Re: [PATCH] mm: vmscan: consistent update to pgsteal and pgscan

[Shakeel Butt <shakeelb@google.com>]

On Fri, May 8, 2020 at 6:38 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Fri, May 08, 2020 at 06:25:14AM -0700, Shakeel Butt wrote:
> > On Fri, May 8, 2020 at 3:34 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> > >
> > > On Fri, May 8, 2020 at 4:49 AM Shakeel Butt <shakeelb@google.com> wrote:
> > > >
> > > > One way to measure the efficiency of memory reclaim is to look at the
> > > > ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are
> > > > not updated consistently at the system level and the ratio of these are
> > > > not very meaningful. The pgsteal and pgscan are updated for only global
> > > > reclaim while pgrefill gets updated for global as well as cgroup
> > > > reclaim.
> > > >
> > >
> > > Hi Shakeel,
> > >
> > > We always use pgscan and pgsteal for monitoring the system level
> > > memory pressure, for example, by using sysstat(sar) or some other
> > > monitor tools.
>
> I'm in the same boat. It's useful to have activity that happens purely
> due to machine capacity rather than localized activity that happens
> due to the limits throughout the cgroup tree.
>
> > Don't you need pgrefill in addition to pgscan and pgsteal to get the
> > full picture of the reclaim activity?
>
> I actually almost never look at pgrefill.
>

Nowadays we are looking at reclaim cost on high utilization
machines/devices and noticed that rmap walk takes more than 60/70% of
the CPU cost of the reclaim. Kernel does rmap walks in
shrink_active_list and shrink_page_list and pgscan and pgrefill are
good approximations of the number of rmap walks during a reclaim.

> > > But with this change, these two counters include the memcg pressure as
> > > well. It is not easy to know whether the pgscan and pgsteal are caused
> > > by system level pressure or only some specific memcgs reaching their
> > > memory limit.
> > >
> > > How about adding  cgroup_reclaim() to pgrefill as well ?
> > >
> >
> > I am looking for all the reclaim activity on the system. Adding
> > !cgroup_reclaim to pgrefill will skip the cgroup reclaim activity.
> > Maybe adding pgsteal_cgroup and pgscan_cgroup would be better.
>
> How would you feel about adding memory.stat at the root cgroup level?
>

Actually I would prefer adding memory.stat at the root cgroup level as
you noted below that more use-cases would benefit from it.

> There are subtle differences between /proc/vmstat and memory.stat, and
> cgroup-aware code that wants to watch the full hierarchy currently has
> to know about these intricacies and translate semantics back and forth.
>
> Generally having the fully recursive memory.stat at the root level
> could help a broader range of usecases.

Thanks for the feedback. I will send the patch with the additional motivation.