Re: bug #5493

[Rik van Riel <riel@redhat.com>]

On Thu, 8 Nov 2007 09:57:04 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> > On Thu, 8 Nov 2007 16:53:20 +0000 mel@skynet.ie (Mel Gorman) wrote:
> > On (07/11/07 19:12), Andrew Morton didst pronounce:
> > > (added linux-mm)
> > > 
> > > > On Wed, 7 Nov 2007 18:00:20 -0800 "Natalie Protasevich" <protasnb@gmail.com> wrote:
> > > > Andrew, this one http://bugzilla.kernel.org/show_bug.cgi?id=5493 looks
> > > > serious, and I'm not sure who to ping now that the reporter can't test
> > > > it anymore.
> > > > This is about mprotect ...
> > > 
> > > No, I don't think anyone knows how to fix that.
> > > 
> > > Fortunately I'm only aware of the one person hitting this problem.
> > > 
> > 
> > I tried out the test program with 1GiB of memory. First, the program could
> > not even run unless mprotect was called again to make pages read-only a
> > second time - otherwise mprotect would report ENOMEM because VMAs were not
> > getting merged. That in itself was a little unexpected.
> > 
> > After fixing that, I ran with varying number of pages and got the
> > following timings
> > 
> > 300000: 68.36 seconds
> > 295000: 55.07 seconds
> > 290000: 41.79 seconds
> > 285000: 31.71 seconds
> > 280000: 22.92 seconds
> > 275000: 11.27 seconds
> > 270000: 5.60 seconds
> > 265000: 5.94 seconds
> > 260000: 5.77 seconds
> > 255000: 5.65 seconds
> > 250000: 5.53 seconds
> > 245000: 5.42 seconds
> > 240000: 5.31 seconds
> > 
> > The system has about 250000 pages and around that mark it seemed fine in
> > terms of time-to-completion. Above that vmstat was showing high figures
> > for si/so which is not a major suprise as such.
> 
> hm, I was able to reproduce it way back when it was first reported.  See
> below.
> 
> 
> > If this only occurs on systems with large amounts of memory, could it be
> > a variation of the excessive page-scanning problem that Rik has been on
> > about?
> 
> No, it was due to linear traversal of very long reverse-mapping lists
> (thousands of elements, irrc).

Traversal at pageout time, or at mprotect time?

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan

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