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

[Paul Hartman <paul.hartman+linux@gmail.com>]

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