Re: [PATCH] sched: wake-affine throttle

[Michael Wang <wangyun@linux.vnet.ibm.com>]

Hi, Peter

Thanks for your reply :)

On 04/10/2013 04:51 PM, Peter Zijlstra wrote:
> On Wed, 2013-04-10 at 11:30 +0800, Michael Wang wrote:
>> | 15 GB   |      32 | 35918 |   | 37632 | +4.77% | 47923 | +33.42% |
>> 52241 | +45.45%
> 
> So I don't get this... is wake_affine() once every milisecond _that_
> expensive?
> 
> Seeing we get a 45%!! improvement out of once every 100ms that would
> mean we're like spending 1/3rd of our time in wake_affine()? that's
> preposterous. So what's happening?

Not all the regression was caused by overhead, adopt curr_cpu not
prev_cpu for select_idle_sibling() is a more important reason for the
regression of pgbench.

In other word, for pgbench, we waste time in wake_affine() and make the
wrong decision at most of the time, the previously patch show
wake_affine() do pull unrelated tasks together, that's good if current
cpu still cached hot data for wakee, but that's not the case of the
workload like pgbench.

The workload just don't satisfied the decision changed by wake-affine,
the more wake-affine active, the more it suffered, that's why 100ms show
better results than 1ms, but when reached some rate, the benefit and
lost of wake-affine will be balanced.

Regards,
Michael Wang

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