runtime: don't burn CPU unnecessarily

Two GC-related functions, scang and casgstatus, wait in an active spin loop.
Active spinning is never a good idea in user-space. Once we wait several
times more than the expected wait time, something unexpected is happenning
(e.g. the thread we are waiting for is descheduled or handling a page fault)
and we need to yield to OS scheduler. Moreover, the expected wait time is
very high for these functions: scang wait time can be tens of milliseconds,
casgstatus can be hundreds of microseconds. It does not make sense to spin
even for that time.

go install -a std profile on a 4-core machine shows that 11% of time is spent
in the active spin in scang:

  6.12%    compile  compile                [.] runtime.scang
  3.27%    compile  compile                [.] runtime.readgstatus
  1.72%    compile  compile                [.] runtime/internal/atomic.Load

The active spin also increases tail latency in the case of the slightest
oversubscription: GC goroutines spend whole quantum in the loop instead of
executing user code.

Here is scang wait time histogram during go install -a std:

13707.0000 - 1815442.7667 [   118]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎...
1815442.7667 - 3617178.5333 [     9]: ∎∎∎∎∎∎∎∎∎
3617178.5333 - 5418914.3000 [    11]: ∎∎∎∎∎∎∎∎∎∎∎
5418914.3000 - 7220650.0667 [     5]: ∎∎∎∎∎
7220650.0667 - 9022385.8333 [    12]: ∎∎∎∎∎∎∎∎∎∎∎∎
9022385.8333 - 10824121.6000 [    13]: ∎∎∎∎∎∎∎∎∎∎∎∎∎
10824121.6000 - 12625857.3667 [    15]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
12625857.3667 - 14427593.1333 [    18]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
14427593.1333 - 16229328.9000 [    18]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
16229328.9000 - 18031064.6667 [    32]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
18031064.6667 - 19832800.4333 [    28]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
19832800.4333 - 21634536.2000 [     6]: ∎∎∎∎∎∎
21634536.2000 - 23436271.9667 [    15]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
23436271.9667 - 25238007.7333 [    11]: ∎∎∎∎∎∎∎∎∎∎∎
25238007.7333 - 27039743.5000 [    27]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
27039743.5000 - 28841479.2667 [    20]: ∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎∎
28841479.2667 - 30643215.0333 [    10]: ∎∎∎∎∎∎∎∎∎∎
30643215.0333 - 32444950.8000 [     7]: ∎∎∎∎∎∎∎
32444950.8000 - 34246686.5667 [     4]: ∎∎∎∎
34246686.5667 - 36048422.3333 [     4]: ∎∎∎∎
36048422.3333 - 37850158.1000 [     1]: ∎
37850158.1000 - 39651893.8667 [     5]: ∎∎∎∎∎
39651893.8667 - 41453629.6333 [     2]: ∎∎
41453629.6333 - 43255365.4000 [     2]: ∎∎
43255365.4000 - 45057101.1667 [     2]: ∎∎
45057101.1667 - 46858836.9333 [     1]: ∎
46858836.9333 - 48660572.7000 [     2]: ∎∎
48660572.7000 - 50462308.4667 [     3]: ∎∎∎
50462308.4667 - 52264044.2333 [     2]: ∎∎
52264044.2333 - 54065780.0000 [     2]: ∎∎

and the zoomed-in first part:

13707.0000 - 19916.7667 [     2]: ∎∎
19916.7667 - 26126.5333 [     2]: ∎∎
26126.5333 - 32336.3000 [     9]: ∎∎∎∎∎∎∎∎∎
32336.3000 - 38546.0667 [     8]: ∎∎∎∎∎∎∎∎
38546.0667 - 44755.8333 [    12]: ∎∎∎∎∎∎∎∎∎∎∎∎
44755.8333 - 50965.6000 [    10]: ∎∎∎∎∎∎∎∎∎∎
50965.6000 - 57175.3667 [     5]: ∎∎∎∎∎
57175.3667 - 63385.1333 [     6]: ∎∎∎∎∎∎
63385.1333 - 69594.9000 [     5]: ∎∎∎∎∎
69594.9000 - 75804.6667 [     6]: ∎∎∎∎∎∎
75804.6667 - 82014.4333 [     6]: ∎∎∎∎∎∎
82014.4333 - 88224.2000 [     4]: ∎∎∎∎
88224.2000 - 94433.9667 [     1]: ∎
94433.9667 - 100643.7333 [     1]: ∎
100643.7333 - 106853.5000 [     2]: ∎∎
106853.5000 - 113063.2667 [     0]:
113063.2667 - 119273.0333 [     2]: ∎∎
119273.0333 - 125482.8000 [     2]: ∎∎
125482.8000 - 131692.5667 [     1]: ∎
131692.5667 - 137902.3333 [     1]: ∎
137902.3333 - 144112.1000 [     0]:
144112.1000 - 150321.8667 [     2]: ∎∎
150321.8667 - 156531.6333 [     1]: ∎
156531.6333 - 162741.4000 [     1]: ∎
162741.4000 - 168951.1667 [     0]:
168951.1667 - 175160.9333 [     0]:
175160.9333 - 181370.7000 [     1]: ∎
181370.7000 - 187580.4667 [     1]: ∎
187580.4667 - 193790.2333 [     2]: ∎∎
193790.2333 - 200000.0000 [     0]:

Here is casgstatus wait time histogram:

  631.0000 -  5276.6333 [     3]: ∎∎∎
 5276.6333 -  9922.2667 [     5]: ∎∎∎∎∎
 9922.2667 - 14567.9000 [     2]: ∎∎
14567.9000 - 19213.5333 [     6]: ∎∎∎∎∎∎
19213.5333 - 23859.1667 [     5]: ∎∎∎∎∎
23859.1667 - 28504.8000 [     6]: ∎∎∎∎∎∎
28504.8000 - 33150.4333 [     6]: ∎∎∎∎∎∎
33150.4333 - 37796.0667 [     2]: ∎∎
37796.0667 - 42441.7000 [     1]: ∎
42441.7000 - 47087.3333 [     3]: ∎∎∎
47087.3333 - 51732.9667 [     0]:
51732.9667 - 56378.6000 [     1]: ∎
56378.6000 - 61024.2333 [     0]:
61024.2333 - 65669.8667 [     0]:
65669.8667 - 70315.5000 [     0]:
70315.5000 - 74961.1333 [     1]: ∎
74961.1333 - 79606.7667 [     0]:
79606.7667 - 84252.4000 [     0]:
84252.4000 - 88898.0333 [     0]:
88898.0333 - 93543.6667 [     0]:
93543.6667 - 98189.3000 [     0]:
98189.3000 - 102834.9333 [     0]:
102834.9333 - 107480.5667 [     1]: ∎
107480.5667 - 112126.2000 [     0]:
112126.2000 - 116771.8333 [     0]:
116771.8333 - 121417.4667 [     0]:
121417.4667 - 126063.1000 [     0]:
126063.1000 - 130708.7333 [     0]:
130708.7333 - 135354.3667 [     0]:
135354.3667 - 140000.0000 [     1]: ∎

Ideally we eliminate the waiting by switching to async
state machine for GC, but for now just yield to OS scheduler
after a reasonable wait time.

To choose yielding parameters I've measured
golang.org/x/benchmarks/http tail latencies with different yield
delays and oversubscription levels.

With no oversubscription (to the degree possible):

scang yield delay = 1, casgstatus yield delay = 1
Latency-50   1.41ms ±15%  1.41ms ± 5%    ~     (p=0.611 n=13+12)
Latency-95   5.21ms ± 2%  5.15ms ± 2%  -1.15%  (p=0.012 n=13+13)
Latency-99   7.16ms ± 2%  7.05ms ± 2%  -1.54%  (p=0.002 n=13+13)
Latency-999  10.7ms ± 9%  10.2ms ±10%  -5.46%  (p=0.004 n=12+13)

scang yield delay = 5000, casgstatus yield delay = 3000
Latency-50   1.41ms ±15%  1.41ms ± 8%    ~     (p=0.511 n=13+13)
Latency-95   5.21ms ± 2%  5.14ms ± 2%  -1.23%  (p=0.006 n=13+13)
Latency-99   7.16ms ± 2%  7.02ms ± 2%  -1.94%  (p=0.000 n=13+13)
Latency-999  10.7ms ± 9%  10.1ms ± 8%  -6.14%  (p=0.000 n=12+13)

scang yield delay = 10000, casgstatus yield delay = 5000
Latency-50   1.41ms ±15%  1.45ms ± 6%    ~     (p=0.724 n=13+13)
Latency-95   5.21ms ± 2%  5.18ms ± 1%    ~     (p=0.287 n=13+13)
Latency-99   7.16ms ± 2%  7.05ms ± 2%  -1.64%  (p=0.002 n=13+13)
Latency-999  10.7ms ± 9%  10.0ms ± 5%  -6.72%  (p=0.000 n=12+13)

scang yield delay = 30000, casgstatus yield delay = 10000
Latency-50   1.41ms ±15%  1.51ms ± 7%  +6.57%  (p=0.002 n=13+13)
Latency-95   5.21ms ± 2%  5.21ms ± 2%    ~     (p=0.960 n=13+13)
Latency-99   7.16ms ± 2%  7.06ms ± 2%  -1.50%  (p=0.012 n=13+13)
Latency-999  10.7ms ± 9%  10.0ms ± 6%  -6.49%  (p=0.000 n=12+13)

scang yield delay = 100000, casgstatus yield delay = 50000
Latency-50   1.41ms ±15%  1.53ms ± 6%  +8.48%  (p=0.000 n=13+12)
Latency-95   5.21ms ± 2%  5.23ms ± 2%    ~     (p=0.287 n=13+13)
Latency-99   7.16ms ± 2%  7.08ms ± 2%  -1.21%  (p=0.004 n=13+13)
Latency-999  10.7ms ± 9%   9.9ms ± 3%  -7.99%  (p=0.000 n=12+12)

scang yield delay = 200000, casgstatus yield delay = 100000
Latency-50   1.41ms ±15%  1.47ms ± 5%    ~     (p=0.072 n=13+13)
Latency-95   5.21ms ± 2%  5.17ms ± 2%    ~     (p=0.091 n=13+13)
Latency-99   7.16ms ± 2%  7.02ms ± 2%  -1.99%  (p=0.000 n=13+13)
Latency-999  10.7ms ± 9%   9.9ms ± 5%  -7.86%  (p=0.000 n=12+13)

With slight oversubscription (another instance of http benchmark
was running in background with reduced GOMAXPROCS):

scang yield delay = 1, casgstatus yield delay = 1
Latency-50    840µs ± 3%   804µs ± 3%  -4.37%  (p=0.000 n=15+18)
Latency-95   6.52ms ± 4%  6.03ms ± 4%  -7.51%  (p=0.000 n=18+18)
Latency-99   10.8ms ± 7%  10.0ms ± 4%  -7.33%  (p=0.000 n=18+14)
Latency-999  18.0ms ± 9%  16.8ms ± 7%  -6.84%  (p=0.000 n=18+18)

scang yield delay = 5000, casgstatus yield delay = 3000
Latency-50    840µs ± 3%   809µs ± 3%  -3.71%  (p=0.000 n=15+17)
Latency-95   6.52ms ± 4%  6.11ms ± 4%  -6.29%  (p=0.000 n=18+18)
Latency-99   10.8ms ± 7%   9.9ms ± 6%  -7.55%  (p=0.000 n=18+18)
Latency-999  18.0ms ± 9%  16.5ms ±11%  -8.49%  (p=0.000 n=18+18)

scang yield delay = 10000, casgstatus yield delay = 5000
Latency-50    840µs ± 3%   823µs ± 5%  -2.06%  (p=0.002 n=15+18)
Latency-95   6.52ms ± 4%  6.32ms ± 3%  -3.05%  (p=0.000 n=18+18)
Latency-99   10.8ms ± 7%  10.2ms ± 4%  -5.22%  (p=0.000 n=18+18)
Latency-999  18.0ms ± 9%  16.7ms ±10%  -7.09%  (p=0.000 n=18+18)

scang yield delay = 30000, casgstatus yield delay = 10000
Latency-50    840µs ± 3%   836µs ± 5%    ~     (p=0.442 n=15+18)
Latency-95   6.52ms ± 4%  6.39ms ± 3%  -2.00%  (p=0.000 n=18+18)
Latency-99   10.8ms ± 7%  10.2ms ± 6%  -5.15%  (p=0.000 n=18+17)
Latency-999  18.0ms ± 9%  16.6ms ± 8%  -7.48%  (p=0.000 n=18+18)

scang yield delay = 100000, casgstatus yield delay = 50000
Latency-50    840µs ± 3%   836µs ± 6%    ~     (p=0.401 n=15+18)
Latency-95   6.52ms ± 4%  6.40ms ± 4%  -1.79%  (p=0.010 n=18+18)
Latency-99   10.8ms ± 7%  10.2ms ± 5%  -4.95%  (p=0.000 n=18+18)
Latency-999  18.0ms ± 9%  16.5ms ±14%  -8.17%  (p=0.000 n=18+18)

scang yield delay = 200000, casgstatus yield delay = 100000
Latency-50    840µs ± 3%   828µs ± 2%  -1.49%  (p=0.001 n=15+17)
Latency-95   6.52ms ± 4%  6.38ms ± 4%  -2.04%  (p=0.001 n=18+18)
Latency-99   10.8ms ± 7%  10.2ms ± 4%  -4.77%  (p=0.000 n=18+18)
Latency-999  18.0ms ± 9%  16.9ms ± 9%  -6.23%  (p=0.000 n=18+18)

With significant oversubscription (background http benchmark
was running with full GOMAXPROCS):

scang yield delay = 1, casgstatus yield delay = 1
Latency-50   1.32ms ±12%  1.30ms ±13%    ~     (p=0.454 n=14+14)
Latency-95   16.3ms ±10%  15.3ms ± 7%  -6.29%  (p=0.001 n=14+14)
Latency-99   29.4ms ±10%  27.9ms ± 5%  -5.04%  (p=0.001 n=14+12)
Latency-999  49.9ms ±19%  45.9ms ± 5%  -8.00%  (p=0.008 n=14+13)

scang yield delay = 5000, casgstatus yield delay = 3000
Latency-50   1.32ms ±12%  1.29ms ± 9%    ~     (p=0.227 n=14+14)
Latency-95   16.3ms ±10%  15.4ms ± 5%  -5.27%  (p=0.002 n=14+14)
Latency-99   29.4ms ±10%  27.9ms ± 6%  -5.16%  (p=0.001 n=14+14)
Latency-999  49.9ms ±19%  46.8ms ± 8%  -6.21%  (p=0.050 n=14+14)

scang yield delay = 10000, casgstatus yield delay = 5000
Latency-50   1.32ms ±12%  1.35ms ± 9%     ~     (p=0.401 n=14+14)
Latency-95   16.3ms ±10%  15.0ms ± 4%   -7.67%  (p=0.000 n=14+14)
Latency-99   29.4ms ±10%  27.4ms ± 5%   -6.98%  (p=0.000 n=14+14)
Latency-999  49.9ms ±19%  44.7ms ± 5%  -10.56%  (p=0.000 n=14+11)

scang yield delay = 30000, casgstatus yield delay = 10000
Latency-50   1.32ms ±12%  1.36ms ±10%     ~     (p=0.246 n=14+14)
Latency-95   16.3ms ±10%  14.9ms ± 5%   -8.31%  (p=0.000 n=14+14)
Latency-99   29.4ms ±10%  27.4ms ± 7%   -6.70%  (p=0.000 n=14+14)
Latency-999  49.9ms ±19%  44.9ms ±15%  -10.13%  (p=0.003 n=14+14)

scang yield delay = 100000, casgstatus yield delay = 50000
Latency-50   1.32ms ±12%  1.41ms ± 9%  +6.37%  (p=0.008 n=14+13)
Latency-95   16.3ms ±10%  15.1ms ± 8%  -7.45%  (p=0.000 n=14+14)
Latency-99   29.4ms ±10%  27.5ms ±12%  -6.67%  (p=0.002 n=14+14)
Latency-999  49.9ms ±19%  45.9ms ±16%  -8.06%  (p=0.019 n=14+14)

scang yield delay = 200000, casgstatus yield delay = 100000
Latency-50   1.32ms ±12%  1.42ms ±10%   +7.21%  (p=0.003 n=14+14)
Latency-95   16.3ms ±10%  15.0ms ± 7%   -7.59%  (p=0.000 n=14+14)
Latency-99   29.4ms ±10%  27.3ms ± 8%   -7.20%  (p=0.000 n=14+14)
Latency-999  49.9ms ±19%  44.8ms ± 8%  -10.21%  (p=0.001 n=14+13)

All numbers are on 8 cores and with GOGC=10 (http benchmark has
tiny heap, few goroutines and low allocation rate, so by default
GC barely affects tail latency).

10us/5us yield delays seem to provide a reasonable compromise
and give 5-10% tail latency reduction. That's what used in this change.

go install -a std results on 4 core machine:

name      old time/op  new time/op  delta
Time       8.39s ± 2%   7.94s ± 2%  -5.34%  (p=0.000 n=47+49)
UserTime   24.6s ± 2%   22.9s ± 2%  -6.76%  (p=0.000 n=49+49)
SysTime    1.77s ± 9%   1.89s ±11%  +7.00%  (p=0.000 n=49+49)
CpuLoad    315ns ± 2%   313ns ± 1%  -0.59%  (p=0.000 n=49+48) # %CPU
MaxRSS    97.1ms ± 4%  97.5ms ± 9%    ~     (p=0.838 n=46+49) # bytes

Update #14396
Update #14189

Change-Id: I3f4109bf8f7fd79b39c466576690a778232055a2
Reviewed-on: https://go-review.googlesource.com/21503
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -721,9 +721,13 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		throw("casgstatus")
 	}
 
+	// See http://golang.org/cl/21503 for justification of the yield delay.
+	const yieldDelay = 5 * 1000
+	var nextYield int64
+
 	// loop if gp->atomicstatus is in a scan state giving
 	// GC time to finish and change the state to oldval.
-	for !atomic.Cas(&gp.atomicstatus, oldval, newval) {
+	for i := 0; !atomic.Cas(&gp.atomicstatus, oldval, newval); i++ {
 		if oldval == _Gwaiting && gp.atomicstatus == _Grunnable {
 			systemstack(func() {
 				throw("casgstatus: waiting for Gwaiting but is Grunnable")
@@ -736,6 +740,18 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		// 		gcphasework(gp)
 		// 	})
 		// }
+		// But meanwhile just yield.
+		if i == 0 {
+			nextYield = nanotime() + yieldDelay
+		}
+		if nanotime() < nextYield {
+			for x := 0; x < 10 && gp.atomicstatus != oldval; x++ {
+				procyield(1)
+			}
+		} else {
+			osyield()
+			nextYield = nanotime() + yieldDelay/2
+		}
 	}
 	if newval == _Grunning {
 		gp.gcscanvalid = false
@@ -773,12 +789,17 @@ func scang(gp *g) {
 
 	gp.gcscandone = false
 
+	// See http://golang.org/cl/21503 for justification of the yield delay.
+	const yieldDelay = 10 * 1000
+	var nextYield int64
+
 	// Endeavor to get gcscandone set to true,
 	// either by doing the stack scan ourselves or by coercing gp to scan itself.
 	// gp.gcscandone can transition from false to true when we're not looking
 	// (if we asked for preemption), so any time we lock the status using
 	// castogscanstatus we have to double-check that the scan is still not done.
-	for !gp.gcscandone {
+loop:
+	for i := 0; !gp.gcscandone; i++ {
 		switch s := readgstatus(gp); s {
 		default:
 			dumpgstatus(gp)
@@ -787,6 +808,7 @@ func scang(gp *g) {
 		case _Gdead:
 			// No stack.
 			gp.gcscandone = true
+			break loop
 
 		case _Gcopystack:
 		// Stack being switched. Go around again.
@@ -802,6 +824,7 @@ func scang(gp *g) {
 					gp.gcscandone = true
 				}
 				restartg(gp)
+				break loop
 			}
 
 		case _Gscanwaiting:
@@ -827,6 +850,16 @@ func scang(gp *g) {
 				casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
 			}
 		}
+
+		if i == 0 {
+			nextYield = nanotime() + yieldDelay
+		}
+		if nanotime() < nextYield {
+			procyield(10)
+		} else {
+			osyield()
+			nextYield = nanotime() + yieldDelay/2
+		}
 	}
 
 	gp.preemptscan = false // cancel scan request if no longer needed