Re: [RFC PATCH v2] blk-mq: Introduce per sw queue time-slice

[Markus Trippelsdorf <markus@trippelsdorf.de>]

On 2016.02.09 at 18:12 +0100, Andreas Herrmann wrote:
> [CC-ing linux-block and linux-scsi and adding some comments]
> 
> On Mon, Feb 01, 2016 at 11:43:40PM +0100, Andreas Herrmann wrote:
> > This introduces a new blk_mq hw attribute time_slice_us which allows
> > to specify a time slice in usecs.
> > 
> > Fio test results are sent in a separate mail to this.
> 
> See http://marc.info/?l=linux-kernel&m=145436682607949&w=2
> 
> In short it shows significant performance gains in some tests,
> e.g. sequential read iops up by >40% with 8 jobs. But it's never on
> par with CFQ when more than 1 job was used during the test.
> 
> > Results for fio improved to some extent with this patch. But in
> > reality the picture is quite mixed. Performance is highly dependend on
> > task scheduling. There is no guarantee that the requests originated
> > from one CPU belong to the same process.
> > 
> > I think for rotary devices CFQ is by far the best choice. A simple
> > illustration is:
> > 
> >   Copying two files (750MB in this case) in parallel on a rotary
> >   device. The elapsed wall clock time (seconds) for this is
> >                                mean    stdev
> >    cfq, slice_idle=8           16.18   4.95
> >    cfq, slice_idle=0           23.74   2.82
> >    blk-mq, time_slice_usec=0   24.37   2.05
> >    blk-mq, time_slice_usec=250 25.58   3.16
> 
> This illustrates that although their was performance gain with fio
> tests, the patch can cause higher variance and lower performance in
> comparison to unmodified blk-mq with other tests. And it underscores
> superiority of CFQ for rotary disks.
> 
> Meanwhile my opinion is that it's not really worth to look further
> into introduction of I/O scheduling support in blk-mq. I don't see the
> need for scheduling support (deadline or something else) for fast
> storage devices. And rotary devices should really avoid usage of blk-mq
> and stick to CFQ.
> 
> Thus I think that introducing some coexistence of blk-mq and the
> legacy block with CFQ is the best option.
> 
> Recently Johannes sent a patch to enable scsi-mq per driver, see
> http://marc.info/?l=linux-scsi&m=145347009631192&w=2
> 
> Probably that is a good solution (at least in the short term) to allow
> users to switch to blk-mq for some host adapters (with fast storage
> attached) but to stick to legacy stuff on other host adapters with
> rotary devices.

I don't think that Johannes' patch is a good solution.

The best solution for the user would be if blk-mq could be toggled per
drive (or even automatically enabled if queue/rotational == 0). Is there
a fundamental reason why this is not feasible?

Your solution is better than nothing, but it requires that the user
finds out the drive <=> host mapping by hand and then runs something
like: 
echo "250" > /sys/devices/pci0000:00/0000:00:11.0/ata2/host1/target1:0:0/1:0:0:0/block/sdb/mq/0/time_slice_us
during boot for spinning rust drives...

-- 
Markus