From ccca9c9cc0fa5b6ea6e5c8276a96eee8c27ebd87 Mon Sep 17 00:00:00 2001 From: Rhys Hiltner Date: Fri, 22 Jul 2016 16:36:30 -0700 Subject: runtime: reduce GC assist extra credit Mutator goroutines that allocate memory during the concurrent mark phase are required to spend some time assisting the garbage collector. The magnitude of this mandatory assistance is proportional to the goroutine's allocation debt and subject to the assistance ratio as calculated by the pacer. When assisting the garbage collector, a mutator goroutine will go beyond paying off its allocation debt. It will build up extra credit to amortize the overhead of the assist. In fast-allocating applications with high assist ratios, building up this credit can take the affected goroutine's entire time slice. Reduce the penalty on each goroutine being selected to assist the GC in two ways, to spread the responsibility more evenly. First, do a consistent amount of extra scan work without regard for the pacer's assistance ratio. Second, reduce the magnitude of the extra scan work so it can be completed within a few hundred microseconds. Commentary on gcOverAssistWork is by Austin Clements, originally in https://golang.org/cl/24704 Updates #14812 Fixes #16432 Change-Id: I436f899e778c20daa314f3e9f0e2a1bbd53b43e1 Reviewed-on: https://go-review.googlesource.com/25155 Run-TryBot: Austin Clements TryBot-Result: Gobot Gobot Reviewed-by: Austin Clements Reviewed-by: Rick Hudson Reviewed-by: Chris Broadfoot --- src/runtime/mgc.go | 9 ++++----- src/runtime/mgcmark.go | 11 ++++++++--- 2 files changed, 12 insertions(+), 8 deletions(-) diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go index 1eabf43d6f..3b238cba1c 100644 --- a/src/runtime/mgc.go +++ b/src/runtime/mgc.go @@ -741,11 +741,10 @@ const gcCreditSlack = 2000 // can accumulate on a P before updating gcController.assistTime. const gcAssistTimeSlack = 5000 -// gcOverAssistBytes determines how many extra allocation bytes of -// assist credit a GC assist builds up when an assist happens. This -// amortizes the cost of an assist by pre-paying for this many bytes -// of future allocations. -const gcOverAssistBytes = 1 << 20 +// gcOverAssistWork determines how many extra units of scan work a GC +// assist does when an assist happens. This amortizes the cost of an +// assist by pre-paying for this many bytes of future allocations. +const gcOverAssistWork = 64 << 10 var work struct { full uint64 // lock-free list of full blocks workbuf diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go index 00b96fd00b..aa7f7a7769 100644 --- a/src/runtime/mgcmark.go +++ b/src/runtime/mgcmark.go @@ -393,10 +393,15 @@ func gcAssistAlloc(gp *g) { } // Compute the amount of scan work we need to do to make the - // balance positive. We over-assist to build up credit for - // future allocations and amortize the cost of assisting. - debtBytes := -gp.gcAssistBytes + gcOverAssistBytes + // 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)) + } retry: // Steal as much credit as we can from the background GC's -- cgit v1.2.3-54-g00ecf