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

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Date:	Wed, 2 Dec 2015 02:11:18 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Hugh Dickins <hughd@...gle.com>,
	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 Mon, 16 Nov 2015, Johannes Weiner wrote:

> Dear Hugh,
> 
> [ sorry, I just noticed this email now ]

No problem: as you can see, I'm very far from keeping up myself.

> 
> 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@...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?
> 
> 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.

Never mind: I thought it worth calling out at the time, but now that
your change is in, I've no reason to make a fuss over it - if it ever
bites us again, we can rework it then.

Besides, when I wrote, I hadn't noticed the comment there about
SWAP_CLUSTER_MAX, just out of sight before the context of your fix:
that would have to be reworded, and I don't know what it should say.

Oh, it's all still mmotm stuff, not in 4.4-rc.  Well, even so,
forget my interjection: we've all got bigger fish to fry.

Hugh
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