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

From: Vitaly V. Bursov
Date: Tue Nov 11 2008 - 14:42:23 EST

Jens Axboe wrote:
> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx> 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 {
>

No patches:

iostat for nfs+cfq read
avg-cpu: %user %nice %system %iowait %steal %idle
0,00 0,00 3,25 52,20 0,00 44,55

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 2820,10 0,00 452,40 0,00 47648,80 0,00 105,32 7,54 16,70 1,96 88,60
sdb 2818,60 0,00 453,90 0,00 47391,20 0,00 104,41 4,13 9,02 1,33 60,30

NFS+cfq, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 57.5762 s, 36.4 MB/s

NFS+deadline, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 23.6672 s, 88.6 MB/s

======================
Above patch applied:

avg-cpu: %user %nice %system %iowait %steal %idle
0,00 0,00 3,60 51,10 0,00 45,30

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 2805,80 0,00 446,20 0,00 47267,20 0,00 105,93 5,61 12,62 1,71 76,50
sdb 2803,90 0,00 448,50 0,00 47246,40 0,00 105,34 5,56 12,46 1,68 75,40


NFS+cfq, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 57.5903 s, 36.4 MB/s

NFS+deadline, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 23.46 s, 89.4 MB/s

======================
Both patches applied:

avg-cpu: %user %nice %system %iowait %steal %idle
0,00 0,00 22,95 24,65 0,00 52,40

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 6504,60 0,00 1089,80 0,00 110359,20 0,00 101,27 4,67 4,29 0,40 43,50
sdb 6495,50 0,00 1097,50 0,00 110312,80 0,00 100,51 4,57 4,17 0,39 43,10


NFS+cfq, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 25.4477 s, 82.4 MB/s

NFS+deadline, file not cached:
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 23.1639 s, 90.5 MB/s

--
Thanks,
Vitaly
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