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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
