Re: [PATCH 3/4] cfq-iosched: idling on deep seeky sync queues

[Corrado Zoccolo <czoccolo@gmail.com>]

Hi Vivek,
On Tue, Nov 24, 2009 at 3:33 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> Hi Corrado,
>
> Thinking more about it. This clearing of flag when idle expires might
> create issues with queues which sent down requests with a burst initially
> forcing to set "deep" flag and then fall back to low depth. In that case,
> enable_idle will continue to be 1 and we will be driving queue depth as 1.
>
> This is a theoritical explanation looking at the patch. I don't know if
> in real life we have workloads who do this frequently. At least for my
> testing, this patch did make sure we don't switch between workload type
> of queue very frequently.
>
I thought at this scenario when developing the patch, but considered
it too infrequent (and not so costly) to justify the added complexity
of having a moving average.

For me, wasting an idle time is something to be punished for, while
driving the queue at lower depth is not, if the requests are coming
timely.

> May be keeping a track of average queue depth of a seeky process might
> help here like thinktime. If average queue depth is less over a period of
> time, we move the queue to sync-noidle group to achieve better throughput
> overall and if average queue depth is high, make is sync-idle.
>
> Currently we seem to be taking queue depth into account only for enabling
> the flag. We don't want too frequent switching of "deep" flag, so some
> kind of slow moving average might help.
>
Averages can still change in the middle of a slice.
A simpler way could be to reset the deep flag after a full slice, if
the depth never reached the threshold during that slice.

> Thanks
> Vivek
>



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dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
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