStartTime = now
// 并发标记
systemstack(func() {
now = startTheWorldWithSema(trace.enabled)
})
work.pauseNS += now - work.pauseStart
work.tMark = now
} else {
t := nanotime()
work.tMark, work.tMarkTerm = t, t
work.heapGoal = work.heap0
// 进行 mark termination 阶段的工作，会 restart the world
gcMarkTermination(memstats.triggerRatio)
}
semrelease(&work.startSema)
}// GC runs a garbage collection and blocks the caller until the
// garbage collection is complete. It may also block the entire
// program.
func GC() {
// We consider a cycle to be: sweep termination, mark, mark
// termination, and sweep. This function shouldn't return
// until a full cycle has been completed, from beginning to
// end. Hence, we always want to finish up the current cycle
// and start a new one. That means:
//
// 1. In sweep termination, mark, or mark termination of cycle
// N, wait until mark termination N completes and transitions
// to sweep N.
//
// 2. In sweep N, help with sweep N.
//
// At this point we can begin a full cycle N+1.
//
// 3. Trigger cycle N+1 by starting sweep termination N+1.
//
// 4. Wait for mark termination N+1 to complete.
//
// 5. Help with sweep N+1 until it's done.
//
// This all has to be written to deal with the fact that the
// GC may move ahead on its own. For example, when we block
// until mark termination N, we may wake up in cycle N+2.
gp := getg()
// Prevent the GC phase or cycle count from changing.
lock(&work.sweepWaiters.lock)
n := atomic.Load(&work.cycles)
if gcphase == _GCmark {
// Wait until sweep termination, mark, and mark
// termination of cycle N complete.
gp.schedlink = work.sweepWaiters.head
work.sweepWaiters.head.set(gp)
goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
} else {
// We're in sweep N already.
unlock(&work.sweepWaiters.lock)
}
// We're now in sweep N or later. Trigger GC cycle N+1, which
// will first finish sweep N if necessary and then enter sweep
// termination N+1.
gcStart(gcBackgroundMode, gcTrigger{kind: gcTriggerCycle, n: n + 1})
// Wait for mark termination N+1 to complete.
lock(&work.sweepWaiters.lock)
if gcphase == _GCmark && atomic.Load(&work.cycles) == n+1 {
gp.schedlink = work.sweepWaiters.head
work.sweepWaiters.head.set(gp)
goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
} else {
unlock(&work.sweepWaiters.lock)
}
// Finish sweep N+1 before returning. We do this both to
// complete the cycle and because runtime.GC() is often used
// as part of tests and benchmarks to get the system into a
// relatively stable and isolated state.
for atomic.Load(&work.cycles) == n+1 && gosweepone() != ^uintptr(0) {
sweep.nbgsweep++
Gosched()
}
// Callers may assume that the heap profile reflects the
// just-completed cycle when this returns (historically this
// happened because this was a STW GC), but right now the
// profile still reflects mark termination N, not N+1.
//
// As soon as all of the sweep frees from cycle N+1 are done,
// we can go ahead and publish the heap profile.
//
// First, wait for sweeping to finish. (We know there are no
// more spans on the sweep queue, but we may be concurrently
// sweeping spans, so we have to wait.)
for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 {
Gosched()
}
// Now we're really done with sweeping, so we can publish the
// stable heap profile. Only do this if we haven't already hit
// another mark termination.
mp := acquirem()
cycle := atomic.Load(&work.cycles)
if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) {
mProf_PostSweep()
}
releasem(mp)
}为什么需要 debug.FreeOsMemory 才能释放堆空间。
concurrent sweep 和普通后台sweep有什么区别，分别在什么时间触发
stw的时候在干什么
优化gc主要是在优化什么
gc时间，stw时间和响应延迟之间是什么关系
宏观来看gc划分为多少个阶段
