Re: [LSF/MM/BPF] Improving MGLRU

[Kairui Song <ryncsn@gmail.com>]

On Fri, Feb 27, 2026 at 11:30 AM Barry Song <21cnbao@gmail.com> wrote:
>
> > 4. MGLRU's swappiness is kind of useless in some situations compared to
> >   Active / Inactive LRU, since its force protects the youngest two gen, so
> >   quite often we can only reclaim one type of folios. To workaround that, the
> >   user usually runs force aging before reclaim. So, can we just remove the
> >   force protection of the youngest two gens?
>
> I guess not—MGLRU needs at least two generations to function,
> similar to active and inactive lists, meaning it requires two lists.

Hi Barry,

You are right. But I think that doesn't mean we can't never reclaim
the folios in the oldest gen? Or maybe, just let the kernel itself
perform aging when one type of folios is not reclaimable.

We have an internal workaround for forces aging, and waits for sync
aging if one type of folios are not reclaimable (without the wait, we
still hit the MIN_NR_GEN protect again since aging is not finished).
And without the MIN_NR_GEN protection we might end up over reclaiming
without aging.

The problem with that is that the OOM killer became very slow to
trigger since aging is costly, so the system will hang for minutes
before OOM is triggered when it should get triggered immediately.

And for the OOM part I saw David Rientjes also mentioned the TTL
config in MGLRU, I do think TTL is a good idea, we just need to figure
out a good way to make better use of that.

I think a feasible solution might be (just idea): implement async
aging; decouple aging and reclaim, reclaim just keep shrinking
whatever is oldest; and optionally improve thrashing and OOM with TTL.