linux-kernel - Re: [PATCH 2/3] vmscan: make mapped executable pages the first class citizen

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Date:	Wed, 20 May 2009 09:44:45 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Elladan <elladan@...imo.com>, Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	"tytso@....edu" <tytso@....edu>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>
Subject: Re: [PATCH 2/3] vmscan: make mapped executable pages the first
	class  citizen

On Tue, May 19, 2009 at 08:28:28PM +0800, KOSAKI Motohiro wrote:
> Hi
> 
> 2009/5/19 Wu Fengguang <fengguang.wu@...el.com>:
> > On Tue, May 19, 2009 at 04:06:35PM +0800, KOSAKI Motohiro wrote:
> >> > > > Like the console mode, the absolute nr_mapped drops considerably - to 1/13 of
> >> > > > the original size - during the streaming IO.
> >> > > >
> >> > > > The delta of pgmajfault is 3 vs 107 during IO, or 236 vs 393 during the whole
> >> > > > process.
> >> > >
> >> > > hmmm.
> >> > >
> >> > > about 100 page fault don't match Elladan's problem, I think.
> >> > > perhaps We missed any addional reproduce condition?
> >> >
> >> > Elladan's case is not the point of this test.
> >> > Elladan's IO is use-once, so probably not a caching problem at all.
> >> >
> >> > This test case is specifically devised to confirm whether this patch
> >> > works as expected. Conclusion: it is.
> >>
> >> Dejection ;-)
> >>
> >> The number should address the patch is useful or not. confirming as expected
> >> is not so great.
> >
> > OK, let's make the conclusion in this way:
> >
> > The changelog analyzed the possible beneficial situation, and this
> > test backs that theory with real numbers, ie: it successfully stops
> > major faults when the active file list is slowly scanned when there
> > are partially cache hot streaming IO.
> >
> > Another (amazing) finding of the test is, only around 1/10 mapped pages
> > are actively referenced in the absence of user activities.
> >
> > Shall we protect the remaining 9/10 inactive ones? This is a question ;-)
> 
> Unfortunately, I don't reproduce again.
> I don't apply your patch yet. but mapped ratio is reduced only very little.

mapped ratio or absolute numbers? The ratio wont change much because
nr_mapped is already small.

> I think smem can show which library evicted.  Can you try it?
> 
> download:  http://www.selenic.com/smem/
> usage:   ./smem -m -r --abbreviate

Sure, but I don't see much change in its output (see attachments).

smem-console-0 is collected after fresh boot,
smem-console-1 is collected after the big IO.

> We can't decide 9/10 is important or not. we need know actual evicted file list.

Right. But what I measured is the activeness. Almost zero major page
faults means the evicted 90% mapped pages are inactive during the
long 300 seconds of IO.

Thanks,
Fengguang

> > Or, shall we take the "protect active VM_EXEC mapped pages" approach,
> > or Christoph's "protect all mapped pages all time, unless they grow
> > too large" attitude?  I still prefer the best effort VM_EXEC heuristics.
> >
> > 1) the partially cache hot streaming IO is far more likely to happen
> >   on (file) servers. For them, evicting the 9/10 inactive mapped
> >   pages over night should be acceptable for sysadms.
> >
> > 2) for use-once IO on desktop, we have Rik's active file list
> >   protection heuristics, so nothing to worry at all.
> >
> > 3) for big working set small memory desktop, the active list will
> >   still be scanned, in this situation, why not evict some of the
> >   inactive mapped pages? If they have not been accessed for 1 minute,
> >   they are not likely be the user focus, and the tight memory
> >   constraint can only afford to cache the user focused working set.
> >
> > Does that make sense?

View attachment "smem-console-0" of type "text/plain" (2376 bytes)

View attachment "smem-console-1" of type "text/plain" (2377 bytes)