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Re: [PATCHSET v5] Make background writeback great again for the first time

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  Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-04-28 14:00 +0200
    Re: [PATCHSET v5] Make background writeback great again for the first  time Jens Axboe <axboe@kernel.dk> - 2016-04-28 20:50 +0200
      Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-05-03 14:20 +0200
        Re: [PATCHSET v5] Make background writeback great again for the  first time Chris Mason <clm@fb.com> - 2016-05-03 14:50 +0200
          Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-05-03 15:10 +0200
            Re: [PATCHSET v5] Make background writeback great again for the  first time Chris Mason <clm@fb.com> - 2016-05-03 15:50 +0200
              Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-05-03 16:00 +0200
        Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-05-11 18:40 +0200

#1390077 — Re: [PATCHSET v5] Make background writeback great again for the first time

On Wed 27-04-16 14:59:15, Jens Axboe wrote:
> On Wed, Apr 27 2016, Jens Axboe wrote:
> > On Wed, Apr 27 2016, Jens Axboe wrote:
> > > On 04/27/2016 12:01 PM, Jan Kara wrote:
> > > >Hi,
> > > >
> > > >On Tue 26-04-16 09:55:23, Jens Axboe wrote:
> > > >>Since the dawn of time, our background buffered writeback has sucked.
> > > >>When we do background buffered writeback, it should have little impact
> > > >>on foreground activity. That's the definition of background activity...
> > > >>But for as long as I can remember, heavy buffered writers have not
> > > >>behaved like that. For instance, if I do something like this:
> > > >>
> > > >>$ dd if=/dev/zero of=foo bs=1M count=10k
> > > >>
> > > >>on my laptop, and then try and start chrome, it basically won't start
> > > >>before the buffered writeback is done. Or, for server oriented
> > > >>workloads, where installation of a big RPM (or similar) adversely
> > > >>impacts database reads or sync writes. When that happens, I get people
> > > >>yelling at me.
> > > >>
> > > >>I have posted plenty of results previously, I'll keep it shorter
> > > >>this time. Here's a run on my laptop, using read-to-pipe-async for
> > > >>reading a 5g file, and rewriting it. You can find this test program
> > > >>in the fio git repo.
> > > >
> > > >I have tested your patchset on my test system. Generally I have observed
> > > >noticeable drop in average throughput for heavy background writes without
> > > >any other disk activity and also somewhat increased variance in the
> > > >runtimes. It is most visible on this simple testcases:
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > > >
> > > >and
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > >
> > > >The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
> > > >created before each dd run on a dedicated disk.
> > > >
> > > >Without your patches I get pretty stable dd runtimes for both cases:
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > > >Runtimes: 87.9611 87.3279 87.2554
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > >Runtimes: 93.3502 93.2086 93.541
> > > >
> > > >With your patches the numbers look like:
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > > >Runtimes: 108.183, 97.184, 99.9587
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > >Runtimes: 104.9, 102.775, 102.892
> > > >
> > > >I have checked whether the variance is due to some interaction with CFQ
> > > >which is used for the disk. When I switched the disk to deadline, I still
> > > >get some variance although, the throughput is still ~10% lower:
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > > >Runtimes: 100.417 100.643 100.866
> > > >
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > >Runtimes: 104.208 106.341 105.483
> > > >
> > > >The disk is rotational SATA drive with writeback cache, queue depth of the
> > > >disk reported in /sys/block/sdb/device/queue_depth is 1.
> > > >
> > > >So I think we still need some tweaking on the low end of the storage
> > > >spectrum so that we don't lose 10% of throughput for simple cases like
> > > >this.
> > > 
> > > Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
> > > you are seeing smaller requests, and that is why it both varies and
> > > you get lower throughput? I'll try and setup a test here similar to
> > > yours.
> > 
> > Jan, care to try the below patch? I can't fully reproduce your issue on
> > a SCSI disk limited to QD=1, but I have a feeling this might help. It's
> > a bit of a hack, but the general idea is to allow one more request to
> > build up for QD=1 devices. That eliminates wait time between one request
> > finishing, and the next being submitted.
> 
> That accidentally added a potentially stall, this one is both cleaner
> and should have that fixed.
> 
..
> -	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> -	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> -	rwb->wb_background = (rwb->wb_max + 3) / 4;
> +	if (rwb->queue_depth == 1) {
> +		rwb->wb_max = rwb->wb_normal = 2;
> +		rwb->wb_background = 1;

This breaks the detection of too big scale_step in scale_up() where we key
of wb_max == 1 value. However even with that fixed no luck :(:

dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
Runtime: 105.126 107.125 105.641

So about the same as before. I'll try to debug this later today...

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

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#1390455 — Re: [PATCHSET v5] Make background writeback great again for the first time

On 04/28/2016 05:54 AM, Jan Kara wrote:
> On Wed 27-04-16 14:59:15, Jens Axboe wrote:
>> On Wed, Apr 27 2016, Jens Axboe wrote:
>>> On Wed, Apr 27 2016, Jens Axboe wrote:
>>>> On 04/27/2016 12:01 PM, Jan Kara wrote:
>>>>> Hi,
>>>>>
>>>>> On Tue 26-04-16 09:55:23, Jens Axboe wrote:
>>>>>> Since the dawn of time, our background buffered writeback has sucked.
>>>>>> When we do background buffered writeback, it should have little impact
>>>>>> on foreground activity. That's the definition of background activity...
>>>>>> But for as long as I can remember, heavy buffered writers have not
>>>>>> behaved like that. For instance, if I do something like this:
>>>>>>
>>>>>> $ dd if=/dev/zero of=foo bs=1M count=10k
>>>>>>
>>>>>> on my laptop, and then try and start chrome, it basically won't start
>>>>>> before the buffered writeback is done. Or, for server oriented
>>>>>> workloads, where installation of a big RPM (or similar) adversely
>>>>>> impacts database reads or sync writes. When that happens, I get people
>>>>>> yelling at me.
>>>>>>
>>>>>> I have posted plenty of results previously, I'll keep it shorter
>>>>>> this time. Here's a run on my laptop, using read-to-pipe-async for
>>>>>> reading a 5g file, and rewriting it. You can find this test program
>>>>>> in the fio git repo.
>>>>>
>>>>> I have tested your patchset on my test system. Generally I have observed
>>>>> noticeable drop in average throughput for heavy background writes without
>>>>> any other disk activity and also somewhat increased variance in the
>>>>> runtimes. It is most visible on this simple testcases:
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>>
>>>>> and
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>>
>>>>> The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
>>>>> created before each dd run on a dedicated disk.
>>>>>
>>>>> Without your patches I get pretty stable dd runtimes for both cases:
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>> Runtimes: 87.9611 87.3279 87.2554
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>> Runtimes: 93.3502 93.2086 93.541
>>>>>
>>>>> With your patches the numbers look like:
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>> Runtimes: 108.183, 97.184, 99.9587
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>> Runtimes: 104.9, 102.775, 102.892
>>>>>
>>>>> I have checked whether the variance is due to some interaction with CFQ
>>>>> which is used for the disk. When I switched the disk to deadline, I still
>>>>> get some variance although, the throughput is still ~10% lower:
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>> Runtimes: 100.417 100.643 100.866
>>>>>
>>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>> Runtimes: 104.208 106.341 105.483
>>>>>
>>>>> The disk is rotational SATA drive with writeback cache, queue depth of the
>>>>> disk reported in /sys/block/sdb/device/queue_depth is 1.
>>>>>
>>>>> So I think we still need some tweaking on the low end of the storage
>>>>> spectrum so that we don't lose 10% of throughput for simple cases like
>>>>> this.
>>>>
>>>> Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
>>>> you are seeing smaller requests, and that is why it both varies and
>>>> you get lower throughput? I'll try and setup a test here similar to
>>>> yours.
>>>
>>> Jan, care to try the below patch? I can't fully reproduce your issue on
>>> a SCSI disk limited to QD=1, but I have a feeling this might help. It's
>>> a bit of a hack, but the general idea is to allow one more request to
>>> build up for QD=1 devices. That eliminates wait time between one request
>>> finishing, and the next being submitted.
>>
>> That accidentally added a potentially stall, this one is both cleaner
>> and should have that fixed.
>>
> ..
>> -	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
>> -	rwb->wb_normal = (rwb->wb_max + 1) / 2;
>> -	rwb->wb_background = (rwb->wb_max + 3) / 4;
>> +	if (rwb->queue_depth == 1) {
>> +		rwb->wb_max = rwb->wb_normal = 2;
>> +		rwb->wb_background = 1;
>
> This breaks the detection of too big scale_step in scale_up() where we key
> of wb_max == 1 value. However even with that fixed no luck :(:

Yeah, I need to look at that. For QD=1, I think the only sensible values 
for max/normal/bg is 2/2/1 and 1/1/1 if we step down.

> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> Runtime: 105.126 107.125 105.641
>
> So about the same as before. I'll try to debug this later today...

Thanks, I'm very interested in what you find!

-- 
Jens Axboe

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#1393372

On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> >>+	if (rwb->queue_depth == 1) {
> >>+		rwb->wb_max = rwb->wb_normal = 2;
> >>+		rwb->wb_background = 1;
> >
> >This breaks the detection of too big scale_step in scale_up() where we key
> >of wb_max == 1 value. However even with that fixed no luck :(:
> 
> Yeah, I need to look at that. For QD=1, I think the only sensible values for
> max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> 
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> >Runtime: 105.126 107.125 105.641
> >
> >So about the same as before. I'll try to debug this later today...
> 
> Thanks, I'm very interested in what you find!

OK, so the reason was relatively standard in the end. I was using ext3 (or
more exactly ext4 without delayed allocation) for the test. The throttling
of background writes gave more priority to writes from the journalling
thread which happen with WRITE_SYNC and thus are not throttled. Thus the
journalling thread ended up having to do more data writeback to be able to
commit a transaction (due to requirements of data=ordered mode) and it is
less efficient at that than the normal flusher thread.

So this is an example where throttling background writeback effectively
just pushes more work into another context which does it less efficiently
and indirectly makes everyone wait for it. ext3 has been always sensitive to
issues like this. ext4 is using delayed allocation and thus only data
writes into holes end up being part of a transaction -> simple dd test case
doesn't hit that path. And indeed when I repeat the same test with ext4,
the numbers with and without your patch are exactly the same.

The question remains how common a pattern where throttling of background
writeback delays also something else is. I'll schedule a couple of
benchmarks to measure impact of your patches for a wider range of workloads
(but sadly pretty limited set of hw). If ext3 is the only one seeing
issues, I would be willing to accept that ext3 takes the hit since it is
doing something rather stupid (but inherent in its journal design) and we
have a way to deal with this either by enabling delayed allocation or by
turning off the writeback throttling...

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

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#1393394

On Tue, May 03, 2016 at 02:17:19PM +0200, Jan Kara wrote:
> On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> > >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> > >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> > >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> > >>+	if (rwb->queue_depth == 1) {
> > >>+		rwb->wb_max = rwb->wb_normal = 2;
> > >>+		rwb->wb_background = 1;
> > >
> > >This breaks the detection of too big scale_step in scale_up() where we key
> > >of wb_max == 1 value. However even with that fixed no luck :(:
> > 
> > Yeah, I need to look at that. For QD=1, I think the only sensible values for
> > max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> > 
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtime: 105.126 107.125 105.641
> > >
> > >So about the same as before. I'll try to debug this later today...
> > 
> > Thanks, I'm very interested in what you find!
> 
> OK, so the reason was relatively standard in the end. I was using ext3 (or
> more exactly ext4 without delayed allocation) for the test. The throttling
> of background writes gave more priority to writes from the journalling
> thread which happen with WRITE_SYNC and thus are not throttled. Thus the
> journalling thread ended up having to do more data writeback to be able to
> commit a transaction (due to requirements of data=ordered mode) and it is
> less efficient at that than the normal flusher thread.
> 
> So this is an example where throttling background writeback effectively
> just pushes more work into another context which does it less efficiently
> and indirectly makes everyone wait for it. ext3 has been always sensitive to
> issues like this. ext4 is using delayed allocation and thus only data
> writes into holes end up being part of a transaction -> simple dd test case
> doesn't hit that path. And indeed when I repeat the same test with ext4,
> the numbers with and without your patch are exactly the same.
> 
> The question remains how common a pattern where throttling of background
> writeback delays also something else is. I'll schedule a couple of
> benchmarks to measure impact of your patches for a wider range of workloads
> (but sadly pretty limited set of hw). If ext3 is the only one seeing
> issues, I would be willing to accept that ext3 takes the hit since it is
> doing something rather stupid (but inherent in its journal design) and we
> have a way to deal with this either by enabling delayed allocation or by
> turning off the writeback throttling...

At least in the case of io that we know is going to be data=ordered, we
can bump the prio of those pages?

-chris

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#1393404

On Tue 03-05-16 08:40:11, Chris Mason wrote:
> On Tue, May 03, 2016 at 02:17:19PM +0200, Jan Kara wrote:
> > On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> > > >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> > > >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> > > >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> > > >>+	if (rwb->queue_depth == 1) {
> > > >>+		rwb->wb_max = rwb->wb_normal = 2;
> > > >>+		rwb->wb_background = 1;
> > > >
> > > >This breaks the detection of too big scale_step in scale_up() where we key
> > > >of wb_max == 1 value. However even with that fixed no luck :(:
> > > 
> > > Yeah, I need to look at that. For QD=1, I think the only sensible values for
> > > max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> > > 
> > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > >Runtime: 105.126 107.125 105.641
> > > >
> > > >So about the same as before. I'll try to debug this later today...
> > > 
> > > Thanks, I'm very interested in what you find!
> > 
> > OK, so the reason was relatively standard in the end. I was using ext3 (or
> > more exactly ext4 without delayed allocation) for the test. The throttling
> > of background writes gave more priority to writes from the journalling
> > thread which happen with WRITE_SYNC and thus are not throttled. Thus the
> > journalling thread ended up having to do more data writeback to be able to
> > commit a transaction (due to requirements of data=ordered mode) and it is
> > less efficient at that than the normal flusher thread.
> > 
> > So this is an example where throttling background writeback effectively
> > just pushes more work into another context which does it less efficiently
> > and indirectly makes everyone wait for it. ext3 has been always sensitive to
> > issues like this. ext4 is using delayed allocation and thus only data
> > writes into holes end up being part of a transaction -> simple dd test case
> > doesn't hit that path. And indeed when I repeat the same test with ext4,
> > the numbers with and without your patch are exactly the same.
> > 
> > The question remains how common a pattern where throttling of background
> > writeback delays also something else is. I'll schedule a couple of
> > benchmarks to measure impact of your patches for a wider range of workloads
> > (but sadly pretty limited set of hw). If ext3 is the only one seeing
> > issues, I would be willing to accept that ext3 takes the hit since it is
> > doing something rather stupid (but inherent in its journal design) and we
> > have a way to deal with this either by enabling delayed allocation or by
> > turning off the writeback throttling...
> 
> At least in the case of io that we know is going to be data=ordered, we
> can bump the prio of those pages?

But how would flusher thread, which is submitting IO, know that? We would
have to somehow mark inodes that are part of the running transaction and
flusher thread could give more priority to such writeback - e.g. by using
WRITE_SYNC or at least plain writes. Hmm, if we use an inode flag for that,
it could be doable.

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

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#1393429

On Tue, May 03, 2016 at 03:06:09PM +0200, Jan Kara wrote:
> On Tue 03-05-16 08:40:11, Chris Mason wrote:
> > On Tue, May 03, 2016 at 02:17:19PM +0200, Jan Kara wrote:
> > > On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> > > > >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> > > > >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> > > > >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> > > > >>+	if (rwb->queue_depth == 1) {
> > > > >>+		rwb->wb_max = rwb->wb_normal = 2;
> > > > >>+		rwb->wb_background = 1;
> > > > >
> > > > >This breaks the detection of too big scale_step in scale_up() where we key
> > > > >of wb_max == 1 value. However even with that fixed no luck :(:
> > > > 
> > > > Yeah, I need to look at that. For QD=1, I think the only sensible values for
> > > > max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> > > > 
> > > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > > >Runtime: 105.126 107.125 105.641
> > > > >
> > > > >So about the same as before. I'll try to debug this later today...
> > > > 
> > > > Thanks, I'm very interested in what you find!
> > > 
> > > OK, so the reason was relatively standard in the end. I was using ext3 (or
> > > more exactly ext4 without delayed allocation) for the test. The throttling
> > > of background writes gave more priority to writes from the journalling
> > > thread which happen with WRITE_SYNC and thus are not throttled. Thus the
> > > journalling thread ended up having to do more data writeback to be able to
> > > commit a transaction (due to requirements of data=ordered mode) and it is
> > > less efficient at that than the normal flusher thread.
> > > 
> > > So this is an example where throttling background writeback effectively
> > > just pushes more work into another context which does it less efficiently
> > > and indirectly makes everyone wait for it. ext3 has been always sensitive to
> > > issues like this. ext4 is using delayed allocation and thus only data
> > > writes into holes end up being part of a transaction -> simple dd test case
> > > doesn't hit that path. And indeed when I repeat the same test with ext4,
> > > the numbers with and without your patch are exactly the same.
> > > 
> > > The question remains how common a pattern where throttling of background
> > > writeback delays also something else is. I'll schedule a couple of
> > > benchmarks to measure impact of your patches for a wider range of workloads
> > > (but sadly pretty limited set of hw). If ext3 is the only one seeing
> > > issues, I would be willing to accept that ext3 takes the hit since it is
> > > doing something rather stupid (but inherent in its journal design) and we
> > > have a way to deal with this either by enabling delayed allocation or by
> > > turning off the writeback throttling...
> > 
> > At least in the case of io that we know is going to be data=ordered, we
> > can bump the prio of those pages?
> 
> But how would flusher thread, which is submitting IO, know that? We would
> have to somehow mark inodes that are part of the running transaction and
> flusher thread could give more priority to such writeback - e.g. by using
> WRITE_SYNC or at least plain writes. Hmm, if we use an inode flag for that,
> it could be doable.

This would be specific to the data=ordered code in the FS.  If there's
some way to test for an inode or a page's status in the data=ordered
list, the FS writepages call could flag the IO as higher prio?

-chris

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#1393433

On Tue 03-05-16 09:42:40, Chris Mason wrote:
> On Tue, May 03, 2016 at 03:06:09PM +0200, Jan Kara wrote:
> > On Tue 03-05-16 08:40:11, Chris Mason wrote:
> > > On Tue, May 03, 2016 at 02:17:19PM +0200, Jan Kara wrote:
> > > > On Thu 28-04-16 12:46:41, Jens Axboe wrote:
> > > > > >>-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> > > > > >>-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> > > > > >>-	rwb->wb_background = (rwb->wb_max + 3) / 4;
> > > > > >>+	if (rwb->queue_depth == 1) {
> > > > > >>+		rwb->wb_max = rwb->wb_normal = 2;
> > > > > >>+		rwb->wb_background = 1;
> > > > > >
> > > > > >This breaks the detection of too big scale_step in scale_up() where we key
> > > > > >of wb_max == 1 value. However even with that fixed no luck :(:
> > > > > 
> > > > > Yeah, I need to look at that. For QD=1, I think the only sensible values for
> > > > > max/normal/bg is 2/2/1 and 1/1/1 if we step down.
> > > > > 
> > > > > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > > > > >Runtime: 105.126 107.125 105.641
> > > > > >
> > > > > >So about the same as before. I'll try to debug this later today...
> > > > > 
> > > > > Thanks, I'm very interested in what you find!
> > > > 
> > > > OK, so the reason was relatively standard in the end. I was using ext3 (or
> > > > more exactly ext4 without delayed allocation) for the test. The throttling
> > > > of background writes gave more priority to writes from the journalling
> > > > thread which happen with WRITE_SYNC and thus are not throttled. Thus the
> > > > journalling thread ended up having to do more data writeback to be able to
> > > > commit a transaction (due to requirements of data=ordered mode) and it is
> > > > less efficient at that than the normal flusher thread.
> > > > 
> > > > So this is an example where throttling background writeback effectively
> > > > just pushes more work into another context which does it less efficiently
> > > > and indirectly makes everyone wait for it. ext3 has been always sensitive to
> > > > issues like this. ext4 is using delayed allocation and thus only data
> > > > writes into holes end up being part of a transaction -> simple dd test case
> > > > doesn't hit that path. And indeed when I repeat the same test with ext4,
> > > > the numbers with and without your patch are exactly the same.
> > > > 
> > > > The question remains how common a pattern where throttling of background
> > > > writeback delays also something else is. I'll schedule a couple of
> > > > benchmarks to measure impact of your patches for a wider range of workloads
> > > > (but sadly pretty limited set of hw). If ext3 is the only one seeing
> > > > issues, I would be willing to accept that ext3 takes the hit since it is
> > > > doing something rather stupid (but inherent in its journal design) and we
> > > > have a way to deal with this either by enabling delayed allocation or by
> > > > turning off the writeback throttling...
> > > 
> > > At least in the case of io that we know is going to be data=ordered, we
> > > can bump the prio of those pages?
> > 
> > But how would flusher thread, which is submitting IO, know that? We would
> > have to somehow mark inodes that are part of the running transaction and
> > flusher thread could give more priority to such writeback - e.g. by using
> > WRITE_SYNC or at least plain writes. Hmm, if we use an inode flag for that,
> > it could be doable.
> 
> This would be specific to the data=ordered code in the FS.  If there's
> some way to test for an inode or a page's status in the data=ordered
> list, the FS writepages call could flag the IO as higher prio?

Oh, right, we could do that. I can experiment with that later.

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

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#1399340

On Tue 03-05-16 14:17:19, Jan Kara wrote:
> The question remains how common a pattern where throttling of background
> writeback delays also something else is. I'll schedule a couple of
> benchmarks to measure impact of your patches for a wider range of workloads
> (but sadly pretty limited set of hw). If ext3 is the only one seeing
> issues, I would be willing to accept that ext3 takes the hit since it is
> doing something rather stupid (but inherent in its journal design) and we
> have a way to deal with this either by enabling delayed allocation or by
> turning off the writeback throttling...

So I've run some benchmarks on a machine with 6 GB of RAM and SSD with
queue depth 32. The filesystem on the disk was XFS this time. I've found
couple of regressions. A clear one is with dbench (version 4). The average
throughput numbers look like:

			Baseline		WBT
Hmean    mb/sec-1         30.26 (  0.00%)       18.67 (-38.28%)
Hmean    mb/sec-2         40.71 (  0.00%)       31.25 (-23.23%)
Hmean    mb/sec-4         52.67 (  0.00%)       46.83 (-11.09%)
Hmean    mb/sec-8         69.51 (  0.00%)       64.35 ( -7.42%)
Hmean    mb/sec-16        91.07 (  0.00%)       86.46 ( -5.07%)
Hmean    mb/sec-32       115.10 (  0.00%)      110.29 ( -4.18%)
Hmean    mb/sec-64       145.14 (  0.00%)      134.97 ( -7.00%)
Hmean    mb/sec-512       93.99 (  0.00%)      133.85 ( 42.41%)

There were also some losses in a filebench webproxy workload (I can give
you exact details of the settings if you want to reproduce it).

Also, and this really puzzles me, I've seen higher read latencies in some
cases (I've verified they are not just noise by rerunning the test for
kernel with writeback throttling patches). For example with the following
fio job file:

[global]
direct=0
ioengine=sync
runtime=300
time_based
invalidate=1
blocksize=4096
size=10g        # Just random value, we are running time based workload
log_avg_msec=10
group_reporting=1

[writer]
nrfiles=1
filesize=1g
fdatasync=256
readwrite=randwrite
numjobs=4

[reader]
# Simulate random reading from different files, switching to different file
# after 16 ios. This somewhat simulates application startup.
new_group
filesize=100m
nrfiles=20
file_service_type=random:16
readwrite=randread

I get the following results:

Throughput			Baseline		WBT
Hmean    kb/sec-writer-write      591.60 (  0.00%)      507.00 (-14.30%)
Hmean    kb/sec-reader-read       211.81 (  0.00%)      137.53 (-35.07%)

So both read and write throughput have suffered. And latencies don't offset
for the loss either:

FIO read latency
Min         latency-read     1383.00 (  0.00%)     1519.00 ( -9.83%)
1st-qrtle   latency-read     3485.00 (  0.00%)     5235.00 (-50.22%)
2nd-qrtle   latency-read     4708.00 (  0.00%)    15028.00 (-219.20%)
3rd-qrtle   latency-read    10286.00 (  0.00%)    57622.00 (-460.20%)
Max-90%     latency-read   195834.00 (  0.00%)   167149.00 ( 14.65%)
Max-93%     latency-read   273145.00 (  0.00%)   200319.00 ( 26.66%)
Max-95%     latency-read   335434.00 (  0.00%)   220695.00 ( 34.21%)
Max-99%     latency-read   537017.00 (  0.00%)   347174.00 ( 35.35%)
Max         latency-read   991101.00 (  0.00%)   485835.00 ( 50.98%)
Mean        latency-read    51282.79 (  0.00%)    49953.95 (  2.59%)

So we have reduced the extra high read latencies which is nice but on
average there is no change.

And another fio jobfile which doesn't look great:

[global]
direct=0
ioengine=sync
runtime=300
blocksize=4096
invalidate=1
time_based
ramp_time=5     # Let the flusher thread start before taking measurements
log_avg_msec=10
group_reporting=1

[writer]
nrfiles=1
filesize=$((MEMTOTAL_BYTES*2))
readwrite=randwrite

[reader]
# Simulate random reading from different files, switching to different file
# after 16 ios. This somewhat simulates application startup.
new_group
filesize=100m
nrfiles=20
file_service_type=random:16
readwrite=randread

The throughput numbers look like:
Hmean    kb/sec-writer-write    24707.22 (  0.00%)    19912.23 (-19.41%)
Hmean    kb/sec-reader-read       886.65 (  0.00%)      905.71 (  2.15%)

So we've got significant hit in writes not really offset by a big increase
in reads. Read latency numbers look like (I show the WBT numbers for two runs
just so that one can see how variable the latency numbers are because I was
puzzled by very high max latency for WBT kernels - quartiles seem rather
stable higher percentiles and min/max are rather variable):

			   Baseline		WBT			WBT
Min         latency-read     1230.00 (  0.00%)     1560.00 (-26.83%)	1100.00 ( 10.57%)
1st-qrtle   latency-read     3357.00 (  0.00%)     3351.00 (  0.18%)	3351.00 (  0.18%)
2nd-qrtle   latency-read     4074.00 (  0.00%)     4056.00 (  0.44%)	4022.00 (  1.28%)
3rd-qrtle   latency-read     5198.00 (  0.00%)     5145.00 (  1.02%)	5095.00 (  1.98%)
Max-90%     latency-read     6594.00 (  0.00%)     6370.00 (  3.40%)	6130.00 (  7.04%)
Max-93%     latency-read    11251.00 (  0.00%)     9410.00 ( 16.36%)	6654.00 ( 40.86%)
Max-95%     latency-read    14769.00 (  0.00%)    13231.00 ( 10.41%)	10306.00 ( 30.22%)
Max-99%     latency-read    27826.00 (  0.00%)    28728.00 ( -3.24%)	25077.00 (  9.88%)
Max         latency-read    80202.00 (  0.00%)   186491.00 (-132.53%)	141346.00 (-76.24%)
Mean        latency-read     5356.12 (  0.00%)     5229.00 (  2.37%)	4927.23 (  8.01%)

I have run also other tests but they have mostly shown no significant
difference.

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

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