View Full Version : Bullet: Simulating Dynamics @12% CPU

08-19-2013, 03:32 PM
Does any one have an answer to why the "Simulating Dynamics" to Bullet only uses 12% of my CPU for calculations. I have 8 threads - only 2 are being used. I've set LW to use all 8 Threads when rendering and that's fine when rendering.

At the moment I have a 250 frame animation of a collapsing building, using 11.5gig of RAM out of my 12gig available. Being at my RAM limit shouldn't effect the CPU? The simulation has been running for 6 hours now and only 62% complete, long time to wait just to see how it looks.

Any ideas on how to commit more CPU to the simulation ?

08-19-2013, 03:43 PM
Annoying isn't it. I think I read somewhere though that it allocates the cores it needs, and using more may not speed up the process. I don't know enough about programming to understand that, but hey-ho...:)

08-19-2013, 04:46 PM
Bullet is a single-threaded Dynamics-engine. That goes for the Hard Body stuff. That said, Softbodies are multithreaded. If you use softbodies, with certain settings, you should be able to do Softbody simulations that looks like Hardbody, but gain more threads to the process. I've not done this myself, but I'm quite sure that I have read about this trick elswere.

08-19-2013, 08:07 PM
You should feel sorry for us poor bastards who have 16, 24 or more, core boxes!

Have you tried turning off hyperthreading, you may get a bit more out of the CPU. A test like this would be interesting.


08-20-2013, 12:22 AM
A simple scene, dropping a bed sheet onto a bed.

8 cores Hyperthreading off.
A rough guess CPU usage 21-27%, average roughly 24%
6 minutes 24 seconds

16 cores Hyperthreading on.
A rough guess CPU usage 18-26%, average roughly 20%
7 minutes 8 seconds

So turning HT off is 10% quicker in this test.

08-20-2013, 02:43 AM
That's interesting - The sad news is that LW froze at 83% with 0% use on the CPU, but the PC has not crashed. Also I notice the .dynacache file is 955MB. Does anyone know if there is a 1Gig limit for the cache file? or is this a very large cache file?

Time is short so I will have to rethink how to do the building.

08-20-2013, 07:37 PM
I had a dynacache file as large as 2 gb, but never larger. I was wondering the same thing. I've done some very poly intense sims and wish I could go higher than 2gb. Limit should only be limited by memory available IMO.