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

On Fri, Apr 13, 2007 at 12:21:25PM +1000, Nick Piggin wrote:
> Matt Mackall wrote:
> >On Fri, Apr 13, 2007 at 11:42:29AM +1000, Nick Piggin wrote:
> 
> >>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.
> 
> Why? Why is a kprobes trap significantly more expensive than a read
> syscall?

I guess I'm not clear on what you're proposing. From my understanding
of kprobes (admittedly not an expert), this is hard to do and not a
very good match.
 
> >>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.
> 
> Traverse the LRU? Which files do they belong to? What process maps them?

-ENOPARSE.

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