Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

[Matt Mackall <mpm@selenic.com>]

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.