From: Lionel Bouton <lionel-subscription@...>
Date: 2008-02-21T04:08:53+09:00
Subject: [QUIZ #157] 32bit vs 64bit vs UML performance

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

Each benchmark as been run at least twice and at least once while 
monitoring the swap/cpu usage with vmstat and top and once without 
monitoring to reduce interference. The results where nearly the same.

First 64bit MRI is slower on the same hardware than 32bit :

* Athlon 64 X2 3800+ (2GHz) 64bit : ~25% faster in a 32bit chroot on the 
same system (~24s instead of ~30s).
* Core2Duo 64bit : ~25% slower than Core2Duo  32bit too (~18s on E6750 
(2.66GHz) instead of ~20s on E6300 (1.83GHz), should be 14s assuming 
perf = k*GHz which results in the "-25%")

That's not unexpected, but I didn't think it would be so large a 
performance hit.

Now for the true surprise:

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.

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 ?

Lionel