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    SubjectRe: [PATCH 2/3] vmscan: make mapped executable pages the first class citizen
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    Hi

    2009/5/19 Wu Fengguang <fengguang.wu@intel.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.

    I think smem can show which library evicted. Can you try it?

    download: http://www.selenic.com/smem/
    usage: ./smem -m -r --abbreviate


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

    Thanks.


    > 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?
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