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

[Jens Axboe <axboe@fb.com>]

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.

-- 
Jens Axboe