Messages in this thread | | | Date | Wed, 27 May 2015 16:09:04 -0400 | From | Josef Bacik <> | Subject | Re: [PATCH] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE |
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On 05/26/2015 05:31 PM, Josef Bacik wrote: > At Facebook we have a pretty heavily multi-threaded application that is > sensitive to latency. We have been pulling forward the old SD_WAKE_IDLE code > because it gives us a pretty significant performance gain (like 20%). It turns > out this is because there are cases where the scheduler puts our task on a busy > CPU when there are idle CPU's in the system. We verify this by reading the > cpu_delay_req_avg_us from the scheduler netlink stuff. With our crappy patch we > get much lower numbers vs baseline. > > SD_BALANCE_WAKE is supposed to find us an idle cpu to run on, however it is just > looking for an idle sibling, preferring affinity over all else. This is not > helpful in all cases, and SD_BALANCE_WAKE's job is to find us an idle cpu, not > garuntee affinity. Fix this by first trying to find an idle sibling, and then > if the cpu is not idle fall through to the logic to find an idle cpu. With this > patch we get slightly better performance than with our forward port of > SD_WAKE_IDLE. Thanks, >
I rigged up a test script to run the perf bench sched tests and give me the numbers. Here are the numbers
4.0
Messaging: 56.934 Total runtime in seconds Pipe: 105620.762 ops/sec
4.0 + my patch
Messaging: 47.374 Pipe: 113691.199
so ~20% better performance out of the Messaging test which is sort of like HHVM and ~8% better pipe performance. This box is a 2 socket 16 core box. I've attached the script I'm using, basically I just run each thing 5 times, and for the perf bench sched pipe run I do NR_CPUS/2 instances of them in parallel.
If you are interested I'd be happy to show you numbers for our HHVM test, but they are less straightforward and require pretty pictures and a book of how to read the numbers. Thanks
Josef
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