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    SubjectRe: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups
    On Fri, Apr 13, 2007 at 11:42:29AM +1000, Nick Piggin wrote:
    > >Instead, one says "what pages are being used by my application", then, for
    >
    > That includes unmapped pagecache being used by my application, doesn't it?
    > Maybe that's too hard to do via /proc so we forget about it...

    It'd be really nice to have a window into the pagecache too. But I for
    one couldn't come up with a sensible scheme for it.

    > >each of those pages "what is that page's state". So the first step is to
    > >collect all the pfns from /proc/$(pidof my-application)/pagemap and then to
    > >use those pfns to look the individual pages up in /proc/kpagemap.
    >
    > OK I realise you could do it that way, but systemtap can definitely be
    > used as a tool for understanding application behaviour in the context of
    > the kernel, I think? The purpose for it is so that various little bits
    > of deep kernel internals do not have to be exposed on a case by case basis.
    >
    > If kprobes is simply crappy and doesn't work properly for this, then I
    > could accept that. I'm not someone trying to get this info. So why can't
    > it be used? (not just for kpagemap, but for clear_refs and all that gunk
    > too).

    kprobes is good for looking at events, but bad for looking at state.
    Especially metric shitloads of state.

    > > If you really want to know "who is using page 123435" then you'd need to
    > > search /proc/*/pagemap. There are possibly legitimate reasons why an
    > > application developer would want to at least pertially perform such an
    > > operation ("who am I sharing with"), but I doubt if it's the common case.
    >
    > Maybe. How about LRU? Reclaim performance is bad, and you want to work out
    > which pages keep going off the end of it, or which pages keep getting
    > written out via it, or who's pages are on the active list, forcing mine
    > out.

    Those are actually probably a good match for systemtap as they're all events.

    --
    Mathematics is the supreme nostalgia of our time.
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