谈论Go 什么时候会触发 GC问题

目录

• 1、什么是 GC
• 2、为什么要 GC
• 3、GC 触发场景

• 3.1系统触发
• 3.2手动触发
• 3.3 基本流程
• 3.4 在哪触发

const ( 
 gcTriggerHeap gcTriggerKind = iota 
 gcTriggerTime 
 gcTriggerCycle 
) 

func GC() { 
 n := atomic.Load(&work.cycles) 
 gcWaitOnMark(n) 
 gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1}) 
 gcWaitOnMark(n + 1) 
 for atomic.Load(&work.cycles) == n+1 && sweepone() != ^uintptr(0) { 
  sweep.nbgsweep++ 
  Gosched() 
 } 
 for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 { 
  Gosched() 
 } 
 mp := acquirem() 
 cycle := atomic.Load(&work.cycles) 
 if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) { 
  mProf_PostSweep() 
 } 
 releasem(mp) 
} 

func init() { 
 go forcegchelper() 
} 
func forcegchelper() { 
 forcegc.g = getg() 
 lockInit(&forcegc.lock, lockRankForcegc) 
 for { 
  lock(&forcegc.lock) 
  if forcegc.idle != 0 { 
   throw("forcegc: phase error") 
  } 
  atomic.Store(&forcegc.idle, 1) 
  goparkunlock(&forcegc.lock, waitReasonForceGCIdle, traceEvGoBlock, 1) 
    // this goroutine is explicitly resumed by sysmon 
  if debug.gctrace > 0 { 
   println("GC forced") 
  } 
  gcStart(gcTrigger{kind: gcTriggerTime, now: nanotime()}) 
 } 
} 

func sysmon() { 
 ... 
 for { 
  ... 
  // 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 
   var list gList 
   list.push(forcegc.g) 
   injectglist(&list) 
   unlock(&forcegc.lock) 
  } 
  if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now { 
   lasttrace = now 
   schedtrace(debug.scheddetail > 0) 
  } 
  unlock(&sched.sysmonlock) 
 } 
} 

func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer { 
 shouldhelpgc := false 
 ... 
 if size <= maxSmallSize { 
  if noscan && size < maxTinySize { 
   ... 
   // Allocate a new maxTinySize block. 
   span = c.alloc[tinySpanClass] 
   v := nextFreeFast(span) 
   if v == 0 { 
    v, span, shouldhelpgc = c.nextFree(tinySpanClass) 
   } 
   ... 
   spc := makeSpanClass(sizeclass, noscan) 
   span = c.alloc[spc] 
   v := nextFreeFast(span) 
   if v == 0 { 
    v, span, shouldhelpgc = c.nextFree(spc) 
   } 
   ... 
  } 
 } else { 
  shouldhelpgc = true 
  span = c.allocLarge(size, needzero, noscan) 
  ... 
 } 
 if shouldhelpgc { 
  if t := (gcTrigger{kind: gcTriggerHeap}); t.test() { 
   gcStart(t) 
  } 
 } 
 return x 
}