linux-kernel - Re: [PATCH] mm: vmpressure: fix scan window after SWAP_CLUSTER

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Date:	Wed, 21 Oct 2015 13:05:53 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmpressure: fix scan window after SWAP_CLUSTER_MAX
 increase

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@...xchg.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?

Hugh
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