Re: [PATCH] mm, memcg: cg2 memory{.swap,}.peak write handlers

[Michal Hocko <mhocko@suse.com>]

On Tue 16-07-24 08:47:59, David Finkel wrote:
> On Tue, Jul 16, 2024 at 3:20 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Mon 15-07-24 16:46:36, David Finkel wrote:
> > > > On Mon, Jul 15, 2024 at 4:38 PM David Finkel <davidf@vimeo.com> wrote:
> > > > >
> > > > > Other mechanisms for querying the peak memory usage of either a process
> > > > > or v1 memory cgroup allow for resetting the high watermark. Restore
> > > > > parity with those mechanisms.
> > > > >
> > > > > For example:
> > > > >  - Any write to memory.max_usage_in_bytes in a cgroup v1 mount resets
> > > > >    the high watermark.
> > > > >  - writing "5" to the clear_refs pseudo-file in a processes's proc
> > > > >    directory resets the peak RSS.
> > > > >
> > > > > This change copies the cgroup v1 behavior so any write to the
> > > > > memory.peak and memory.swap.peak pseudo-files reset the high watermark
> > > > > to the current usage.
> > > > >
> > > > > This behavior is particularly useful for work scheduling systems that
> > > > > need to track memory usage of worker processes/cgroups per-work-item.
> > > > > Since memory can't be squeezed like CPU can (the OOM-killer has
> > > > > opinions),
> >
> > I do not understand the OOM-killer reference here. Why does it matter?
> > Could you explain please?
> 
> Sure, we're attempting to bin-packing work based on past items of the same type.
> With CPU, we can provision for the mean CPU-time per-wall-time to get
> a lose "cores"
> concept that we use for binpacking. With CPU, if we end up with a bit
> of contention,
> everything just gets a bit slower while the schedule arbitrates among cgroups.
> 
> However, with memory, you only have so much physical memory for the outer memcg.
> If we pack things too tightly on memory, the OOM-killer is going to kill
> something to free up memory. In some cases that's fine, but provisioning for the
> peak memory for that "type" of work-item mostly avoids this issue.

It is still not clear to me how the memory reclaim falls into that. Are
your workloads mostly unreclaimable (e.g. anon mostly consumers without
any swap)? Why I am asking? Well, if the workload's memory is
reclaimable then the peak memory consumption is largely misleading
because an unknown portion of that memory consumption is hidden by the
reclaimed portion of it. This is not really specific to the write
handlers to reset the value though so I do not want to digress this
patch too much. I do not have objections to the patch itself. Clarifying
the usecase with your followup here would be nice.

Thanks for the clarification!
-- 
Michal Hocko
SUSE Labs