From 08bf64a81e4c3e2199d0e879089880f538fafbb9 Mon Sep 17 00:00:00 2001 From: Michael Anthony Knyszek Date: Fri, 24 Apr 2020 20:06:57 +0000 Subject: [PATCH] runtime: bound small object sweeping to 100 spans when allocating Currently, the small object sweeper will sweep until it finds a free slot or there are no more spans of that size class to sweep. In dense heaps, this can cause sweeping for a given size class to take unbounded time, and gets worse with larger heaps. This CL limits the small object sweeper to try at most 100 spans before giving up and allocating a fresh span. Since it's already shown that 100 spans are completely full at that point, the space overhead of this fresh span is at most 1%. This CL is based on an experimental CL by Austin Clements (CL 187817) and is updated to be part of the mcentral implementation, gated by go115NewMCentralImpl. Updates #18155. Change-Id: I37a72c2dcc61dd6f802d1d0eac3683e6642b6ef8 Reviewed-on: https://go-review.googlesource.com/c/go/+/229998 Run-TryBot: Michael Knyszek Reviewed-by: Austin Clements --- src/runtime/mcentral.go | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go index 8a39f601bf..ed49d86d0c 100644 --- a/src/runtime/mcentral.go +++ b/src/runtime/mcentral.go @@ -106,14 +106,31 @@ func (c *mcentral) cacheSpan() *mspan { if trace.enabled { traceGCSweepStart() } + + // If we sweep spanBudget spans without finding any free + // space, just allocate a fresh span. This limits the amount + // of time we can spend trying to find free space and + // amortizes the cost of small object sweeping over the + // benefit of having a full free span to allocate from. By + // setting this to 100, we limit the space overhead to 1%. + // + // TODO(austin,mknyszek): This still has bad worst-case + // throughput. For example, this could find just one free slot + // on the 100th swept span. That limits allocation latency, but + // still has very poor throughput. We could instead keep a + // running free-to-used budget and switch to fresh span + // allocation if the budget runs low. + spanBudget := 100 + var s *mspan // Try partial swept spans first. if s = c.partialSwept(sg).pop(); s != nil { goto havespan } + // Now try partial unswept spans. - for { + for ; spanBudget >= 0; spanBudget-- { s = c.partialUnswept(sg).pop() if s == nil { break @@ -132,7 +149,7 @@ func (c *mcentral) cacheSpan() *mspan { } // Now try full unswept spans, sweeping them and putting them into the // right list if we fail to get a span. - for { + for ; spanBudget >= 0; spanBudget-- { s = c.fullUnswept(sg).pop() if s == nil { break