Hey, I previously posted this on StackOverflow, but I was told this mailing
list would be a better forum for discussion.
I am attempting to benchmark the maximum STW GC pause time for different
numbers of heap objects. To do this I have written a simple benchmark that
pushes and pops messages from a map:
package main
type message []byte
type channel map[int]message
const (
windowSize = 200000
msgCount = 1000000
)
func mkMessage(n int) message {
m := make(message, 1024)
for i := range m {
m[i] = byte(n)
}
return m
}
func pushMsg(c *channel, highID int) {
lowID := highID - windowSize
m := mkMessage(highID)
(*c)[highID] = m
if lowID >= 0 {
delete(*c, lowID)
}
}
func main() {
c := make(channel)
for i := 0; i < msgCount; i++ {
pushMsg(&c, i)
}
}
I ran this with GODEBUG=gctrace=1 <https://golang.org/pkg/runtime/>, and on
my machine the output is:
gc 1 @0.004s 2%: 0.007+0.44+0.032 ms clock, 0.029+0.22/0.20/0.28+0.12 ms cpu
, 4->4->3 MB, 5 MB goal, 4 P
gc 2 @0.009s 3%: 0.007+0.64+0.042 ms clock, 0.030+0/0.53/0.18+0.17 ms cpu, 7
->7->7 MB, 8 MB goal, 4 P
gc 3 @0.019s 1%: 0.007+0.99+0.037 ms clock, 0.031+0/0.13/1.0+0.14 ms cpu, 13
->13->13 MB, 14 MB goal, 4 P
gc 4 @0.044s 2%: 0.009+2.3+0.032 ms clock, 0.039+0/2.3/0.30+0.13 ms cpu, 25
->25->25 MB, 26 MB goal, 4 P
gc 5 @0.081s 1%: 0.009+9.2+0.082 ms clock, 0.039+0/0.32/9.7+0.32 ms cpu, 49
->49->48 MB, 50 MB goal, 4 P
gc 6 @0.162s 0%: 0.020+10+0.078 ms clock, 0.082+0/0.28/11+0.31 ms cpu, 93->
93->91 MB, 96 MB goal, 4 P
gc 7 @0.289s 0%: 0.020+27+0.092 ms clock, 0.080+0/0.95/28+0.37 ms cpu, 178->
178->173 MB, 182 MB goal, 4 P
gc 8 @0.557s 1%: 0.023+38+0.086 ms clock, 0.092+0/38/10+0.34 ms cpu, 337->
339->209 MB, 346 MB goal, 4 P
gc 9 @0.844s 1%: 0.008+40+0.077 ms clock, 0.032+0/5.6/46+0.30 ms cpu, 407->
409->211 MB, 418 MB goal, 4 P
gc 10 @1.100s 1%: 0.009+43+0.047 ms clock, 0.036+0/6.6/50+0.19 ms cpu, 411->
414->212 MB, 422 MB goal, 4 P
gc 11 @1.378s 1%: 0.008+45+0.093 ms clock, 0.033+0/6.5/52+0.37 ms cpu, 414->
417->213 MB, 425 MB goal, 4 P
My version of Go is:
$ go version
go version go1.7.1 darwin/amd64
>From the above results, the longest wall clock STW pause time is 0.093ms.
Great!
However as a sanity check I also manually timed how long it took to create
a new message by wrapping mkMessage with
start := time.Now()
m := mkMessage(highID)
elapsed := time.Since(start)
and printed the slowest `elapsed` time. The time I get for this was
38.573036ms!
I was instantly suspicious because this correlated strongly with the wall
clock times in the supposedly concurrent mark/scan phase, and in particular
with "idle GC time".
*My question is: why does this supposedly concurrent phase of the GC appear
to block the mutator?*
If I force the GC to run at regular intervals, my manually calculated pause
times go way down to <1ms, so it appears to be hitting some kind of limit
of non-live heap objects. If so, I'm not sure what that limit is, and why
it would cause a concurrent phase of the GC to appear to block the mutator.
Thanks!
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