Re: Slow file transfer speeds with CFQ IO scheduler in some cases

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Nov 11 2008, Vitaly V. Bursov wrote:
> Jens Axboe wrote:
> > 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?
> > 
> 
> No crashes so far. Transfer speed is quiet good also.
> 
> 
> NFS+deadline, file not cached:
> 
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            0,00    0,00   25,50   19,40    0,00   55,10
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
> sda            6648,80     0,00 1281,70    0,00 115179,20     0,00    89,86     5,35    4,18   0,35  45,20
> sdb            6672,30     0,00 1257,00    0,00 115292,80     0,00    91,72     5,09    4,06   0,35  44,60
> 
> 
> 
> NFS+cfq, file not cached:
> 
> avg-cpu:  %user   %nice %system %iowait  %steal   %idle
>            0,05    0,00   25,30   23,95    0,00   50,70
> 
> Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
> sda            6403,00     0,00 1089,90    0,00 108655,20     0,00    99,69     4,50    4,13   0,41  44,50
> sdb            6394,90     0,00 1099,60    0,00 108639,20     0,00    98,80     4,53    4,12   0,39  42,50
> 
> 
> Just for reference: 10 sec interval average, gigabit network,
> no tcp/udp hardware checksumming may lead to high system cpu load.
> 
> 
> Also, a few more test (server has 4G RAM):
> 
> NFS+cfq, file not cached:
> $ dd if=test of=/dev/null bs=1M count=2000
> 2000+0 records in
> 2000+0 records out
> 2097152000 bytes (2.1 GB) copied, 24.9147 s, 84.2 MB/s
> 
> NFS+deadline, file not cached:
> 2000+0 records in
> 2000+0 records out
> 2097152000 bytes (2.1 GB) copied, 23.2999 s, 90.0 MB/s
> 
> file cached on server:
> 2000+0 records in
> 2000+0 records out
> 2097152000 bytes (2.1 GB) copied, 21.9784 s, 95.4 MB/s
> 
> 
> Local single dd read leads to 193 MB/s for deadline and
> 167 MB/s for cfq.

OK, that looks better. Can I talk you into just trying this little
patch, just to see what kind of performance that yields? Remove the cfq
patch first. I would have patched nfsd only, but this is just a quick'n
dirty.

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 8e7a7ce..3aacf48 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -92,7 +92,7 @@ static void create_kthread(struct kthread_create_info *create)
 	int pid;
 
 	/* We want our own signal handler (we take no signals by default). */
-	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
+	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | CLONE_IO | SIGCHLD);
 	if (pid < 0) {
 		create->result = ERR_PTR(pid);
 	} else {

-- 
Jens Axboe