Re: dramatic I/O slowdown after upgrading 2.6.38->3.0+

[Jan Kara <jack@suse.cz>]

On Wed 11-04-12 00:55:44, Suresh Jayaraman wrote:
> On 04/10/2012 08:43 PM, Jan Kara wrote:
> > On Tue 10-04-12 10:00:38, Michael Tokarev wrote:
> >> On 10.04.2012 06:26, Dave Chinner wrote:
> >>
> >>> Barriers. Turn them off, and see if that fixes your problem.
> >>
> >> Thank you Dave for a hint.  And nope, that's not it, not at all... ;)
> >> While turning off barriers helps a tiny bit, to gain a few %% from
> >> the huge slowdown, it does not cure the issue.
> >>
> >> Meanwhile, I observed the following:
> >>
> >> 1) the issue persists on more recent kernels too, I tried 3.3
> >>    and it is also as slow as 3.0.
> >>
> >> 2) at least 2.6.38 kernel works fine, as fast as 2.6.32, I'll
> >>    try 2.6.39 next.
> >>
> >>    I updated $subject accordingly.
> >>
> >> 3) the most important thing I think: this is general I/O speed
> >>    issue.  Here's why:
> >>
> >>   2.6.38:
> >>   # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
> >>   100+0 records in
> >>   100+0 records out
> >>   104857600 bytes (105 MB) copied, 1.73126 s, 60.6 MB/s
> >>
> >>   3.0:
> >>   # dd if=/dev/sdb of=/dev/null bs=1M iflag=direct count=100
> >>   100+0 records in
> >>   100+0 records out
> >>   104857600 bytes (105 MB) copied, 29.4508 s, 3.6 MB/s
> >>
> >> That's about 20 times difference on direct read from the
> >> same - idle - device!!
> >   Huh, that's a huge difference for such a trivial load. So we can rule out
> > filesystems, writeback, mm. I also wouldn't think it's IO scheduler but
> > you can always check by comparing dd numbers after
> >   echo none >/sys/block/sdb/queue/scheduler
> 
> s/none/noop
> you meant noop, of course?
  Yeah. Thanks for correction!

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR