Re: Random high CPU utilization in blk-mq with the none scheduler

Ming Lei
2021-12-14T00:54:06Z

On Tue, Dec 14, 2021 at 12:31:23AM +0000, Dexuan Cui wrote:
> > From: Ming Lei <ming.lei@redhat.com>
> > Sent: Sunday, December 12, 2021 11:38 PM
>
> Ming, thanks so much for the detailed analysis!
>
> > From the log:
> >
> > 1) dm-mpath:
> > - queue depth: 2048
> > - busy: 848, and 62 of them are in sw queue, so run queue is often
> > caused
> > - nr_hw_queues: 1
> > - dm-2 is in use, and dm-1/dm-3 is idle
> > - dm-2's dispatch busy is 8, that should be the reason why excessive CPU
> > usage is observed when flushing plug list without commit dc5fc361d891 in
> > which hctx->dispatch_busy is just bypassed
> >
> > 2) iscsi
> > - dispatch_busy is 0
> > - nr_hw_queues: 1
> > - queue depth: 113
> > - busy=~33, active_queues is 3, so each LUN/iscsi host is saturated
> > - 23 active LUNs, 23 * 33 = 759 in-flight commands
> >
> > The high CPU utilization may be caused by:
> >
> > 1) big queue depth of dm mpath, the situation may be improved much if it
> > is reduced to 1024 or 800. The max allowed inflight commands from iscsi
> > hosts can be figured out, if dm's queue depth is much more than this number,
> > the extra commands need to dispatch, and run queue can be scheduled
> > immediately, so high CPU utilization is caused.
>
> I think you're correct:
> with dm_mod.dm_mq_queue_depth=256, the max CPU utilization is 8%.
> with dm_mod.dm_mq_queue_depth=400, the max CPU utilization is 12%.
> with dm_mod.dm_mq_queue_depth=800, the max CPU utilization is 88%.
>
> The performance with queue_depth=800 is poor.
> The performance with queue_depth=400 is good.
> The performance with queue_depth=256 is also good, and there is only a
> small drop comared with the 400 case.

That should be the reason why the issue isn't triggered in case of real
io scheduler.

So far blk-mq doesn't provide way to adjust tags queue depth
dynamically.

But not understand reason of default dm_mq_queue_depth(2048), in this
situation, each LUN can just queue 113/3 requests at most, and 3 LUNs
are attached to single iscsi host.

Mike, can you share why the default dm_mq_queue_depth is so big? And
seems it doesn't consider the underlying queue's queue depth. What is
the biggest dm rq queue depth? which need to saturate all underlying paths?

>
> > 2) single hw queue, so contention should be big, which should be avoided
> > in big machine, nvme-tcp might be better than iscsi here
> >
> > 3) iscsi io latency is a bit big
> >
> > Even CPU utilization is reduced by commit dc5fc361d891, io performance
> > can't be good too with v5.16-rc, I guess.
> >
> > Thanks,
> > Ming
>
> Actually the I/O performance of v5.16-rc4 (commit dc5fc361d891 is included)
> is good -- it's about the same as the case where v5.16-rc4 + reverting
> dc5fc361d891 + dm_mod.dm_mq_queue_depth=400 (or 256).

The single hw queue may be the root cause of your issue, and there
is only single run_work, which can be touched by all CPUs(~200) almost, so cache
ping-pong could be very serious.

Jens patch may improve it more or less, please test it.

Thanks,
Ming