linux-kernel - Re: I/O and pdflush

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Date:	Tue, 1 Sep 2009 16:14:58 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Fernando Silveira <fsilveira@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: I/O and pdflush

On Mon, Aug 31, 2009 at 10:33:43PM +0800, Fernando Silveira wrote:
> On Mon, Aug 31, 2009 at 11:07, Wu Fengguang<fengguang.wu@...el.com> wrote:
> > On Mon, Aug 31, 2009 at 10:01:13PM +0800, Wu Fengguang wrote:
> >> On Mon, Aug 31, 2009 at 10:00:06PM +0800, Wu Fengguang wrote:
> >> > Hi Fernando,
> >> >
> >> > What's your SSD's IO parameters? Ie. output of this command:
> >> >
> >> >         grep -r . /sys/block/sda/queue/
> >> >
> >> > Please replace 'sda' with your SSD device name.
> >>
> >> Oh I guess it's sdc:
> >>
> >>          grep -r . /sys/block/sdc/queue/
> 
> Here is it:
> 
> # grep -r . /sys/block/sdc/queue/
> /sys/block/sdc/queue/nr_requests:128
> /sys/block/sdc/queue/read_ahead_kb:128
> /sys/block/sdc/queue/max_hw_sectors_kb:128
> /sys/block/sdc/queue/max_sectors_kb:128
> /sys/block/sdc/queue/scheduler:noop anticipatory [deadline] cfq
> /sys/block/sdc/queue/hw_sector_size:512
> /sys/block/sdc/queue/rotational:0
> /sys/block/sdc/queue/nomerges:0
> /sys/block/sdc/queue/rq_affinity:0
> /sys/block/sdc/queue/iostats:1
> /sys/block/sdc/queue/iosched/read_expire:500
> /sys/block/sdc/queue/iosched/write_expire:5000
> /sys/block/sdc/queue/iosched/writes_starved:2
> /sys/block/sdc/queue/iosched/front_merges:1
> /sys/block/sdc/queue/iosched/fifo_batch:16
> #
> 
> These are probably default settings.
> 
> > BTW, would you run "iostat -x 1 5" (which will run 5 seconds) when
> > doing I/O in ideal throughput, and when in 25MB/s thoughput state?
> 
> Both files are attached (25mbps = 25MB/s, 80mbps = 80MB/s).

The iostat reported IO size is 64kb, which is half of max_sectors_kb=128.
It is strange why the optimal 128kb IO size is not reached in both cases:

        Device:         rrqm/s   wrqm/s     r/s     w/s    rsec/s   wsec/s  avgrq-sz avgqu-sz   await  svctm  %util
case 1: sdc               0.00 69088.00     0.00  552.00     0.00 70656.00    128.00   142.75  386.39   1.81 100.10
case 2: sdc               0.00 153504.00    0.00 1200.00     0.00 153600.00   128.00   138.35  115.76   0.83 100.10

Fernando, could you try increasing these deadline parameters by 10
times?

        echo 160   > /sys/block/sdc/queue/iosched/fifo_batch
        echo 50000 > /sys/block/sdc/queue/iosched/write_expire

And try cfq iosched if that still fails? The iostat outputs would be
enough during the tests.

Thanks,
Fengguang
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