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SubjectRe: Slow file transfer speeds with CFQ IO scheduler in some cases
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Jens Axboe <jens.axboe@oracle.com> writes:

> On Tue, Nov 11 2008, Jens Axboe wrote:
>> On Tue, Nov 11 2008, Jens Axboe wrote:
>> > On Mon, Nov 10 2008, Jeff Moyer wrote:
>> > > "Vitaly V. Bursov" <vitalyb@telenet.dn.ua> writes:
>> > >
>> > > > Jens Axboe wrote:
>> > > >> On Mon, Nov 10 2008, Vitaly V. Bursov wrote:
>> > > >>> Jens Axboe wrote:
>> > > >>>> On Mon, Nov 10 2008, Jeff Moyer wrote:
>> > > >>>>> Jens Axboe <jens.axboe@oracle.com> writes:
>> > > >>>>>
>> > > >>>>>> http://bugzilla.kernel.org/attachment.cgi?id=18473&action=view
>> > > >>>>> Funny, I was going to ask the same question. ;) The reason Jens wants
>> > > >>>>> you to try this patch is that nfsd may be farming off the I/O requests
>> > > >>>>> to different threads which are then performing interleaved I/O. The
>> > > >>>>> above patch tries to detect this and allow cooperating processes to get
>> > > >>>>> disk time instead of waiting for the idle timeout.
>> > > >>>> Precisely :-)
>> > > >>>>
>> > > >>>> The only reason I haven't merged it yet is because of worry of extra
>> > > >>>> cost, but I'll throw some SSD love at it and see how it turns out.
>> > > >>>>
>> > > >>> Sorry, but I get "oops" same moment nfs read transfer starts.
>> > > >>> I can get directory list via nfs, read files locally (not
>> > > >>> carefully tested, though)
>> > > >>>
>> > > >>> Dumps captured via netconsole, so these may not be completely accurate
>> > > >>> but hopefully will give a hint.
>> > > >>
>> > > >> Interesting, strange how that hasn't triggered here. Or perhaps the
>> > > >> version that Jeff posted isn't the one I tried. Anyway, search for:
>> > > >>
>> > > >> RB_CLEAR_NODE(&cfqq->rb_node);
>> > > >>
>> > > >> and add a
>> > > >>
>> > > >> RB_CLEAR_NODE(&cfqq->prio_node);
>> > > >>
>> > > >> just below that. It's in cfq_find_alloc_queue(). I think that should fix
>> > > >> it.
>> > > >>
>> > > >
>> > > > Same problem.
>> > > >
>> > > > I did make clean; make -j3; sync; on (2 times) patched kernel and it went OK
>> > > > but It won't boot anymore with cfq with same error...
>> > > >
>> > > > Switching cfq io scheduler at runtime (booting with "as") appears to work with
>> > > > two parallel local dd reads.
>> > >
>> > > Strange, I can't reproduce a failure. I'll keep trying. For now, these
>> > > are the results I see:
>> > >
>> > > [root@maiden ~]# mount megadeth:/export/cciss /mnt/megadeth/
>> > > [root@maiden ~]# dd if=/mnt/megadeth/file1 of=/dev/null bs=1M
>> > > 1024+0 records in
>> > > 1024+0 records out
>> > > 1073741824 bytes (1.1 GB) copied, 26.8128 s, 40.0 MB/s
>> > > [root@maiden ~]# umount /mnt/megadeth/
>> > > [root@maiden ~]# mount megadeth:/export/cciss /mnt/megadeth/
>> > > [root@maiden ~]# dd if=/mnt/megadeth/file1 of=/dev/null bs=1M
>> > > 1024+0 records in
>> > > 1024+0 records out
>> > > 1073741824 bytes (1.1 GB) copied, 23.7025 s, 45.3 MB/s
>> > > [root@maiden ~]# umount /mnt/megadeth/
>> > >
>> > > Here is the patch, with the suggestion from Jens to switch the cfqq to
>> > > the right priority tree when the priority is changed.
>> >
>> > I don't see the issue here either. Vitaly, are you using any openvz
>> > kernel patches? IIRC, they patch cfq so it could just be that your cfq
>> > version is incompatible with Jeff's patch.
>>
>> Heh, got it to trigger about 3 seconds after sending that email! I'll
>> look more into it.
>
> OK, found the issue. A few bugs there... cfq_prio_tree_lookup() doesn't
> even return a hit, since it just breaks and returns NULL always. That
> can cause cfq_prio_tree_add() to screw up the rbtree. The code to
> correct on ioprio change wasn't correct either, I changed that as well.
> New patch below, Vitaly can you give it a spin?

Thanks for doing that! Yeah, that was a stupid bug with the lookup
routine. I don't know that I agree with you that the ioprio change code
was wrong. I looked at all of the callers and that seemed the code path
that was used for I/O priority *changes*. The initial creation was
already okay, wasn't it?

Anwyay, I'll test this new version.

Thanks!

Jeff


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