Re: False waker detection in BFQ

[Jan Kara <jack@suse.cz>]

On Mon 23-08-21 18:06:18, Jan Kara wrote:
> On Mon 23-08-21 15:58:25, Paolo Valente wrote:
> > > Currently I'm running wider set of benchmarks for the patches to see
> > > whether I didn't regress anything else. If not, I'll post the patches to
> > > the list.
> > 
> > Any news?
> 
> It took a while for all those benchmarks to run. Overall results look sane,
> I'm just verifying by hand now whether some of the localized regressions
> (usually specific to a particular fs+machine config) are due to a measurement
> noise or real regressions...

OK, so after some manual analysis I've found out that dbench indeed becomes
more noisy with my changes for high numbers of processes. I'm leaving for
vacation soon so I will not be probably able to debug it before I leave but
let me ask you one thing: The problematic change seems to be mostly a
revert of 7cc4ffc55564 ("block, bfq: put reqs of waker and woken in
dispatch list") and I'm currently puzzled why it has such an effect. What
I've found out is that 7cc4ffc55564 results in IO of higher priority
process being injected into the time slice of lower priority process and
thus there's always only single busy queue (of the lower priority process)
and thus higher priority process queue never gets scheduled. As a result
higher priority IO always competes with lower priority IO and there's no
service differentiation (we get 50/50 split of throughput between the
processes despite different IO priorities).  And this scenario shows that
always injecting IO of waker/wakee isn't desirable, especially in a way as
done in 7cc4ffc55564 where injected IO isn't accounted within BFQ at all
(which easily allows for service degradation unnoticed by BFQ).  That's why
I've basically reverted that commit on the ground that on next dispatch we
call bfq_select_queue() which will see waker/wakee has IO to do and can
decide to inject the IO anyway. We do more CPU work but the IO pattern
should be similar. But apparently I was wrong :) I just wanted to bounce
this off of you if you have any suggestion what to look for or any tips
regarding why 7cc4ffc55564 apparently achieves much more reliable request
injection for dbench.
								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR