linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Fri, 06 Aug 2010 13:14:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On Fri, 2010-08-06 at 12:11 +0200, Peter Zijlstra wrote:
> > You need to read the whole trace to find these cookies (even if it is just once
> > at the beginning if you create an index).

Even if you want to index all sync points you can quickly skip through
the file using the sync-distance, after which you'll have, on average,
only 1/2 avg-event-size to read before you find your next sync point.

So suppose you have a 1M sync-distance, and an effective average event
size of 128 bytes, then for a 4G file, you can find all sync points by
only reading ~262144 bytes (not counting for the fact that the pagecache
will bring in full pages, which would result in something like 16M to be
read in total or somesuch -- which, again assumes read-ahead isn't going
to play tricks on you).

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