Re: fio mmap randread 64k more than 40% regression with 2.6.33-rc1

From: Corrado Zoccolo
Date: Tue Jan 19 2010 - 15:10:41 EST

On Mon, Jan 18, 2010 at 4:06 AM, Zhang, Yanmin
<yanmin_zhang@xxxxxxxxxxxxxxx> wrote:
> On Sat, 2010-01-16 at 17:27 +0100, Corrado Zoccolo wrote:
>> Hi Yanmin
>> On Mon, Jan 4, 2010 at 7:28 PM, Corrado Zoccolo <czoccolo@xxxxxxxxx> wrote:
>> > Hi Yanmin,
>> >> When low_latency=1, we get the biggest number with kernel 2.6.32.
>> >> Comparing with low_latency=0's result, the prior one is about 4% better.
>> > Ok, so 2.6.33 + corrado (with low_latency =0) is comparable with
>> > fastest 2.6.32, so we can consider the first part of the problem
>> > solved.
>> >
>> I think we can return now to your full script with queue merging.
>> I'm wondering if (in arm_slice_timer):
>> - Â Â Â if (cfqq->dispatched)
>> + Â Â Âif (cfqq->dispatched || (cfqq->new_cfqq && rq_in_driver(cfqd)))
>> Â Â Â Â Â Â Â Âreturn;
>> gives the same improvement you were experiencing just reverting to rq_in_driver.
> I did a quick testing against 2.6.33-rc1. With the new method, fio mmap randread 46k
> has about 20% improvement. With just checking rq_in_driver(cfqd), it has
> about 33% improvement.
Jeff, do you have an idea why in arm_slice_timer, checking
rq_in_driver instead of cfqq->dispatched gives so much improvement in
presence of queue merging, while it doesn't have noticeable effect
when there are no merges?


>> We saw that cfqq->dispatched worked fine when there was no queue
>> merging happening, so it must be something concerning merging,
>> probably dispatched is not accurate when we set up for a merging, but
>> the merging was not yet done.
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