linux-kernel - Re: Bug in kernel 2.6.31, Slow wb

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Date:	Sat, 1 Aug 2009 13:03:54 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Martin Bligh <mbligh@...gle.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Chad Talbott <ctalbott@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael Rubin <mrubin@...gle.com>, sandeen@...hat.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Bug in kernel 2.6.31, Slow wb_kupdate writeout

On Sat, Aug 01, 2009 at 12:53:46PM +0800, Wu Fengguang wrote:
> On Sat, Aug 01, 2009 at 12:03:13PM +0800, Wu Fengguang wrote:

> I can see the growth when I increased the dd size to 2GB,
> and the dd throughput decreased from 82.5MB/s to 60.9MB/s.

The raw disk write throughput seems to be 60MB/s:

        wfg ~% dd if=/dev/zero of=/opt/vm/200M bs=1M count=200 oflag=direct
        200+0 records in
        200+0 records out
        209715200 bytes (210 MB) copied, 3.48137 s, 60.2 MB/s

read throughput is a bit better:

        wfg ~% dd of=/dev/null if=/opt/vm/200M bs=1M count=200 iflag=direct
        200+0 records in
        200+0 records out
        209715200 bytes (210 MB) copied, 2.66606 s, 78.7 MB/s

        # hdparm -tT /dev/hda

        /dev/hda:
         Timing cached reads:   10370 MB in  1.99 seconds = 5213.70 MB/sec
         Timing buffered disk reads:  216 MB in  3.03 seconds =  71.22 MB/sec

And sync writes are pretty slow:

        wfg ~% dd if=/dev/zero of=/opt/vm/200M bs=1M count=200 oflag=sync
        200+0 records in
        200+0 records out
        209715200 bytes (210 MB) copied, 10.4741 s, 20.0 MB/s

Thanks,
Fengguang
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