Parallels 4 > XP > Sony Vegas 8c: 2 CPUs is best - Why?

Context: 8-core Mac Pro > Parallels 4.0.3810 > XP(32)Pro-SP2 > Sony Vegas (8.0c), running the RenderTest-HDV benchmark (Ref given at end of this post), which is heavy video stuff, including HDV render to Mpeg2. This all works fine, the point of this post/thread is the optimum config of the VM and App (together).

Although BootCamp is obviously fastest, the benchmark still ran usably fast in a Parallels 4.0 virtual machine, despite being about 3 times slower than BootCamp and in consequence comparable to a dual-core.

Following these successful tests, I upgraded my license to Parallels 4.0, and so far it seems fine. But can it do better?

THE TWO-CPU MYSTERY:

The optimum performance was when virtual machine CPUs=2. Of course I also tried 4 (Vegas' maximum threads) and 8 (the whole Mac), but these were much slower. One might naively expect that increasing the number of CPUs for the virtual machine would speed things up ... but NO, for the described test situation it was the opposite! I spent a day tweaking all possibilities of Parallels 4 and of Vegas that I could imagine or discover from forum posts (including virtual machine CPU settings, HyperVisor, memory; even a trick involving the Vegas "hidden" internal prefs setting its threads min=4 max=8). The very best performance was with virtual machine CPUs=2 and, bizarrely, Vegas' (own preferences) processing threads = 3 or 4. The tests were repeatable, the results were always consistent.

Any idea why the magic number 2 CPUs worked best and why, with this config, it was best to specify more than 2 processing threads in the application (Vegas) ?

Ref: (my benchmark test results) http://www.sonycreativesoftware.com/forums/ShowMessage.asp?MessageID=526098&Replies=321

 

It is definitely depends on software you are using.
For example BOINC is running faster with 8CPU settins