[dm-devel] dm: dm-mpath: Provide high-resolution timer to HST with bio-mpath

[Mike Snitzer]

On Mon, May 09 2022 at  3:47P -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Wed, Apr 27 2022 at 12:57P -0400,
> Gabriel Krisman Bertazi <krisman at collabora.com> 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@collabora.com/T/
> > 
> > Signed-off-by: Gabriel Krisman Bertazi <krisman at collabora.com>
> > Acked-by: Gabriel Krisman Bertazi <krisman at collabora.com>
> 
> 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.

s/healer/helper/ ;)