From: David Kastrup <dak@gnu.org>
To: emacs-devel@gnu.org
Subject: Re: [PATCH] use tail pointer for LOOP
Date: Fri, 18 Jun 2010 09:07:06 +0200 [thread overview]
Message-ID: <87eig4rf8l.fsf@lola.goethe.zz> (raw)
In-Reply-To: jwvpqzpcrqe.fsf-monnier+emacs@gnu.org
Stefan Monnier <monnier@IRO.UMontreal.CA> writes:
>> Reverse: (1.006534 3 0.682734)
>> Tail pointer: (1.305476 4 0.9159619999999986)
>
>> Still, reversing seems to be worth it (by some 30 percent). Unless we
>> find some way to streamline the tail pointer better.
>
> That's really not surprising: count the number of function calls and
> you'll see that the nreverse way is likely to win every time, simply
> because it suffers about half as much from the interpretation overhead.
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.
--
David Kastrup
next prev parent reply other threads:[~2010-06-18 7:07 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 [this message]
2010-06-18 13:40 ` Stefan Monnier
2010-05-30 17:05 ` Štěpán Němec
2010-05-30 17:09 ` Daniel Colascione
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