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

[Johannes Weiner]

On Mon, Aug 26, 2019 at 02:06:30PM +0200, Michal Hocko wrote:
> 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.

Rootcausing additional (re)faults is really difficult. We're talking
about a slight trend change in caching behavior in a sea of millions
of pages. There could be so many factors causing this, and for most
you have to patch debugging stuff into the kernel to rule them out.

A CPU regression you can figure out with perf.

> > > > 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...

Personally, I'm not convinced by this patch. I think it's a pretty
drastic change in aging heuristics just to address a CPU overhead
problem that has simpler, easier to verify, alternative solutions.

It WOULD be great to clarify and improve the aging model for mapped
cache, to make it a bit easier to reason about. But this patch does
not really get there either. Instead of taking a serious look at
mapped cache lifetime and usage scenarios, the changelog is more in
"let's see what breaks if we take out this screw here" territory.

So I'm afraid I don't think the patch & changelog in its current shape
should go upstream.