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

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Date:	Tue, 5 Jan 2010 10:07:22 -0600
From:	Paul Hartman <paul.hartman+linux@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Multi-file USB mass-storage copy from PC to Nokia N900 slow when 
	using CFQ

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!

Thanks
Paul
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