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

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Date:	Tue, 19 Jan 2010 15:42:56 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"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

Corrado Zoccolo <czoccolo@...il.com> writes:

> 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?

It's tough to say.  Is there any chance I could get some blktrace data
for the run?

Cheers,
Jeff
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