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    SubjectRe: [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@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.

    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?
    Map PIDs AVGPSS PSS
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    Map PIDs AVGPSS PSS
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    /lib/libutil-2.9.so 1 16.0K 16.0K
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    /lib/libcap.so.2.11 1 12.0K 12.0K
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    [vsyscall] 2 0 0
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