Fix excess work stealing under low loads #2254
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SeanTAllen
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changelog - fixed
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We perf tested this over at @WallarooLabs. It looks good. |
Theodus
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jemc
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Sep 29, 2017
src/libponyrt/sched/cpu.c
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| @@ -314,6 +314,10 @@ void ponyint_cpu_core_pause(uint64_t tsc, uint64_t tsc2, bool yield) | |||
| // If it has been 1 billion cycles, pause 1 ms. | |||
| ts.tv_nsec = 1000000; | |||
| } | |||
| else | |||
| { | |||
| ts.tv_nsec = 100000; | |||
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Fixes #1787 Interestingly, all the info needed to solve this issue a while ago was already in the issue but it wasn't until @slfritchie put his additional comments in #1787 (comment) that it all clicked for me. The excess CPU time is from us doing too much work stealing. In a normal scenario, with nothing to do, we'd not doing anything for a long time and we'd end up sleeping for quite a while. With the timer that goes off every few seconds as seen in the issue, that isn't what happens. We regularly get woken and end up in a work stealing cycle. Then, due to the lack of an `else` block for yielding, on OSX, we'd nanosleep for 0 which is the same as an immediate return. To see what the impact of that would be on any platform change the: ```c // 10m cycles is about 3ms if((tsc2 - tsc) < 10000000) return; ``` to ```c // 10m cycles is about 3ms if((tsc2 - tsc) < 1000000000) return; ``` This is effectively what we were running. That's a lot more work-stealing. And, not the increased CPU usage. The reason this was happening more on OSX is that on Linux, nanosleep 0 will sleep for at least a bit. Here we remove the variability and do a small nanosleep that will be the same across all platforms.
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Fixes #1787
Interestingly, all the info needed to solve this issue a while ago was
already in the issue but it wasn't until @slfritchie put his additional
comments in
#1787 (comment)
that it all clicked for me.
The excess CPU time is from us doing too much work stealing. In a normal
scenario, with nothing to do, we'd not doing anything for a long time
and we'd end up sleeping for quite a while.
With the timer that goes off every few seconds as seen in the issue,
that isn't what happens. We regularly get woken and end up in a work
stealing cycle.
Then, due to the lack of an
elseblock for yielding, on OSX, we'dnanosleep for 0 which is the same as an immediate return. To see what
the impact of that would be on any platform change the:
to
This is effectively what we were running. That's a lot more
work-stealing. And, not the increased CPU usage. The reason this was
happening more on OSX is that on Linux, nanosleep 0 will sleep for at
least a bit. Here we remove the variability and do a small nanosleep
that will be the same across all platforms.