Re: [RFC PROPOSAL] memcg: per-memcg user space reclaim interface

[Roman Gushchin <guro@fb.com>]

On Fri, Jul 03, 2020 at 09:27:19AM -0700, Shakeel Butt wrote:
> On Fri, Jul 3, 2020 at 8:50 AM Roman Gushchin <guro@fb.com> wrote:
> >
> > On Fri, Jul 03, 2020 at 07:23:14AM -0700, Shakeel Butt wrote:
> > > On Thu, Jul 2, 2020 at 11:35 PM Michal Hocko <mhocko@kernel.org> wrote:
> > > >
> > > > On Thu 02-07-20 08:22:22, Shakeel Butt wrote:
> > > > [...]
> > > > > Interface options:
> > > > > ------------------
> > > > >
> > > > > 1) memcg interface e.g. 'echo 10M > memory.reclaim'
> > > > >
> > > > > + simple
> > > > > + can be extended to target specific type of memory (anon, file, kmem).
> > > > > - most probably restricted to cgroup v2.
> > > > >
> > > > > 2) fadvise(PAGEOUT) on cgroup_dir_fd
> > > > >
> > > > > + more general and applicable to other FSes (actually we are using
> > > > > something similar for tmpfs).
> > > > > + can be extended in future to just age the LRUs instead of reclaim or
> > > > > some new use cases.
> > > >
> > > > Could you explain why memory.high as an interface to trigger pro-active
> > > > memory reclaim is not sufficient. Also memory.low limit to protect
> > > > latency sensitve workloads?
> >
> > I initially liked the proposal, but after some thoughts I've realized
> > that I don't know a good use case where memory.high is less useful.
> > Shakeel, what's the typical use case you thinking of?
> > Who and how will use the new interface?
> >
> > >
> > > Yes, we can use memory.high to trigger [proactive] reclaim in a memcg
> > > but note that it can also introduce stalls in the application running
> > > in that memcg. Let's suppose the memory.current of a memcg is 100MiB
> > > and we want to reclaim 20MiB from it, we can set the memory.high to
> > > 80MiB but any allocation attempt from the application running in that
> > > memcg can get stalled/throttled. I want the functionality of the
> > > reclaim without potential stalls.
> >
> > But reclaiming some pagecache/swapping out anon pages can always
> > generate some stalls caused by pagefaults, no?
> >
> 
> Thanks for looking into the proposal. Let me answer both of your
> questions together. I have added the two use-cases but let me explain
> the proactive reclaim a bit more as we actually use that in our
> production.
> 
> We have defined tolerable refault rates for the applications based on
> their type (latency sensitive or not). Proactive reclaim is triggered
> in the application based on their current refault rates and usage. If
> the current refault rate exceeds the tolerable refault rate then
> stop/slowdown the proactive reclaim.
> 
> For the second question, yes, each individual refault can induce the
> stall as well but we have more control on that stall as compared to
> stalls due to reclaim. For us almost all the reclaimable memory is
> anon and we use compression based swap, so, the cost of each refault
> is fixed and a couple of microseconds.
> 
> I think the next question is what about the refaults from disk or
> source with highly variable cost. Usually the latency sensitive
> applications remove such uncertainty by mlocking the pages backed by
> such backends (e.g. mlocking the executable) or at least that is the
> case for us.

Got it.

It feels like you're suggesting something similar to memory.high with
something similar to a different gfp flags. In other words, the
difference is only which pages can be reclaimed and which not. I don't
have a definitive answer here, but I wonder if we can somehow
generalize the existing interface? E.g. if the problem is with artificially
induced delays, we can have a config option/sysctl/sysfs knob/something else
which would disable it. Otherwise we risk ending up with many different kinds
of soft memory limits.

Thanks!