On Tue, Feb 09, 2016 at 06:41:56PM +0100, Markus Trippelsdorf wrote: > 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. Why? Because it's not per device? > The best solution for the user would be if blk-mq could be toggled > per drive (or even automatically enabled if queue/rotational == 0). Yes, I aggree, but ... > Is there a fundamental reason why this is not feasible? ... it's not possible (*) with the current implementation. Tag handling/command allocation differs. Respective functions are set per host. (*) Or maybe it's possible but just hard to achieve and I didn't look long enough into relevant code to get an idea how to do it. > 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... Or it could automatically be set in case of rotational device. (Once we know for sure that it doesn't cause performance degradation.) > -- > Markus Andreas -- 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
