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

[Jens Axboe]

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