Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

[Rik van Riel]

On 07/16/2013 03:21 PM, Jason Low wrote:
> When running benchmarks on an 8 socket 80 core machine with a 3.10 kernel,
> there can be a lot of contention in idle_balance() and related functions.
> On many AIM7 workloads in which CPUs go idle very often and idle balance
> gets called a lot, it is actually lowering performance.
>
> Since idle balance often helps performance (when it is not overused), I
> looked into trying to avoid attempting idle balance only when it is
> occurring too frequently.
>
> This RFC patch attempts to keep track of the approximate "average" time between
> idle balance attempts per CPU. Each time the idle_balance() function is
> invoked, it will compute the duration since the last idle_balance() for
> the current CPU. The avg time between idle balance attempts is then updated
> using a very similar method as how rq->avg_idle is computed.
>
> Once the average time between idle balance attempts drops below a certain
> value (which in this patch is sysctl_sched_idle_balance_limit), idle_balance
> for that CPU will be skipped. The average time between idle balances will
> continue to be updated, even if it ends up getting skipped. The
> initial/maximum average is set a lot higher though to make sure that the
> avg doesn't fall below the threshold until the sample size is large and to
> prevent the avg from being overestimated.
>
> This change improved the performance of many AIM7 workloads at 1, 2, 4, 8
> sockets on the 3.10 kernel. The most significant differences were at
> 8 sockets HT-enabled. The table below compares the average jobs per minute
> at 1100-2000 users between the vanilla 3.10 kernel and 3.10 kernel with this
> patch. I included data for both hyperthreading disabled and enabled. I used
> numactl to restrict AIM7 to run on certain number of nodes. I only included
> data in which the % difference was beyond a 2% noise range.

Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>

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