Re: bug #5493

[mel@skynet.ie (Mel Gorman)]

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.

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?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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