[TOC]
TODO:
// runtime/mgc.go
func gcinit() {
(...)
// 第一个周期没有扫描。
mheap_.sweepdone = 1
// 设置合理的初始 GC 触发比率。
memstats.triggerRatio = 7 / 8.0
// 伪造一个 heap_marked 值，使它看起来像一个触发器
// heapminimum 是 heap_marked的 适当增长。
// 这将用于计算初始 GC 目标。
memstats.heap_marked = uint64(float64(heapminimum) / (1 + memstats.triggerRatio))
// 从环境中设置 gcpercent。这也将计算并设置 GC 触发器和目标。
_ = 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
}
//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
// 更新步调来响应 gcpercent 变化
gcSetTriggerRatio(memstats.triggerRatio)
unlock(&mheap_.lock)
// 如果我们刚好禁用了 GC，则等待任何并发 GC 标记完成，从而我们总是能够在没有 GC 的情况下返回
if in < 0 {
gcWaitOnMark(atomic.Load(&work.cycles))
}
return out
}func isSweepDone() bool {
return mheap_.sweepdone != 0
}
func gcSetTriggerRatio(triggerRatio float64) {
// Set the trigger ratio, capped to reasonable bounds.
if triggerRatio < 0 {
// This can happen if the mutator is allocating very
// quickly or the GC is scanning very slowly.
triggerRatio = 0
} else if gcpercent >= 0 {
// Ensure there's always a little margin so that the
// mutator assist ratio isn't infinity.
maxTriggerRatio := 0.95 * float64(gcpercent) / 100
if triggerRatio > maxTriggerRatio {
triggerRatio = maxTriggerRatio
}
}
memstats.triggerRatio = triggerRatio
// Compute the absolute GC trigger from the trigger ratio.
//
// We trigger the next GC cycle when the allocated heap has
// grown by the trigger ratio over the marked heap size.
trigger := ^uint64(0)
if gcpercent >= 0 {
trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio))
// Don't trigger below the minimum heap size.
minTrigger := heapminimum
if !isSweepDone() {
// Concurrent sweep happens in the heap growth
// from heap_live to gc_trigger, so ensure
// that concurrent sweep has some heap growth
// in which to perform sweeping before we
// start the next GC cycle.
sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance*uint64(gcpercent)/100
if sweepMin > minTrigger {
minTrigger = sweepMin
}
}
if trigger < minTrigger {
trigger = minTrigger
}
if int64(trigger) < 0 {
print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n")
throw("gc_trigger underflow")
}
}
memstats.gc_trigger = trigger
// Compute the next GC goal, which is when the allocated heap
// has grown by GOGC/100 over the heap marked by the last
// cycle.
goal := ^uint64(0)
if gcpercent >= 0 {
goal = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
if goal < trigger {
// The trigger ratio is always less than GOGC/100, but
// other bounds on the trigger may have raised it.
// Push up the goal, too.
goal = trigger
}
}
memstats.next_gc = goal
if trace.enabled {
traceNextGC()
}
// Update mark pacing.
if gcphase != _GCoff {
gcController.revise()
}
// Update sweep pacing.
if isSweepDone() {
mheap_.sweepPagesPerByte = 0
} else {
// Concurrent sweep needs to sweep all of the in-use
// pages by the time the allocated heap reaches the GC
// trigger. Compute the ratio of in-use pages to sweep
// per byte allocated, accounting for the fact that
// some might already be swept.
heapLiveBasis := atomic.Load64(&memstats.heap_live)
heapDistance := int64(trigger) - int64(heapLiveBasis)
// Add a little margin so rounding errors and
// concurrent sweep are less likely to leave pages
// unswept when GC starts.
heapDistance -= 1024 * 1024
if heapDistance < _PageSize {
// Avoid setting the sweep ratio extremely high
heapDistance = _PageSize
}
pagesSwept := atomic.Load64(&mheap_.pagesSwept)
sweepDistancePages := int64(mheap_.pagesInUse) - int64(pagesSwept)
if sweepDistancePages <= 0 {
mheap_.sweepPagesPerByte = 0
} else {
mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
mheap_.sweepHeapLiveBasis = heapLiveBasis
// Write pagesSweptBasis last, since this
// signals concurrent sweeps to recompute
// their debt.
atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept)
}
}
}func (c *gcControllerState) revise() {
gcpercent := gcpercent
if gcpercent < 0 {
// If GC is disabled but we're running a forced GC,
// act like GOGC is huge for the below calculations.
gcpercent = 100000
}
live := atomic.Load64(&memstats.heap_live)
var heapGoal, scanWorkExpected int64
if live <= memstats.next_gc {
// We're under the soft goal. Pace GC to complete at
// next_gc assuming the heap is in steady-state.
heapGoal = int64(memstats.next_gc)
// Compute the expected scan work remaining.
//
// This is estimated based on the expected
// steady-state scannable heap. For example, with
// GOGC=100, only half of the scannable heap is
// expected to be live, so that's what we target.
//
// (This is a float calculation to avoid overflowing on
// 100*heap_scan.)
scanWorkExpected = int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent))
} else {
// We're past the soft goal. Pace GC so that in the
// worst case it will complete by the hard goal.
const maxOvershoot = 1.1
heapGoal = int64(float64(memstats.next_gc) * maxOvershoot)
// Compute the upper bound on the scan work remaining.
scanWorkExpected = int64(memstats.heap_scan)
}
// Compute the remaining scan work estimate.
//
// Note that we currently count allocations during GC as both
// scannable heap (heap_scan) and scan work completed
// (scanWork), so allocation will change this difference will
// slowly in the soft regime and not at all in the hard
// regime.
scanWorkRemaining := scanWorkExpected - c.scanWork
if scanWorkRemaining < 1000 {
// We set a somewhat arbitrary lower bound on
// remaining scan work since if we aim a little high,
// we can miss by a little.
//
// We *do* need to enforce that this is at least 1,
// since marking is racy and double-scanning objects
// may legitimately make the remaining scan work
// negative, even in the hard goal regime.
scanWorkRemaining = 1000
}
// Compute the heap distance remaining.
heapRemaining := heapGoal - int64(live)
if heapRemaining <= 0 {
// This shouldn't happen, but if it does, avoid
// dividing by zero or setting the assist negative.
heapRemaining = 1
}
// Compute the mutator assist ratio so by the time the mutator
// allocates the remaining heap bytes up to next_gc, it will
// have done (or stolen) the remaining amount of scan work.
c.assistWorkPerByte = float64(scanWorkRemaining) / float64(heapRemaining)
c.assistBytesPerWork = float64(heapRemaining) / float64(scanWorkRemaining)
}
var gcController gcControllerState
type gcControllerState struct {
scanWork int64
bgScanCredit int64
assistTime int64
dedicatedMarkTime int64
fractionalMarkTime int64
idleMarkTime int64
markStartTime int64
dedicatedMarkWorkersNeeded int64
assistWorkPerByte float64
assistBytesPerWork float64
fractionalUtilizationGoal float64
_ cpu.CacheLinePad
}func gcWaitOnMark(n uint32) {
for {
// Disable phase transitions.
lock(&work.sweepWaiters.lock)
nMarks := atomic.Load(&work.cycles)
if gcphase != _GCmark {
// We've already completed this cycle's mark.
nMarks++
}
if nMarks > n {
// We're done.
unlock(&work.sweepWaiters.lock)
return
}
// Wait until sweep termination, mark, and mark
// termination of cycle N complete.
work.sweepWaiters.list.push(getg())
goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1)
}
}
// 将当前 goroutine 置于等待状态并解锁 lock。
// 通过调用 goready(gp) 可让 goroutine 再次 runnable
func goparkunlock(lock *mutex, reason waitReason, traceEv byte, traceskip int) {
gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceEv, traceskip)
}// 在我们即将开始让用户代码运行之前，在大量运行时初始化之后调用 gcenable。
// 它启动后台的 sweeper goroutine 并启用 GC。
func gcenable() {
c := make(chan int, 1)
go bgsweep(c)
<-c
memstats.enablegc = true // 现在运行时已经初始化完毕了，GC 已就绪
}Go under the hood | CC-BY-NC-ND 4.0 & MIT © changkun