linux-kernel - Re: [RFC][PATCH] mm: remove unnecessary loop inside shrink_inactive

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Message-ID: <20090821112228.GA6457@localhost>
Date:	Fri, 21 Aug 2009 19:22:28 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Dike, Jeffrey G" <jeffrey.g.dike@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"menage@...gle.com" <menage@...gle.com>
Subject: Re: [RFC][PATCH] mm: remove unnecessary loop inside
	shrink_inactive_list()

On Fri, Aug 21, 2009 at 07:09:17PM +0800, KOSAKI Motohiro wrote:
> 2009/8/20 Wu Fengguang <fengguang.wu@...el.com>:
> > shrink_inactive_list() won't be called to scan too much pages
> > (unless in hibernation code which is fine) or too few pages (ie.
> > batching is taken care of by the callers).  So we can just remove the
> > big loop and isolate the exact number of pages requested.
> >
> > Just a RFC, and a scratch patch to show the basic idea.
> > Please kindly NAK quick if you don't like it ;)
> 
> Hm, I think this patch taks only cleanups. right?
> if so, I don't find any objection reason.

Mostly cleanups, but one behavior change here: 

> > -               nr_taken = sc->isolate_pages(sc->swap_cluster_max,
> > +               nr_taken = sc->isolate_pages(nr_to_scan,
> >                             &page_list, &nr_scan, sc->order, mode,
> >                                zone, sc->mem_cgroup, 0, file);

The new behavior is to scan exactly the number of pages that
shrink_zone() or other callers tell it. It won't try to "round it up"
to 32 pages. This new behavior is in line with shrink_active_list()'s
current status as well as shrink_zone()'s expectation.

shrink_zone() may still submit scan requests for <32 pages, which is
suboptimal. I'll try to eliminate that totally with more patches.

> >                nr_active = clear_active_flags(&page_list, count);
> > @@ -1093,7 +1095,6 @@ static unsigned long shrink_inactive_lis
> >
> >                spin_unlock_irq(&zone->lru_lock);
> >
> > -               nr_scanned += nr_scan;
> >                nr_freed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
> >
> >                /*
> > @@ -1117,7 +1118,7 @@ static unsigned long shrink_inactive_lis
> >                                                        PAGEOUT_IO_SYNC);
> >                }
> >
> > -               nr_reclaimed += nr_freed;
> > +               nr_reclaimed = nr_freed;
> 
> maybe, nr_freed can be removed perfectly. it have the same meaning as
> nr_reclaimed.

Yes, good spot!

Thanks,
Fengguang
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