Re: [PATCH] mm: vmpressure: fix scan window after SWAP_CLUSTER_MAX increase

[Johannes Weiner <hannes@cmpxchg.org>]

Dear Hugh,

[ sorry, I just noticed this email now ]

On Wed, Oct 21, 2015 at 01:05:53PM -0700, Hugh Dickins wrote:
> On Wed, 21 Oct 2015, Johannes Weiner wrote:
> > On Mon, Oct 19, 2015 at 02:13:01PM -0400, Johannes Weiner wrote:
> > > mm-increase-swap_cluster_max-to-batch-tlb-flushes.patch changed
> > > SWAP_CLUSTER_MAX from 32 pages to 256 pages, inadvertantly switching
> > > the scan window for vmpressure detection from 2MB to 16MB. Revert.
> > > 
> > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > > ---
> > >  mm/vmpressure.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> > > index c5afd57..74f206b 100644
> > > --- a/mm/vmpressure.c
> > > +++ b/mm/vmpressure.c
> > > @@ -38,7 +38,7 @@
> > >   * TODO: Make the window size depend on machine size, as we do for vmstat
> > >   * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
> > >   */
> > > -static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
> > > +static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX;
> > 
> > Argh, Mel's patch sets SWAP_CLUSTER_MAX to 256, so this should be
> > SWAP_CLUSTER_MAX * 2 to retain the 512 pages scan window.
> > 
> > Andrew could you please update this fix in-place? Otherwise I'll
> > resend a corrected version.
> > 
> > Thanks, and sorry about that.
> 
> I don't understand why "SWAP_CLUSTER_MAX * 2" is thought better than "512".
> Retaining a level of obscurity, that just bit us twice, is a good thing?

I'm not sure it is. But it doesn't seem entirely wrong to link it to
the reclaim scan window, either--at least be a multiple of it so that
the vmpressure reporting happens cleanly at the end of a scan cycle?

I don't mind changing it to 512, but it doesn't feel like an obvious
improvement, either.

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