Thanks for confirming that CL 23540 has reduced STW delays. On Friday, 28 October 2016 03:52:29 UTC+11, Will Sewell wrote: > > Are you referring to https://go-review.googlesource.com/#/c/23540/ or > https://github.com/golang/go/issues/16528? If it's the former, then yes I > have tried the benchmark with the HEAD version on master of the compiler, > and it did bring the pause down to ~7.7ms. I was under the impression the > latter issue has not been fixed yet, and is the reason the pause time was > not even lower. > > On Wednesday, 26 October 2016 21:45:59 UTC+1, Dave Cheney wrote: >> >> Will, the changes has been in master for a few months now, are you able >> to build master from source and see if this has addressed the issue? I'm >> sure Rick and Austin would appreciate the feedback. >> >> On Thursday, 27 October 2016 01:46:47 UTC+11, Will Sewell wrote: >>> >>> Thanks for the information. I think it could well be caused by that. >>> Below is the screenshot of one of the periods of time where the mutator is >>> blocked. >>> >>> >>> <https://lh3.googleusercontent.com/-9oESdo48mbY/WBDBYsoWB2I/AAAAAAAAHYA/d32gkJjxjXU0G-_4to7SdUxcI6GOGgFSACLcB/s1600/Screen%2BShot%2B2016-10-26%2Bat%2B15.39.07.png> >>> >>> >>> Note: pause times were as high as 15ms with the tracer enabled. >>> >>> Similar sizes occur every ~100ms. >>> >>> Let's hope this gets resolved in Go1.8 :) >>> >>> On Monday, 24 October 2016 17:06:59 UTC+1, rhys.h...@gmail.com wrote: >>>> >>>> Yes, this sounds like https://golang.org/issue/16528. During the >>>> concurrent mark phase (the "27 [ms]" of "0.008+27+0.072 ms clock"), both >>>> your code and the garbage collector are running. The program is allowed to >>>> use four OS threads ("4 P"), which might be executing your code in your >>>> goroutines, or might be running GC code in dedicated GC goroutines. >>>> >>>> There's plenty of work for the GC to do, so when a GC helper goroutine >>>> is allowed to have some processing time it'll keep running until it has >>>> used up all of its allowed time—ten milliseconds. If all four threads end >>>> up running GC goroutines at the same time, your goroutine will need to >>>> wait >>>> until one of them has run for about 10ms before it can be scheduled again. >>>> This can lead to individual goroutines being paused for up to 10ms. >>>> >>>> You might be able to see this with the execution tracer, but it's not >>>> an easy tool to use. See package "runtime/trace" and the command "go tool >>>> trace" for some hints. Issue 16528 includes some screenshots of the tool. >>>> >>>> On Thursday, October 20, 2016 at 6:36:27 AM UTC-7, Will Sewell wrote: >>>>> >>>>> Interesting, that does sound like it could be the cause. >>>>> >>>>> I just tried running the same benchmark on master >>>>> (692df217ca21b6df8e4dc65538fcc90733e8900e), and I get the following >>>>> results: >>>>> >>>>> gc 1 @0.004s 3%: 0.009+0.41+0.049 ms clock, 0.036+0.11/0.36/0.12+0.19 >>>>> ms cpu, 4->4->3 MB, 5 MB goal, 4 P >>>>> gc 2 @0.008s 4%: 0.008+0.80+0.035 ms clock, 0.034+0.097/0.67/0.16+0.14 >>>>> ms cpu, 7->7->7 MB, 8 MB goal, 4 P >>>>> gc 3 @0.016s 3%: 0.010+0.91+0.044 ms clock, 0.041+0/0.31/0.79+0.17 ms >>>>> cpu, 13->15->14 MB, 15 MB goal, 4 P >>>>> gc 4 @0.032s 3%: 0.009+2.3+0.10 ms clock, 0.037+0.60/2.0/0.12+0.40 ms >>>>> cpu, 27->28->27 MB, 29 MB goal, 4 P >>>>> gc 5 @0.070s 3%: 0.010+7.6+0.068 ms clock, 0.043+0.79/5.4/8.5+0.27 ms >>>>> cpu, 51->53->51 MB, 54 MB goal, 4 P >>>>> gc 6 @0.149s 3%: 0.020+8.2+0.12 ms clock, 0.081+0.56/7.2/9.7+0.48 ms >>>>> cpu, 98->102->99 MB, 103 MB goal, 4 P >>>>> gc 7 @0.282s 4%: 0.028+21+0.082 ms clock, 0.11+10/20/1.9+0.32 ms cpu, >>>>> 190->195->190 MB, 198 MB goal, 4 P >>>>> gc 8 @0.568s 3%: 0.024+24+0.080 ms clock, 0.098+0/23/41+0.32 ms cpu, >>>>> 364->376->214 MB, 381 MB goal, 4 P >>>>> gc 9 @0.816s 3%: 0.008+27+0.072 ms clock, 0.035+0/25/34+0.29 ms cpu, >>>>> 412->420->213 MB, 428 MB goal, 4 P >>>>> gc 10 @1.064s 3%: 0.009+31+0.10 ms clock, 0.039+6.1/26/33+0.41 ms cpu, >>>>> 415->427->216 MB, 427 MB goal, 4 P >>>>> >>>>> My manually calculated worst time for a call to mkMessage is 7.73812ms, >>>>> which is much better than before. It's significantly faster than the >>>>> worst >>>>> wall clock time for the concurrent mark/scan phase, but it's also much >>>>> slower than the worst STW phase. Do you know why this might be? >>>>> >>>>> Best, >>>>> Will >>>>> >>>>> On Wednesday, 19 October 2016 17:29:23 UTC+1, rhys.h...@gmail.com >>>>> wrote: >>>>>> >>>>>> Yes, this sounds a lot like https://golang.org/issue/16293, where >>>>>> goroutines that allocate memory while the garbage collector is running >>>>>> can >>>>>> end up stalled for nearly the entire GC cycle, in programs where a large >>>>>> amount of the memory is in a single allocation. For the program you've >>>>>> shared, that would be the "channel" map. The bug is present in Go >>>>>> 1.5–1.7, >>>>>> and is fixed in tip (via CL 23540). >>>>>> >>>>>> Do you still see the problem if you run the program with the current >>>>>> development version of Go? >>>>>> >>>>>> On Wednesday, October 19, 2016 at 6:10:23 AM UTC-7, r...@golang.org >>>>>> wrote: >>>>>>> >>>>>>> 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! >>>>>>>> >>>>>>>
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