Re: [PATCH] cfq-iosched: avoid slice overrun when idling

[Corrado Zoccolo]

On Wed, Oct 7, 2009 at 7:51 PM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> On Wed, Oct 07 2009, Corrado Zoccolo wrote:
>> Idle window for a queue is reduced when the queue is about to finish
>> its slice.
>>
>> Signed-off-by: Corrado Zoccolo <czoccolo@xxxxxxxxx>
>> ---
>> Âblock/cfq-iosched.c | Â Â4 +++-
>> Â1 files changed, 3 insertions(+), 1 deletions(-)
>>
>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>> index 4ab33d8..55bb8ca 100644
>> --- a/block/cfq-iosched.c
>> +++ b/block/cfq-iosched.c
>> @@ -1105,8 +1105,10 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
>> Â Â Â Â* we don't want to idle for seeks, but we do want to allow
>> Â Â Â Â* fair distribution of slice time for a process doing back-to-back
>> Â Â Â Â* seeks. so allow a little bit of time for him to submit a new rq
>> + Â Â Â* but avoid overrunning its timeslice
>> Â Â Â Â*/
>> - Â Â sl = cfqd->cfq_slice_idle;
>> + Â Â sl = min_t(unsigned long, cfqd->cfq_slice_idle,
>> + Â Â Â Â Â Â cfqq->slice_end - jiffies);
>> Â Â Â if (sample_valid(cic->seek_samples) && CIC_SEEKY(cic))
>> Â Â Â Â Â Â Â sl = min(sl, msecs_to_jiffies(CFQ_MIN_TT));
>
> This was actually done this way on purpose, since shorter idling more
> often don't suceed. So the logic was rather overrun the slice slightly
> than wait shortly and just miss the incoming IO.
>
> Of course this will overrun the slice even more, which the above will
> also do since it wants to do IO within that time frame too.
>
> So I think we should either leave it as-is, OR simply not arm the idle
> timer when it has less than slice_idle time left and immediately select
> a new queue.

Maybe we can arm the timer only if the think time is less than the
remaining slice, otherwise skip it.

Corrado
>
> --
> Jens Axboe
>
>



-- 
__________________________________________________________________________

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