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
