linux-kernel - Re: Multi-file USB mass-storage copy from PC to Nokia N900 slow when using CFQ

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Message-ID: <4e5e476b1001070541y30cae741ofe16ab82670b2633@mail.gmail.com>
Date:	Thu, 7 Jan 2010 14:41:51 +0100
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Paul Hartman <paul.hartman+linux@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Multi-file USB mass-storage copy from PC to Nokia N900 slow when 
	using CFQ

On Tue, Jan 5, 2010 at 5:07 PM, Paul Hartman
<paul.hartman+linux@...il.com> wrote:
> Hi,
>
> Copying more than one file from my PC (kernel 2.6.32) to my Nokia N900
> over USB mass storage mode is very slow when CFQ is the i/o scheduler.
> The target uses vfat filesystem.
>
> I am using iotop to monitor the I/O in general, plus I performed the
> following test. file1 and file2 are each 700M and both housed on a
> ramdrive for this test. They were deleted from the destination between
> runs.
>
> # one file at a time with sync in-between, fast speeds:
> $ sync; time sh -c "cp file1 /mnt/usb; sync; cp file2 /mnt/usb; sync"
>
> real    1m25.697s
> user    0m0.005s
> sys     0m2.509s
>
> # copy two files in a row, then sync, speed is bad:
> $ sync; time sh -c "cp file1 file2 /mnt/usb; sync"
>
> real    6m51.439s
> user    0m0.007s
> sys     0m2.615s
>
>
> Using all I/O schedulers, the speed of the first test was the same. So
> it's only related to writing more than 1 file to the N900. The timing
> results for the second test ended up as such:
>
> cfq: 6m51.439s
> noop: 3m0.733s
> anticipatory: 1m44.348s
> deadline: 1m36.804s
>
>
> Also, in 2.6.31 the speed was four times slower, so the removal of old
> pdflush code may have made a difference in this case. Copying 1
> gigabyte takes about 1 minute at optimal speed, about 5 minutes using
> CFQ in kernel 2.6.32, and took about 20 minutes using CFQ in kernel
> 2.6.31.
>
> I thought you may be interested in case there's room to improve the
> scheduler.  If you want any other info let me know!

Can you try setting:
/sys/block/**your_device** /queue/iosched/fifo_expire_sync
to a large number, e.g. 5000 ?
CFQ operates pretty much like deadline w.r.t. normal writes, but uses
a much shorter interval to switch between two streams of writes, to
reduce latency of data hitting disk, and this could cause low
performance on flash devices, where writes that do not cover whole
blocks are painfully slow.

Corrado
>
> Thanks
> Paul
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-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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