From: Stefan Monnier <monnier@iro.umontreal.ca>
To: David Kastrup <dak@gnu.org>
Cc: emacs-devel@gnu.org
Subject: Re: [PATCH] use tail pointer for LOOP
Date: Fri, 18 Jun 2010 09:40:01 -0400 [thread overview]
Message-ID: <jwv1vc4a2e9.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <87eig4rf8l.fsf@lola.goethe.zz> (David Kastrup's message of "Fri, 18 Jun 2010 09:07:06 +0200")
> nreverse is also cons-neutral. The only situation where I'd expect it
> to be able to lose is under virtual memory pressure, where the nreverse
> approach has to cons the list forwards when building, and again
> backwards when reversing the conses. A tail pointer approach will
> traverse the list just once. So if it does not fit in real memory, tail
> pointers may cause half the page faults than nreverse does.
Doesn't have to be as drastic as "virtual memory". If the list is long
enough to overflow some level of the memory hierarchy, then the
tail-pointer approach has a fighting change. Hopefully such lists are
really rare (lists that overflow the L1 cache are probably fairly common
(about 1000 elements), but the speed difference between L1 and L2 cache
is probably not sufficient to defeat nreverse).
Stefan
next prev parent reply other threads:[~2010-06-18 13:40 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-29 21:56 O(N^2) behavior in LOOP Daniel Colascione
2010-05-29 22:06 ` Daniel Colascione
2010-05-29 22:14 ` Lennart Borgman
2010-05-29 22:35 ` Geoff Gole
2010-05-29 23:58 ` [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP) Daniel Colascione
2010-05-30 0:45 ` Ken Raeburn
2010-05-30 0:49 ` Daniel Colascione
2010-06-16 17:44 ` tomas
2010-06-16 18:10 ` [PATCH] use tail pointer for LOOP David Kastrup
2010-06-17 5:10 ` tomas
2010-06-17 7:18 ` Thien-Thi Nguyen
2010-06-17 9:22 ` tomas
2010-06-17 10:03 ` Thien-Thi Nguyen
2010-06-17 14:05 ` tomas
2010-06-17 15:16 ` Thien-Thi Nguyen
2010-06-17 10:12 ` Thien-Thi Nguyen
2010-06-17 20:48 ` Stefan Monnier
2010-06-18 7:07 ` David Kastrup
2010-06-18 13:40 ` Stefan Monnier [this message]
2010-05-30 17:05 ` Štěpán Němec
2010-05-30 17:09 ` Daniel Colascione
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