linux-kernel - Re: Performance regression in IO scheduler still there

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Message-ID: <x49pr7npj71.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 12 Nov 2009 15:44:02 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	jens.axboe@...cle.com, LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>, mszeredi@...e.de
Subject: Re: Performance regression in IO scheduler still there

Jan Kara <jack@...e.cz> writes:

> On Wed 11-11-09 12:43:30, Jeff Moyer wrote:
>> Jan Kara <jack@...e.cz> writes:
>> 
>> >   Sadly, I don't see the improvement you can see :(. The numbers are the
>> > same regardless low_latency set to 0:
>> > 2.6.32-rc5 low_latency = 0:
>> > 37.39 36.43 36.51 -> 36.776667 0.434920
>> >   But my testing environment is a plain SATA drive so that probably
>> > explains the difference...
>> 
>> I just retested (10 runs for each kernel) on a SATA disk with no NCQ
>> support and I could not see a difference.  I'll try to dig up a disk
>> that support NCQ.  Is that what you're using for testing?
>   I don't think I am. How do I find out?

Good question.  ;-)  I grep for NCQ in dmesg output and make sure it's
greater than 0/32.  There may be a better way, though.

>>               2.6.29    2.6.32-rc6,low_latency=0
>> ----------------------------------
>> Average:      34.6648   34.4475
>> Pop.Std.Dev.: 0.55523   0.21981
>   Hmm, strange. Miklos Szeredi tried tiobench on his machine and he also
> saw the regression. I'll try to think what could make the difference.

OK, I'll try again.

Cheers,
Jeff
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