[Beowulf] cluster building advice?

Christopher Samuel samuel at unimelb.edu.au
Wed Sep 26 17:50:47 PDT 2012


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On 27/09/12 07:59, Vincent Diepeveen wrote:

> Linus posted something like: "there is no excuse to not generate
> CMOV instructions now that also intel core2 can execute them
> fast".

No, what he actually said was:

http://ondioline.org/mail/cmov-a-bad-idea-on-out-of-order-cpus

#  - on a P4, branch mispredictions are expensive, but so is cmov,
#    so all the above is to some degree exaggerated. On nicer
#    microarchitectures (the Intel Core 2 in particular is something
#    I have to say is very nice indeed), the difference will be a lot
#    less noticeable. The loss from cmov isn't very big (it's not as
#    sucky as P4), but neither is the win (branch misprediction isn't
#    that expensive either).
[...]
[example showing CMOV version slower than non-CMOV on Core2 elided]
[...]
# So don't use cmov. Except for non-performance-critical code, or
# if you  really care about code-size, and it helps (which is actually
# fairly rare: quite often cmov isn't even smaller than a conditional
# jump and a regular move, partly because a regular move can take
# arguments that a cmov cannot: move to memory, move from an immediate
# etc etc, so depending on what  you're moving, cmov simply isn't good
# even if it's _just_ a move).

cheers,
Chris
- -- 
 Christopher Samuel        Senior Systems Administrator
 VLSCI - Victorian Life Sciences Computation Initiative
 Email: samuel at unimelb.edu.au Phone: +61 (0)3 903 55545
 http://www.vlsci.org.au/      http://twitter.com/vlsci

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