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runtime: break up large calls to memclrNoHeapPointers to allow preemption

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If something "huge" is allocated, and the zeroing is trivial (no pointers
involved) then zero it by chunks in a loop so that preemption can occur,
not all in a single non-preemptible call.

Benchmarking suggests that 256K is the best chunk size.

Updates #42642.

Change-Id: I94015e467eaa098c59870e479d6d83bc88efbfb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/270943
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
David Chase 2020-11-17 19:54:31 -05:00
parent 41afd3af42
commit 0bbfc5c31e
4 changed files with 51 additions and 7 deletions
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40
src/runtime/malloc.go
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@ -979,6 +979,9 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
var span *mspan
var x unsafe.Pointer
noscan := typ == nil || typ.ptrdata == 0
// In some cases block zeroing can profitably (for latency reduction purposes)
// be delayed till preemption is possible; isZeroed tracks that state.
isZeroed := true
if size <= maxSmallSize {
if noscan && size < maxTinySize {
// Tiny allocator.
@ -1074,7 +1077,9 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
}
} else {
shouldhelpgc = true
span = c.allocLarge(size, needzero, noscan)
// For large allocations, keep track of zeroed state so that
// bulk zeroing can be happen later in a preemptible context.
span, isZeroed = c.allocLarge(size, needzero && !noscan, noscan)
span.freeindex = 1
span.allocCount = 1
x = unsafe.Pointer(span.base())
@ -1133,6 +1138,12 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
mp.mallocing = 0
releasem(mp)
// Pointerfree data can be zeroed late in a context where preemption can occur.
// x will keep the memory alive.
if !isZeroed && needzero {
memclrNoHeapPointersChunked(size, x)
}
if debug.malloc {
if debug.allocfreetrace != 0 {
tracealloc(x, size, typ)
@ -1185,6 +1196,33 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
return x
}
// memclrNoHeapPointersChunked repeatedly calls memclrNoHeapPointers
// on chunks of the buffer to be zeroed, with opportunities for preemption
// along the way. memclrNoHeapPointers contains no safepoints and also
// cannot be preemptively scheduled, so this provides a still-efficient
// block copy that can also be preempted on a reasonable granularity.
//
// Use this with care; if the data being cleared is tagged to contain
// pointers, this allows the GC to run before it is all cleared.
func memclrNoHeapPointersChunked(size uintptr, x unsafe.Pointer) {
v := uintptr(x)
// got this from benchmarking. 128k is too small, 512k is too large.
const chunkBytes = 256 * 1024
vsize := v + size
for voff := v; voff < vsize; voff = voff + chunkBytes {
if getg().preempt {
// may hold locks, e.g., profiling
goschedguarded()
}
// clear min(avail, lump) bytes
n := vsize - voff
if n > chunkBytes {
n = chunkBytes
}
memclrNoHeapPointers(unsafe.Pointer(voff), n)
}
}
// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function