Re: bug #5493

[Andrew Morton <akpm@linux-foundation.org>]

> On Thu, 8 Nov 2007 13:15:18 -0500 Rik van Riel <riel@redhat.com> wrote:
> 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?
> 

pageout, iirc.  For each page we were walking a linear list of I think
~10,000 elements.

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