Re: [QUIZ #157] 32bit vs 64bit vs UML performance

Lionel Bouton wrote:
I've run the QUIZ benchmark on several systems to check their relative
performance and had quite a surprise.
I'll take Frank's algorithm as reference for simplicity as it spends
roughly the same time for each point distribution in 157_benchmark_2.rb
but this is true for all of them.

This is on Gentoo Linux, all systems are compiled with gcc 4.1.2
on 32bit : -O3 -march=i686 -fomit-frame-pointer -pipe
on 64bit : -O2 -pipe

Hold on!! Try recompiling the 32-bit version with "march=pentium4" and
the 64-bit version with "-O3 -march=athlon64" and *then* compare the
timings! You've saddled the 64-bit version with some default
architecture and one less optimization level than the 32-bit version.

[snip]

The 32bit Core2Duo E6300 is a system with several user-mode-linux based
systems which are idling around (low trafic mail, DNS server and test
systems). It sits at a constant 0 load and zero swap activity.
The benchmark is 2x faster *in* the virtual machine (with the very same
compilation options, the virtual machine is mostly a clone of the host)
: 10s instead of 20s ! That's not a system timer problem, I actually
watched it spend the 10s and 20s.

I don't know what's going on... Maybe Ruby is making a very specific
system call that happens to be faster with user-mode-linux even though
the benchmark consists of nearly 99% floating point operations.

On the Athlon, you can figure out what's going on with CodeAnalyst. I
would guess it's something to do with cache thrashing or lack thereof.
On the Intel, you might be able to get some results from CodeAnalyst --
it's basically a wrapper around "oprofile". But you might end up needing
Intel's VTune. If you do this for a living, it's worth spending the
money. :)


The UML kernel is a 2.6.18 with the UML patches, the host is a 2.6.23.9
with the skas3 patch (designed to help the UML performance). The UML
sees 512MB, the host has 1GB. As I said, nearly no concurrent system
activity, 0 swapping before during and after the benchmark on both the
host and the virtual machine.

Anyone seen something remotely like this ?

Not me ... but I'm an Intel-free zone. ;)


.

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