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

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Message-ID: <461EE925.1000301@yahoo.com.au>
Date:	Fri, 13 Apr 2007 12:21:25 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Matt Mackall <mpm@...enic.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups

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?

>>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?

-- 
SUSE Labs, Novell Inc.
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