On Wed, Apr 27 2022 at 12:57P -0400, Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> wrote: > The precision loss of reading IO start_time with jiffies_to_nsecs > instead of using a high resolution timer degrades HST path prediction > for BIO-based mpath on high load workloads. > > Below, I show the utilization percentage of a 10 disk multipath with > asymmetrical disk access cost, while being exercised by a randwrite FIO > benchmark with high submission queue depth (depth=64). It is possible > to see that the HST path selection degrades heavily for high-iops in > BIO-mpath, underutilizing the slower paths way beyond expected. This > seems to be caused by the start_time truncation, which makes some IO to > seem much slower than they actually is. In this scenario ST outperforms > HST for bio-mpath, but not for mq-mpath, which already uses ktime_get_ns(). > > The third column shows utilization with this patch applied. It is easy > to see that now HST prediction is much closer to the ideal distribution > (calculated considering the real cost of each path). > > | | ST | HST (orig) | HST(ktime) | Best | > | sdd | 0.17 | 0.20 | 0.17 | 0.18 | > | sde | 0.17 | 0.20 | 0.17 | 0.18 | > | sdf | 0.17 | 0.20 | 0.17 | 0.18 | > | sdg | 0.06 | 0.00 | 0.06 | 0.04 | > | sdh | 0.03 | 0.00 | 0.03 | 0.02 | > | sdi | 0.03 | 0.00 | 0.03 | 0.02 | > | sdj | 0.02 | 0.00 | 0.01 | 0.01 | > | sdk | 0.02 | 0.00 | 0.01 | 0.01 | > | sdl | 0.17 | 0.20 | 0.17 | 0.18 | > | sdm | 0.17 | 0.20 | 0.17 | 0.18 | > > This issue was originally discussed [1] when we first merged HST, and > this patch was left as a low hanging fruit to be solved later. I don't > think anyone is using HST with BIO mpath, but it'd be neat to get it > sorted out. > > Regarding the implementation, as suggested by Mike in that mail thread, > in order to avoid the overhead of ktime_get_ns for other selectors, this > patch adds a flag for the selector code to request the high-resolution > timer. > > I tested this using the same benchmark used in the original HST submission. > > Full test and benchmark scripts are available here: > > https://people.collabora.com/~krisman/HST-BIO-MPATH/ > > [1] https://lore.kernel.org/lkml/85tv0am9de.fsf@xxxxxxxxxxxxx/T/ > > Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> > Acked-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx> Overall your code was OK, but I nudged it a bit further to be inkeeping with how 'features' flags have been implemented elsewhere (e.g. dm_target_type's features) -- by using a healer to test the flag, etc. I also tweaked some other small implementation details. Please see: https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.19&id=c06dfd124d46df9c482fbd1319b5fe19bcb1a110 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
