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    SubjectRe: [PATCH 1/1] pagemap clear_refs: modify to specify anon or mapped vma clearing
    On Tue, Jul 28, 2009 at 05:00:54PM -0700, Moussa Ba wrote:
    > The patch makes the clear_refs more versatile in adding the option to
    > select anonymous pages or file backed pages for clearing. This
    > addition has a measurable impact on user space application performance
    > as it decreases the number of pagewalks in scenarios where one is only
    > interested in a specific type of page (anonymous or file mapped).

    I think what Andrew might be looking for is a repeat of the use-case for
    using clear_refs at all and what the additional usecase is that makes this
    patch beneficial. To be honest, I had to go digging for a bit to find out
    why clear_refs is used at all and the potential benefits of this patch were
    initially unclear to me although they were obvious to others. I confess I
    haven't read the patch closely to determine if it's behaving as advertised
    or not.

    Bonus points for a wee illustration of a script that measures the apparent
    working set of a process using clear_refs, showing the working set for anon
    vs filebacked and the difference of measuring just anon versus the full
    process for example. Such a script could be added to Documentation/vm as
    I didn't spot any example in there already.

    Total aside, is there potential to really mess with LRU aging by abusing
    clear_refs a lot, partcularly as clear_refs is writable by the process
    owner? Does it matter?

    > On Tue, Jul 28, 2009 at 4:52 PM, Andrew Morton<> wrote:
    > > On Tue, 28 Jul 2009 15:52:05 -0700
    > > "Moussa A. Ba" <> wrote:
    > >
    > >> This patch adds anonymous and file backed filters to the clear_refs interface.
    > >> echo 1 > /proc/PID/clear_refs resets the bits on all pages
    > >> echo 2 > /proc/PID/clear_refs resets the bits on anonymous pages only
    > >> echo 3 > /proc/PID/clear_refs resets the bits on file backed pages only
    > >> Any other value is ignored
    > >
    > > The changelog is missing any rationale for making this change.
    > > Originally we were told that it "makes the clear_refs proc interface a
    > > bit more versatile", but that's a bit thin.
    > >
    > > How do we justify making this change to Linux?  What value does it
    > > have?  Can you describe a usage scenario which would help people
    > > understand the usefulness of the change?
    > >
    > > Thanks.
    > >

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