Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Tom Zanussi <tzanussi@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        "Frank Ch. Eigler" <fche@redhat.com>, Tejun Heo <htejun@gmail.com>
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Date: Fri, 06 Aug 2010 13:14:58 +0200
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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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