Re: [RFC PATCH] mm: drop mark_page_access from the unmap path

[Michal Hocko]

On Tue 13-08-19 12:51:43, Michal Hocko wrote:
> On Mon 12-08-19 11:07:25, Johannes Weiner wrote:
> > On Mon, Aug 12, 2019 at 10:09:47AM +0200, Michal Hocko wrote:
[...]
> > > > Maybe the refaults will be fine - but latency expectations around
> > > > mapped page cache certainly are a lot higher than unmapped cache.
> > > >
> > > > So I'm a bit reluctant about this patch. If Minchan can be happy with
> > > > the lock batching, I'd prefer that.
> > > 
> > > Yes, it seems that the regular lock drop&relock helps in Minchan's case
> > > but this is a kind of change that might have other subtle side effects.
> > > E.g. will-it-scale has noticed a regression [1], likely because the
> > > critical section is shorter and the overal throughput of the operation
> > > decreases. Now, the w-i-s is an artificial benchmark so I wouldn't lose
> > > much sleep over it normally but we have already seen real regressions
> > > when the locking pattern has changed in the past so I would by a bit
> > > cautious.
> > 
> > I'm much more concerned about fundamentally changing the aging policy
> > of mapped page cache then about the lock breaking scheme. With locking
> > we worry about CPU effects; with aging we worry about additional IO.
> 
> But the later is observable and debuggable little bit easier IMHO.
> People are quite used to watch for major faults from my experience
> as that is an easy metric to compare.
>  
> > > As I've said, this RFC is mostly to open a discussion. I would really
> > > like to weigh the overhead of mark_page_accessed and potential scenario
> > > when refaults would be visible in practice. I can imagine that a short
> > > lived statically linked applications have higher chance of being the
> > > only user unlike libraries which are often being mapped via several
> > > ptes. But the main problem to evaluate this is that there are many other
> > > external factors to trigger the worst case.
> > 
> > We can discuss the pros and cons, but ultimately we simply need to
> > test it against real workloads to see if changing the promotion rules
> > regresses the amount of paging we do in practice.
> 
> Agreed. Do you see other option than to try it out and revert if we see
> regressions? We would get a workload description which would be helpful
> for future regression testing when touching this area. We can start
> slower and keep it in linux-next for a release cycle to catch any
> fallouts early.
> 
> Thoughts?

ping...

-- 
Michal Hocko
SUSE Labs