This is likely 23540 <https://go-review.googlesource.com/#/c/23540/>.
On Wednesday, October 19, 2016 at 8:32:18 AM UTC-4, Will Sewell wrote: > > 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! > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.