runtime: enlarge GC work buffer size

Currently the GC work buffers are only 256 bytes and hence can record
only 24 64-bit pointer. They were reduced from 4K in commits db7fd1c1
and a15818fe as a way to minimize the amount of work the per-P workbuf
caches could "hide" from the mark phase and carry in to the mark
termination phase. However, this approach wasn't very robust and we
later added a "mark 2" phase to address this problem head-on.

Because of mark 2, there's now no benefit to having very small work
buffers. But there are plenty of downsides: small work buffers
increase contention on the work lists, increase the frequency and
hence net overhead of acquiring and releasing work buffers, and
somewhat increase memory overhead of the GC.

This commit expands work buffers back to 4K (504 64-bit pointers).
This reduces the rate of writes to work.full in the garbage benchmark
from a peak of ~780,000 writes/sec to a peak of ~32,000 writes/sec.

This has negligible effect on the go1 benchmarks. It slightly slows
down the garbage benchmark.

name              old time/op  new time/op  delta
XBenchGarbage-12  5.37ms ± 5%  5.60ms ± 2%  +4.37%  (p=0.000 n=20+20)

Change-Id: Ic9cc28e7a125d23d9faf4f5e690fb8aa9bcdfb28
Reviewed-on: https://go-review.googlesource.com/15893Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgcwork.go

@@ -7,8 +7,8 @@ package runtime
 import "unsafe"
 
 const (
-	_Debugwbufs  = false   // if true check wbufs consistency
-	_WorkbufSize = 1 * 256 // in bytes - if small wbufs are passed to GC in a timely fashion.
+	_Debugwbufs  = false // if true check wbufs consistency
+	_WorkbufSize = 4096  // in bytes; larger values result in less contention
 )
 
 // Garbage collector work pool abstraction.