Re: [RFC] Mechanism to induce memory reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 10, 2022 at 09:33:48AM -0800, Wei Xu wrote:
> On Thu, Mar 10, 2022 at 8:58 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Wed, Mar 09, 2022 at 02:03:21PM -0800, David Rientjes wrote:
> > > On Tue, 8 Mar 2022, Michal Hocko wrote:
> > >
> > > > > Let me take a stab at this. The specific reasons why high limit is not a
> > > > > good interface to implement proactive reclaim:
> > > > >
> > > > > 1) It can cause allocations from the target application to get
> > > > > throttled.
> > > > >
> > > > > 2) It leaves a state (high limit) in the kernel which needs to be reset
> > > > > by the userspace part of proactive reclaimer.
> > > > >
> > > > > If I remember correctly, Facebook actually tried to use high limit to
> > > > > implement the proactive reclaim but due to exactly these limitations [1]
> > > > > they went the route [2] aligned with this proposal.
> > > >
> > > > I do remember we have discussed this in the past. There were proposals
> > > > for an additional limit to trigger a background reclaim [3] or to add a
> > > > pressure based memcg knob [4]. For the nr_to_reclaim based interface
> > > > there were some challenges outlined in that email thread. I do
> > > > understand that practical experience could have confirmed or diminished
> > > > those concerns.
> > > >
> > > > I am definitely happy to restart those discussion but it would be really
> > > > great to summarize existing options and why they do not work in
> > > > practice. It would be also great to mention why concerns about nr_to_reclaim
> > > > based interface expressed in the past are not standing out anymore wrt.
> > > > other proposals.
> > > >
> > >
> > > Johannes, since you had pointed out that the current approach used at Meta
> > > and described in the TMO paper works well in practice and is based on
> > > prior discussions of memory.reclaim[1], do you have any lingering concerns
> > > from that 2020 thread?
> >
> > I'd be okay with merging the interface proposed in that thread as-is.
> 
> We will need a nodemask argument for the memory tiering use case. We
> can add it as an optional argument to memory.reclaim later.  Or do you
> think we should add a different interface (e.g. memory.demote) for
> memory tiering instead?

Yes, good point. We can add an optional parameter later on, methinks,
as the behavior for when it's omitted shouldn't change.