Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

[Corrado Zoccolo]

On Fri, Nov 27, 2009 at 12:48 PM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> On Fri, Nov 27 2009, Corrado Zoccolo wrote:
>> Hi Jens,
>> let me explain why my improved formula should work better.
>>
>> The original problem was that, even if an async queue had a slice of 40ms,
>> it could take much more to complete since it could have up to 31
>> requests dispatched at the moment of expiry.
>> In total, it could take up to 40 + 16 * 8 = 168 ms (worst case) to
>> complete all dispatched requests, if they were seeky (I'm taking 8ms
>> average service time of a seeky request).
>>
>> With your patch, within the first 200ms from last sync, the max depth
>> will be 1, so a slice will take at most 48ms.
>> My patch still ensures that a slice will take at most 48ms within the
>> first 200ms from last sync, but lifts the restriction that depth will
>> be 1 at all time.
>> In fact, after the first 100ms, a new async slice will start allowing
>> 5 requests (async_slice/slice_idle). Then, whenever a request
>> completes, we compute remaining_slice / slice_idle, and compare this
>> with the number of dispatched requests. If it is greater, it means we
>> were lucky, and the requests were sequential, so we can allow more
>> requests to be dispatched. The number of requests dispatched will
>> decrease when reaching the end of the slice, and at the end we will
>> allow only depth 1.
>> For next 100ms, you will allow just depth 2, and my patch will allow
>> depth 2 at the end of the slice (but larger at the beginning), and so
>> on.
>>
>> I think the numbers by Mel show that this idea can give better and
>> more stable timings, and they were just with a single NCQ rotational
>> disk. I wonder how much improvement we can get on a raid, where
>> keeping the depth at 1 hits performance really hard.
>> Probably, waiting until memory reclaiming is noticeably active (since
>> in CFQ we will be sampling) may be too late.
>
> I'm not saying it's a no-go, just that it invalidates the low latency
> testing done through the 2.6.32 cycle and we should re-run those tests
> before committing and submitting anything.
Agreed, but it should be ok for 2.6.33.
BTW, when Jeff investigated the write performance drop in 2.6.32, he
found, too, that low_latency should be set to 0 to get performance
comparable with previous kernels.
>
> If the 'check for reclaim' hack isn't good enough, then that's probably
> what we have to do.
There is still something puzzling me. The write performance drop
affects mostly NCQ disks, so it can't be the only cause for hitting
OOM condition, otherwise we should have observed it also on previous
kernels, on non-NCQ disks, in which the phenomenon I described above
doesn't happen.
So probably there is something else to look at, and even this patch
can only be a palliative.

Thanks
Corrado

>
> --
> Jens Axboe
>
>



-- 
__________________________________________________________________________

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