Re: [PATCH v4] sched: Fast idling of CPU when system is partially loaded

[Rik van Riel]

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On 06/23/2014 03:16 PM, Tim Chen wrote:
> Thanks to the review from Jason, Andi and Peter. I've updated the
> code as Peter suggested with simplified logic.
> 
> When a system is lightly loaded (i.e. no more than 1 job per cpu), 
> attempt to pull job to a cpu before putting it to idle is
> unnecessary and can be skipped.  This patch adds an indicator so
> the scheduler can know when there's no more than 1 active job is on
> any CPU in the system to skip needless job pulls.
> 
> On a 4 socket machine with a request/response kind of workload
> from clients, we saw about 0.13 msec delay when we go through a
> full load balance to try pull job from all the other cpus.  While
> 0.1 msec was spent on processing the request and generating a
> response, the 0.13 msec load balance overhead was actually more
> than the actual work being done. This overhead can be skipped much
> of the time for lightly loaded systems.
> 
> With this patch, we tested with a netperf request/response workload
> that has the server busy with half the cpus in a 4 socket system.
> We found the patch eliminated 75% of the load balance attempts
> before idling a cpu.
> 
> The overhead of setting/clearing the indicator is low as we already
> gather the necessary info while we call add_nr_running and
> update_sd_lb_stats. We switch to full load balance load immediately
> if any cpu got more than one job on its run queue in
> add_nr_running.  We'll clear the indicator to avoid load balance
> when we detect no cpu's have more than one job when we scan the
> work queues in update_sg_lb_stats.  We are aggressive in turning on
> the load balance and opportunistic in skipping the load balance.
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> Acked-by:
> Jason Low <jason.low2@xxxxxx>

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


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