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

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Date:	Mon, 16 Nov 2009 11:47:44 +0100
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, 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

On Thu 12-11-09 15:44:02, Jeff Moyer wrote:
> 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.
  Message in the logs:
ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
ata1.00: ATA-8: Hitachi HTS722016K9SA00, DCDOC54P, max UDMA/133
ata1.00: 312581808 sectors, multi 16: LBA48 NCQ (depth 0/32)
ata1.00: configured for UDMA/133
  So apparently no NCQ. /sys/block/sda/device/queue_depth shows 1 but I
guess that's just it's way of saying "no NCQ".

  What I thought might make a difference why I'm seeing the drop and you
are not is size of RAM or number of CPUs vs the tiobench file size or
number of threads. I'm running on a machine with 2 GB of RAM, using 4 GB
filesize. The machine has 2 cores and I'm using 16 tiobench threads. I'm
now rerunning tests with various numbers of threads to see how big
difference it makes.

								Honza

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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