SMP doesn't scale correctly

[popej]

Hi,

my host is Vista x64 with Quad Core, guest is Centos 5.3 32-bit version with 4 vCPU. On guest I'm running make for program compilation. I'm changing the number of concurrent jobs and measuring execute time. This are results, time in seconds:

Code: Select all

jobs  real   user  system
1         34       9         25
2         25     15         38
3         45     38       108
4         67     44       214
5         67     44       213

There is something wrong with SMP. Using more then 2 vCPU concurrently increase execute time compared to single vCPU.
For comparison, similar test on VB and RedHat 7.3 with kernel 2.4 gives results as expected. Also VMware and Centos 5.3 scale correctly up to 4 vCPU.

I'm using VirtualBox 3.0.10 but the previous versions behave the same. Any idea what is wrong? I tried to add boot option divider=10 but it changed nothing.

[sej7278]

there's always been something funky about compile times on virtualbox - some of the problem is disk speed, some is smp, some is how you benchmark (don't use the timer on the guest).

search the forums, although its never been answered, its been asked plenty.