performance "regression" in cfq compared to anticipatory, deadline and noop

[Matthew <jackdachef@gmail.com>]

Hi Ingo, hi everybody,

I've encountered sort of a performance "regression" in using cfq (and
the cfq-based bfq) in comparison with the other io-schedulers:

1) interactivity during load is much better compared to the others
(thanks a lot for that, that made me appreciate this scheduler) BUT
2) everything seems to take somewhat longer to load (big applications
like firefox, etc. )
3) hdparm shows the same behavior

since I've started using cfq only for a few days (approx. 1-2 weeks
now) I didn't really notice it until I tested "performance" via
hdparm:


/dev/sdd:
 Timing buffered disk reads:  308 MB in  3.01 seconds = 102.22 MB/sec

/dev/sdd:
 Timing buffered disk reads:  306 MB in  3.01 seconds = 101.66 MB/sec

/dev/sdd:
 Timing buffered disk reads:  304 MB in  3.02 seconds = 100.77 MB/sec
noop [anticipatory] deadline cfq

deadline & noop are similar, the test of noop finishes pretty fast ...


/dev/sdd:
 Timing buffered disk reads:  170 MB in  3.02 seconds =  56.27 MB/sec

/dev/sdd:
 Timing buffered disk reads:  176 MB in  3.02 seconds =  58.21 MB/sec

/dev/sdd:
 Timing buffered disk reads:  176 MB in  3.02 seconds =  58.22 MB/sec
noop anticipatory deadline [cfq]

this behavior occurs on an jmicron sata-controller (JMB363/361) and
the probably the 4th port of the Intel ICH7R but only with cfq
selected, the first (?) and second (?) port of the Intel ICH7R are
fine performance-wise, don't know why it's that picky
with the other schedulers it's fine

I've tested: 2.6.24-gentoo-r7 (+ 2.6.24.7), 2.6.24-gentoo-r3, 2.6.25,
2.6.25.2 (+ 2.6.25-zen1), 2.6.25-rc8, the kernel of the ubuntu
desktop-livecd amd64 (ubuntu 8.04) (cfq enabled)
all show this worse "performance" compared to the other schedulers
all kernels are amd64 on gentoo ~amd64, glibc-2.7.1, gcc-4.2.3 hardened

hardware:
Asus P5W DH Deluxe

I unfortuantely can't test earlier kernel-versions due to the fact
that I'm using reiser4 for /(root) and the earlier kernels + reiser4
aren't that stable in terms of data safety

hopefully this is reproducable & you guys can explain if this is
something to "worry" about (performance) and/or a real regression or
just some kind of placebo effect

Many thanks in advance & thanks a lot for this great scheduler (cfq;
I'm looking forward to bfq in mainline which seems to work even better
under load)

Regards

Mat