[TOC]
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
if gcphase == _GCmarktermination {
throw("mallocgc called with gcphase == _GCmarktermination")
}
(...)
// assistG is the G to charge for this allocation, or nil if
// GC is not currently active.
var assistG *g
if gcBlackenEnabled != 0 {
// Charge the current user G for this allocation.
assistG = getg()
if assistG.m.curg != nil {
assistG = assistG.m.curg
}
// Charge the allocation against the G. We'll account
// for internal fragmentation at the end of mallocgc.
assistG.gcAssistBytes -= int64(size)
if assistG.gcAssistBytes < 0 {
// This G is in debt. Assist the GC to correct
// this before allocating. This must happen
// before disabling preemption.
gcAssistAlloc(assistG)
}
}
// Set mp.mallocing to keep from being preempted by GC.
mp := acquirem()
if mp.mallocing != 0 {
throw("malloc deadlock")
}
if mp.gsignal == getg() {
throw("malloc during signal")
}
mp.mallocing = 1
shouldhelpgc := false
dataSize := size
// 获取 mcache
c := gomcache()
var x unsafe.Pointer
noscan := typ == nil || typ.kind&kindNoPointers != 0
if size <= maxSmallSize {
if noscan && size < maxTinySize {
(...)
} else {
(...)
}
} else {
(...)
}
var scanSize uintptr
if !noscan {
// If allocating a defer+arg block, now that we've picked a malloc size
// large enough to hold everything, cut the "asked for" size down to
// just the defer header, so that the GC bitmap will record the arg block
// as containing nothing at all (as if it were unused space at the end of
// a malloc block caused by size rounding).
// The defer arg areas are scanned as part of scanstack.
if typ == deferType {
dataSize = unsafe.Sizeof(_defer{})
}
heapBitsSetType(uintptr(x), size, dataSize, typ)
if dataSize > typ.size {
// Array allocation. If there are any
// pointers, GC has to scan to the last
// element.
if typ.ptrdata != 0 {
scanSize = dataSize - typ.size + typ.ptrdata
}
} else {
scanSize = typ.ptrdata
}
c.local_scan += scanSize
}
// Ensure that the stores above that initialize x to
// type-safe memory and set the heap bits occur before
// the caller can make x observable to the garbage
// collector. Otherwise, on weakly ordered machines,
// the garbage collector could follow a pointer to x,
// but see uninitialized memory or stale heap bits.
publicationBarrier()
// Allocate black during GC.
// All slots hold nil so no scanning is needed.
// This may be racing with GC so do it atomically if there can be
// a race marking the bit.
if gcphase != _GCoff {
gcmarknewobject(uintptr(x), size, scanSize)
}
(...)
mp.mallocing = 0
releasem(mp)
(...)
if rate := MemProfileRate; rate > 0 {
if rate != 1 && int32(size) < c.next_sample {
c.next_sample -= int32(size)
} else {
mp := acquirem()
profilealloc(mp, x, size)
releasem(mp)
}
}
if assistG != nil {
// Account for internal fragmentation in the assist
// debt now that we know it.
assistG.gcAssistBytes -= int64(size - dataSize)
}
if shouldhelpgc {
if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
gcStart(t)
}
}
return x
}func gcAssistAlloc(gp *g) {
// Don't assist in non-preemptible contexts. These are
// generally fragile and won't allow the assist to block.
if getg() == gp.m.g0 {
return
}
if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
return
}
traced := false
retry:
// Compute the amount of scan work we need to do to make the
// balance positive. When the required amount of work is low,
// we over-assist to build up credit for future allocations
// and amortize the cost of assisting.
debtBytes := -gp.gcAssistBytes
scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
if scanWork < gcOverAssistWork {
scanWork = gcOverAssistWork
debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork))
}
// Steal as much credit as we can from the background GC's
// scan credit. This is racy and may drop the background
// credit below 0 if two mutators steal at the same time. This
// will just cause steals to fail until credit is accumulated
// again, so in the long run it doesn't really matter, but we
// do have to handle the negative credit case.
bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
stolen := int64(0)
if bgScanCredit > 0 {
if bgScanCredit < scanWork {
stolen = bgScanCredit
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen))
} else {
stolen = scanWork
gp.gcAssistBytes += debtBytes
}
atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
scanWork -= stolen
if scanWork == 0 {
// We were able to steal all of the credit we
// needed.
if traced {
traceGCMarkAssistDone()
}
return
}
}
(...)
// Perform assist work
systemstack(func() {
gcAssistAlloc1(gp, scanWork)
// The user stack may have moved, so this can't touch
// anything on it until it returns from systemstack.
})
completed := gp.param != nil
gp.param = nil
if completed {
gcMarkDone()
}
if gp.gcAssistBytes < 0 {
// We were unable steal enough credit or perform
// enough work to pay off the assist debt. We need to
// do one of these before letting the mutator allocate
// more to prevent over-allocation.
//
// If this is because we were preempted, reschedule
// and try some more.
if gp.preempt {
Gosched()
goto retry
}
// Add this G to an assist queue and park. When the GC
// has more background credit, it will satisfy queued
// assists before flushing to the global credit pool.
//
// Note that this does *not* get woken up when more
// work is added to the work list. The theory is that
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
if !gcParkAssist() {
goto retry
}
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
}
(...)
}//go:systemstack
func gcAssistAlloc1(gp *g, scanWork int64) {
// Clear the flag indicating that this assist completed the
// mark phase.
gp.param = nil
if atomic.Load(&gcBlackenEnabled) == 0 {
// The gcBlackenEnabled check in malloc races with the
// store that clears it but an atomic check in every malloc
// would be a performance hit.
// Instead we recheck it here on the non-preemptable system
// stack to determine if we should perform an assist.
// GC is done, so ignore any remaining debt.
gp.gcAssistBytes = 0
return
}
// Track time spent in this assist. Since we're on the
// system stack, this is non-preemptible, so we can
// just measure start and end time.
startTime := nanotime()
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
throw("nwait > work.nprocs")
}
// gcDrainN requires the caller to be preemptible.
casgstatus(gp, _Grunning, _Gwaiting)
gp.waitreason = waitReasonGCAssistMarking
// drain own cached work first in the hopes that it
// will be more cache friendly.
gcw := &getg().m.p.ptr().gcw
workDone := gcDrainN(gcw, scanWork)
casgstatus(gp, _Gwaiting, _Grunning)
// Record that we did this much scan work.
//
// Back out the number of bytes of assist credit that
// this scan work counts for. The "1+" is a poor man's
// round-up, to ensure this adds credit even if
// assistBytesPerWork is very low.
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone))
// If this is the last worker and we ran out of work,
// signal a completion point.
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait,
"work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This has reached a background completion point. Set
// gp.param to a non-nil value to indicate this. It
// doesn't matter what we set it to (it just has to be
// a valid pointer).
gp.param = unsafe.Pointer(gp)
}
duration := nanotime() - startTime
_p_ := gp.m.p.ptr()
_p_.gcAssistTime += duration
if _p_.gcAssistTime > gcAssistTimeSlack {
atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
_p_.gcAssistTime = 0
}
}Go under the hood | CC-BY-NC-ND 4.0 & MIT © changkun