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

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Message-ID: <4e5e476b1001191210s37423e4av6673225d5078a88e@mail.gmail.com>
Date:	Tue, 19 Jan 2010 21:10:33 +0100
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Shaohua Li <shaohua.li@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fio mmap randread 64k more than 40% regression with 2.6.33-rc1

On Mon, Jan 18, 2010 at 4:06 AM, Zhang, Yanmin
<yanmin_zhang@...ux.intel.com> 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@...il.com> 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):
>> -       if (cfqq->dispatched)
>> +      if (cfqq->dispatched || (cfqq->new_cfqq && rq_in_driver(cfqd)))
>>                return;
>> 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?

Thanks,
Corrado

>
>>
>> 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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