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    SubjectRe: [PATCH 4/4] maps: /proc/<pid>/pmaps interface - memory maps in granularity of pages
    On Sat, Aug 18, 2007 at 12:22:26PM -0500, Matt Mackall wrote:
    > > > > So VSZ:RSS ratio actually goes up with memory pressure.
    > > >
    > > > And yes.
    > > >
    > > > But that's not what I'm talking about. You're likely to have more
    > > > holes in your ranges with memory pressure as things that aren't active
    > > > get paged or swapped out and back in. And because we're walking the
    > > > LRU more rapidly, we'll flip over a lot of the active bits more often
    > > > which will mean more output.
    > > >
    > > > > - page range is a good unit of locality. They are more likely to be
    > > > > reclaimed as a whole. So (RSS:page_ranges) wouldn't degrade as much.
    > > >
    > > > There is that. The relative magnitude of the different effects is
    > > > unclear. But it is clear that the worst case for pmap is much worse
    > >
    > > > than pagemap (two lines per page of RSS?).
    > > It's one line per page. No sane app will make vmas proliferate.
    >
    > Sane apps are few and far between.

    Very likely, and they will bloat maps/smaps/pmaps alike :(

    > > So let's talk about the worst case.
    > >
    > > pagemap's data set size is determined by VSZ.
    > > 4GB VSZ means 1M PFNs, hence 8MB pagemap data.
    > >
    > > pmaps's data set size is bounded by RSS hence physical memory.
    > > 4GB RSS means up to 1M page ranges, hence ~20M pmaps data.
    > > Not too bad :)
    >
    > Hmmm, I've been misreading the output.
    >
    > What does it do with nonlinear VMAs?

    The implementation gets offset from page_index(page), so will work
    the same way in linear/nonlinear VMAs. Depending how one does the
    remap_file_ranges() calls, the output lines may be not strictly
    ordered by offset, or overlap, or have small page ranges.

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