runtime: make putfull start mark workers

Currently we depend on the good graces and timing of the scheduler to
get opportunities to start dedicated mark workers. In the worst case,
it may take 10ms to get dedicated mark workers going at the beginning
of mark 1 and mark 2 or after the amount of available work has dropped
and gone back up.

Instead of waiting for the regular preemption logic to get around to
us, make putfull enlist a random P if we're not already running enough
dedicated workers. This should improve performance stability of the
garbage collector and is likely to improve the overall performance
somewhat.

No overall effect on the go1 benchmarks. It speeds up the garbage
benchmark by 12%, which more than counters the performance loss from
the previous commit.

name              old time/op  new time/op  delta
XBenchGarbage-12  6.32ms ± 4%  5.58ms ± 2%  -11.68%  (p=0.000 n=20+16)

name                      old time/op    new time/op    delta
BinaryTree17-12              3.18s ± 5%     3.12s ± 4%  -1.83%  (p=0.021 n=20+20)
Fannkuch11-12                2.50s ± 2%     2.46s ± 2%  -1.57%  (p=0.000 n=18+19)
FmtFprintfEmpty-12          50.8ns ± 3%    50.4ns ± 3%    ~     (p=0.184 n=20+20)
FmtFprintfString-12          167ns ± 2%     171ns ± 1%  +2.46%  (p=0.000 n=20+19)
FmtFprintfInt-12             161ns ± 2%     163ns ± 2%  +1.81%  (p=0.000 n=20+20)
FmtFprintfIntInt-12          269ns ± 1%     266ns ± 1%  -0.81%  (p=0.002 n=19+20)
FmtFprintfPrefixedInt-12     237ns ± 2%     231ns ± 2%  -2.86%  (p=0.000 n=20+20)
FmtFprintfFloat-12           313ns ± 2%     313ns ± 1%    ~     (p=0.681 n=20+20)
FmtManyArgs-12              1.05µs ± 2%    1.03µs ± 1%  -2.26%  (p=0.000 n=20+20)
GobDecode-12                8.66ms ± 1%    8.67ms ± 1%    ~     (p=0.380 n=19+20)
GobEncode-12                6.56ms ± 1%    6.56ms ± 2%    ~     (p=0.607 n=19+20)
Gzip-12                      317ms ± 1%     314ms ± 2%  -1.10%  (p=0.000 n=20+19)
Gunzip-12                   42.1ms ± 1%    42.2ms ± 1%  +0.27%  (p=0.044 n=20+19)
HTTPClientServer-12         62.7µs ± 1%    62.0µs ± 1%  -1.04%  (p=0.000 n=19+18)
JSONEncode-12               16.7ms ± 1%    16.8ms ± 2%  +0.59%  (p=0.021 n=20+20)
JSONDecode-12               58.2ms ± 1%    61.4ms ± 2%  +5.43%  (p=0.000 n=18+19)
Mandelbrot200-12            3.84ms ± 1%    3.87ms ± 2%  +0.79%  (p=0.008 n=18+20)
GoParse-12                  3.86ms ± 2%    3.76ms ± 2%  -2.60%  (p=0.000 n=20+20)
RegexpMatchEasy0_32-12       100ns ± 2%     100ns ± 1%  -0.68%  (p=0.005 n=18+15)
RegexpMatchEasy0_1K-12       332ns ± 1%     342ns ± 1%  +3.16%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12      82.9ns ± 3%    83.0ns ± 2%    ~     (p=0.906 n=19+20)
RegexpMatchEasy1_1K-12       487ns ± 1%     494ns ± 1%  +1.50%  (p=0.000 n=17+20)
RegexpMatchMedium_32-12      131ns ± 2%     130ns ± 1%    ~     (p=0.686 n=19+20)
RegexpMatchMedium_1K-12     39.6µs ± 1%    39.2µs ± 1%  -1.09%  (p=0.000 n=18+19)
RegexpMatchHard_32-12       2.04µs ± 1%    2.04µs ± 2%    ~     (p=0.804 n=20+20)
RegexpMatchHard_1K-12       61.7µs ± 2%    61.3µs ± 2%    ~     (p=0.052 n=18+20)
Revcomp-12                   529ms ± 2%     533ms ± 1%  +0.83%  (p=0.003 n=20+19)
Template-12                 70.7ms ± 2%    71.0ms ± 2%    ~     (p=0.065 n=20+19)
TimeParse-12                 351ns ± 2%     355ns ± 1%  +1.25%  (p=0.000 n=19+20)
TimeFormat-12                362ns ± 2%     373ns ± 1%  +2.83%  (p=0.000 n=18+20)
[Geo mean]                  62.2µs         62.3µs       +0.13%

name                      old speed      new speed      delta
GobDecode-12              88.6MB/s ± 1%  88.5MB/s ± 1%    ~     (p=0.392 n=19+20)
GobEncode-12               117MB/s ± 1%   117MB/s ± 1%    ~     (p=0.622 n=19+20)
Gzip-12                   61.1MB/s ± 1%  61.8MB/s ± 2%  +1.11%  (p=0.000 n=20+19)
Gunzip-12                  461MB/s ± 1%   460MB/s ± 1%  -0.27%  (p=0.044 n=20+19)
JSONEncode-12              116MB/s ± 1%   115MB/s ± 2%  -0.58%  (p=0.022 n=20+20)
JSONDecode-12             33.3MB/s ± 1%  31.6MB/s ± 2%  -5.15%  (p=0.000 n=18+19)
GoParse-12                15.0MB/s ± 2%  15.4MB/s ± 2%  +2.66%  (p=0.000 n=20+20)
RegexpMatchEasy0_32-12     317MB/s ± 2%   319MB/s ± 2%    ~     (p=0.052 n=20+20)
RegexpMatchEasy0_1K-12    3.08GB/s ± 1%  2.99GB/s ± 1%  -3.07%  (p=0.000 n=19+19)
RegexpMatchEasy1_32-12     386MB/s ± 3%   386MB/s ± 2%    ~     (p=0.939 n=19+20)
RegexpMatchEasy1_1K-12    2.10GB/s ± 1%  2.07GB/s ± 1%  -1.46%  (p=0.000 n=17+20)
RegexpMatchMedium_32-12   7.62MB/s ± 2%  7.64MB/s ± 1%    ~     (p=0.702 n=19+20)
RegexpMatchMedium_1K-12   25.9MB/s ± 1%  26.1MB/s ± 2%  +0.99%  (p=0.000 n=18+20)
RegexpMatchHard_32-12     15.7MB/s ± 1%  15.7MB/s ± 2%    ~     (p=0.723 n=20+20)
RegexpMatchHard_1K-12     16.6MB/s ± 2%  16.7MB/s ± 2%    ~     (p=0.052 n=18+20)
Revcomp-12                 481MB/s ± 2%   477MB/s ± 1%  -0.83%  (p=0.003 n=20+19)
Template-12               27.5MB/s ± 2%  27.3MB/s ± 2%    ~     (p=0.062 n=20+19)
[Geo mean]                99.4MB/s       99.1MB/s       -0.35%

Change-Id: I914d8cadded5a230509d118164a4c201601afc06
Reviewed-on: https://go-review.googlesource.com/16298
Reviewed-by: Rick Hudson <rlh@golang.org>

2 files changed, 39 insertions, 0 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 1ab42a8105..88cee5b8f6 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -574,6 +574,39 @@ func (c *gcControllerState) endCycle() {
 	}
 }
 
+// enlistWorker encourages another dedicated mark worker to start on
+// another P if there are spare worker slots. It is used by putfull
+// when more work is made available.
+//
+//go:nowritebarrier
+func (c *gcControllerState) enlistWorker() {
+	if c.dedicatedMarkWorkersNeeded <= 0 {
+		return
+	}
+	// Pick a random other P to preempt.
+	if gomaxprocs <= 1 {
+		return
+	}
+	gp := getg()
+	if gp == nil || gp.m == nil || gp.m.p == 0 {
+		return
+	}
+	myID := gp.m.p.ptr().id
+	for tries := 0; tries < 5; tries++ {
+		id := int32(fastrand1() % uint32(gomaxprocs-1))
+		if id >= myID {
+			id++
+		}
+		p := allp[id]
+		if p.status != _Prunning {
+			continue
+		}
+		if preemptone(p) {
+			return
+		}
+	}
+}
+
 // findRunnableGCWorker returns the background mark worker for _p_ if it
 // should be run. This must only be called when gcBlackenEnabled != 0.
 func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
diff --git a/src/runtime/mgcwork.go b/src/runtime/mgcwork.go
index 43d5db2fab..1d66200bec 100644
--- a/src/runtime/mgcwork.go
+++ b/src/runtime/mgcwork.go
@@ -363,6 +363,12 @@ func putfull(b *workbuf, entry int) {
 	b.checknonempty()
 	b.logput(entry)
 	lfstackpush(&work.full, &b.node)
+
+	// We just made more work available. Let the GC controller
+	// know so it can encourage more workers to run.
+	if gcphase == _GCmark {
+		gcController.enlistWorker()
+	}
 }
 
 // trygetfull tries to get a full or partially empty workbuffer.