On Thu, 6 Dec 2007, Emmanuel Charpentier wrote:
Prof Brian Ripley a écrit :
Note that Ottorino has only 1GB of RAM installed, which makes a 64-bit
version of R somewhat moot. See chapter 8 of
http://cran.r-project.org/doc/manuals/R-admin.html
Thank you for this reminder|tip ! I didn't re-read this document since
... oh, my ... very early (1.x ?) versions. At which time my favorite
peeve against R was the fixed memory allocation scheme.
I would have thought that 64 bits machines could take advantage of a
wider bus and (marginally ?) faster instructions to balance larger
pointers. Am I mistaken ?
Yes, it is more complex than that. If you run 32-bit instructions on a
x86_64, the physical bus is the same as when you run 64-bit instructions.
The larger code usually means the CPU caches spill more often, and some
64-bit chips have more 32-bit than 64-bit registers which allows better
scheduling.
The R-admin manual reports on some empirical testing. But when you have
limited RAM the larger code and data for a 64-bit build will cause more
swapping and that is likely to dominate performance issues on large
problems.
Note that the comparisons depend on both the chip and the OS: it seems
that on Mac OS 10.5 on a Core 2 Duo the 64-bit version is faster (on small
examples). The original enquiry was about 'amd64 linux', but I've checked
Intel Core 2 Duo as well: on my box 64-bit builds are faster than 32-bit
ones, whereas the reverse is true for Opterons. So it seems that the
architectural differences of Core 2 Duo vs AMD64 do affect the issue.
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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