Hyperthreading and render times

[Titus]

Hi, I recently bought a dual Xeon server and there is something I can't understand well.

I'm playing with some scenes and I don't think hyperthreading is helping to reduce render times, actually sometimes are gretater. I'm using Windows XP with HT enabled (I can see 4 processors running). An example is this simple scene I find these times:

multithreading: 1 Time: 13.5 sec Proc Workload: 25%
multithreading: 2 Time: 16.5 sec Proc workload: 20%
multithreading: 4 Time: 13.4 sec Proc Workload: 26%

What is happening?
when I can make the most of this dual machine?

[bloontz]

I noticed the same with hyperthreading so I turned it off. I think it's more for running multiple apps at once. I guess lightwaves multithreading takes care of processor usage.

[BeeVee]

Try something a bit more complex! The scene you are rendering in takes nearly no time at all, so the overhead of passing the information back and forth between real and virtual processors means that your rendering times are extended - try a scene like Tracer-Radiosity to see what results you get then.

B

[Titus]

Beevee: If this is the issue then I don't have nothing to worrie, just wanted to know why and when happen this. This a test I made yesterday trying to figure what's going on and the render times are 8 minutes for 4 threads ans 16 minutes for 1 thread.

[papou]

hi titus,
can you do one more render with 2 threads please?
arf-

[bloontz]

I think with one thread you are only using one processor.

[facial deluxe]

Using a dual Xeon 2.6Ghz and a PIV 3Ghz (Haven't tested the PIV really yet). What I do is this :
For subpatched model I use 8 threads
For planar polys things, such as typos, I use one thread. Those last compute so fast that the CPUs are loosing time dealing with each others. In that config, got 400% speed with 1 thread.

Haven't tested the with and without hyperthreading. Where do I find control of this ?

[bloontz]

Wow, 400% increase, that's a great tip. On my machine I have to enable hyperthreading in the bios.

[Titus]

Papou: Sorry, I think I deleted the scene file and if try to replicate it I have different rendertimes, I will start from the beginnig.

facial deluxe: Thanks for the advice, very interesting results.