From: malc <malc@pulsesoft.com>
To: Xavier Leroy <xavier.leroy@inria.fr>
Cc: William Chesters <williamc@paneris.org>, <caml-list@inria.fr>
Subject: Re: float array alignment; was Re: [Caml-list] Bigarray access speed
Date: Mon, 19 Aug 2002 17:07:33 +0400 (MSD) [thread overview]
Message-ID: <Pine.LNX.4.44.0208191704170.1584-100000@home.oyster.ru> (raw)
In-Reply-To: <20020819145605.A6276@pauillac.inria.fr>
On Mon, 19 Aug 2002, Xavier Leroy wrote:
> malc writes:
> > To get maximal speed of the inner loops data needs to be naturally
> > aligned. OCaml does nothing to enforce it for non-big arrays. Bigarrays on
> > the other hand are mmaped(4k on IA32) and you get perfectly aligned data
> > for free. I was thinking that maybe Array can be extended with
> > make[create]_aligned, for speed/space tradeoff.
>
> As William Chester said, allocating 8-aligned arrays isn't really
> hard, but keeping them 8-aligned across copying collection,
> compaction, and structured I/O is quite a pain.
>
> My experiments indicate that the lack of alignment on float arrays
> (or more precisely the fact that they are 4-aligned instead of
> 8-aligned) has negligible impact on performance for the IA32 (Pentium)
> and PowerPC processors, but non-negligible for SPARC and MIPS.
> And of course on a 64-bit architecture the problem goes away because
> everything in the Caml heap is then 8-aligned. Since I expect IA32
> and PowerPC to remain dominant until we massively switch to 64-bit
> processors, there's no urgent need to do something about float array
> alignment.
IA32 is now much bigger family, and unlucky owners of AMD 7th generation
machines, such as myself, do pay a price for unaligned double precission
float accesses.
--
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next prev parent reply other threads:[~2002-08-19 13:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-08-15 22:50 Richard Nyberg
2002-08-16 10:27 ` malc
2002-08-16 10:40 ` Markus Mottl
2002-08-16 11:08 ` float array alignment; was " William Chesters
2002-08-19 12:56 ` Xavier Leroy
2002-08-19 13:07 ` malc [this message]
2002-08-16 18:46 ` Richard Nyberg
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