CFQ Idle class slowing down everything?

[Thomas Guyot-Sionnest <dermoth@aei.ca>]

Hey,

I tried to use CFQ and ionice to help some I/O intensive tasks on a
MySQL slave, and it turned out to makes thing much worse to the point I
can figure out what it could be beside a bug. Tested on 2.6.20.1, but I
could eventually upgrade if there's CFQ bugfixes I'm missing. The
filesystem is ReiserFS.

When "idle", the slave do mostly random writes: about 15 rtps and 200
wtps. For testing I ended up running dd to read files from disk to
/dev/null while avoiding the system cache (had 42GB of data to read
from, and <1GB of ram cache/buffers). Here's some sample results (I
tried them multiple times with similar results every time). I monitored
the iops with "sar -b 1 0"

Under deadline-iosched:

262144000 bytes (262 MB) copied, 9.68511 seconds, 27.1 MB/s
rtps raised over 200, wtps gets down to ~100 and then back up after the
read operation (as MySQL catch up)

Under cfq-ipoched without ionice:

262144000 bytes (262 MB) copied, 4.78834 seconds, 54.7 MB/s
rtps raise over 400, wtps gets down near 0 then back up after the read.
I can expect it as cfq does not manage the write queue, so it gives most
bandwidth to dd

Under cfq-ipoched with ionice -c3 (mysqld was "best-effort: prio 4"):
13369344 bytes (13 MB) copied, 39.7619 seconds, 336 kB/s (After pressing
CTRL-C to avoid tripping off alerting on MySQL replication)

Mysql was lagging behind at pretty much the same rate with and without
"ionice -c3", but with the latter the copy operation was also incredibly
slower. During the read operation rtps was around 3 and wtps was between
10 and 20, far beyond what that raid array is able to do with pure
random operations.

Any ides what's going on with this scheduler?

FWIW the dd command was:

dd if=<file> of=/dev/null bs=128k skip=NNNN count=2000
Where NNNN is a multiple of 2000 incremented by 1 on every run.

Thanks,

Thomas