Re: [Lsf-pc] [LSF/MM TOPIC] Congestion

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On Tue 07-01-20 10:21:00, Dave Chinner wrote:
> On Mon, Jan 06, 2020 at 12:55:14PM +0100, Michal Hocko wrote:
> > On Tue 31-12-19 04:59:08, Matthew Wilcox wrote:
> > > 
> > > I don't want to present this topic; I merely noticed the problem.
> > > I nominate Jens Axboe and Michael Hocko as session leaders.  See the
> > > thread here:
> > 
> > Thanks for bringing this up Matthew! The change in the behavior came as
> > a surprise to me. I can lead the session for the MM side.
> > 
> > > https://lore.kernel.org/linux-mm/20190923111900.GH15392@xxxxxxxxxxxxxxxxxxxxxx/
> > > 
> > > Summary: Congestion is broken and has been for years, and everybody's
> > > system is sleeping waiting for congestion that will never clear.
> > > 
> > > A good outcome for this meeting would be:
> > > 
> > >  - MM defines what information they want from the block stack.
> > 
> > The history of the congestion waiting is kinda hairy but I will try to
> > summarize expectations we used to have and we can discuss how much of
> > that has been real and what followed up as a cargo cult. Maybe we just
> > find out that we do not need functionality like that anymore. I believe
> > Mel would be a great contributor to the discussion.
> 
> We most definitely do need some form of reclaim throttling based on
> IO congestion, because it is trivial to drive the system into swap
> storms and OOM killer invocation when there are large dirty slab
> caches that require IO to make reclaim progress and there's little
> in the way of page cache to reclaim.

Just to clarify. I do agree that we need some form of throttling. Sorry
if my wording was confusing. What I meant is that I am not sure whether
wait_iff_congested as it is implemented now is the right way. We
definitely have to slow/block the reclaim when there is a lot of dirty
(meta)data. How to do that is a good topic to discuss.

[skipping the rest of the email which has many good points]

-- 
Michal Hocko
SUSE Labs




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