Re: [PATCH] Revert "mm:vmscan: fix inaccurate reclaim during proactive reclaim"

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Jan 30, 2024 at 12:58:12PM -0800, T.J. Mercier wrote:
> On Fri, Jan 26, 2024 at 8:41 AM T.J. Mercier <tjmercier@google.com> wrote:
> >
> > On Fri, Jan 26, 2024 at 8:34 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Wed, Jan 24, 2024 at 09:46:23AM -0800, T.J. Mercier wrote:
> > > > In the meantime, instead of a revert how about changing the batch size
> > > > geometrically instead of the SWAP_CLUSTER_MAX constant:
> > > >
> > > >                 reclaimed = try_to_free_mem_cgroup_pages(memcg,
> > > > -                                       min(nr_to_reclaim -
> > > > nr_reclaimed, SWAP_CLUSTER_MAX),
> > > > +                                       (nr_to_reclaim - nr_reclaimed)/2,
> > > >                                         GFP_KERNEL, reclaim_options);
> > > >
> > > > I think that should address the overreclaim concern (it was mentioned
> > > > that the upper bound of overreclaim was 2 * request), and this should
> > > > also increase the reclaim rate for root reclaim with MGLRU closer to
> > > > what it was before.
> > >
> > > Hahaha. Would /4 work for you?
> > >
> > > I genuinely think the idea is worth a shot. /4 would give us a bit
> > > more margin for error, since the bailout/fairness cutoffs have changed
> > > back and forth over time. And it should still give you a reasonable
> > > convergence on MGLRU.
> > >
> > > try_to_free_reclaim_pages() already does max(nr_to_reclaim,
> > > SWAP_CLUSTER_MAX) which will avoid the painful final approach loops
> > > the integer division would produce on its own.
> > >
> > > Please add a comment mentioning the compromise between the two reclaim
> > > implementations though.
> >
> > I'll try it out and get back to you. :)
> 
> Right, so (nr_to_reclaim - nr_reclaimed)/4 looks pretty good to me:
> 
> root - full reclaim       pages/sec   time (sec)
> pre-0388536ac291      :    68047        10.46
> post-0388536ac291     :    13742        inf
> (reclaim-reclaimed)/4 :    67352        10.51
> 
> /uid_0 - 1G reclaim       pages/sec   time (sec)  overreclaim (MiB)
> pre-0388536ac291      :    258822       1.12            107.8
> post-0388536ac291     :    105174       2.49            3.5
> (reclaim-reclaimed)/4 :    233396       1.12            -7.4
> 
> /uid_0 - full reclaim     pages/sec   time (sec)
> pre-0388536ac291      :    72334        7.09
> post-0388536ac291     :    38105        14.45
> (reclaim-reclaimed)/4 :    72914        6.96
> 
> So I'll put up a new patch.

That looks great, thanks for giving it a shot.

Looking forward to your patch.