View Full Version : Yeah, so I'm obviously doing something wrong.

07-26-2005, 10:21 PM
I am running a dual core setup (AMD 64 X2 4400+), 2 gigs of ram,XP Pro with SP2, LW 8.3 yada yada. I ran a render on it and it rendered out in 1 minute and 10 seconds, so I up the threads to 8 and render again... 1 minute and 6 seconds. Wow... huge drop. So, I'm guessing that I've missed changing some settings somewhere from the 8.0 configs or something. Anyone have any help to offer?

07-27-2005, 12:24 AM
Try with a second processor, that usually helps. Being dual core doesn't count.

07-27-2005, 02:33 AM
Yes it does, but with scenes that simple it doesn't make sense to render on more than one processor. Try again with a scene that takes half an hour to render a frame for a single processor and then see what the render times are if you up the threads.

The reason it doesn't make sense to multi-thread the simple scene is the overhead of both processors talking to each other. Let's say, purely hypothetically, that the over head is 35 seconds, and that because you are using two processors, the render times are halved. If you take a one minute render, that should be thirty seconds, but uh-oh, there's that overhead making the final render times one minute and five seconds - slower than if you'd just rendered on a single processor. Instead, take a frame that takes an hour. With two processors you can render it in half an hour, then add the overhead of 35 seconds...


07-27-2005, 11:25 AM
Also, how many threads were you using on your first render? If you have one dual-core cpu, 8 threads could be excessive, ESPECIALLY on a simple scene. Try it with 2 threads, then 4. Technically, you have 2 cpu's, so sending them eight threads will most likely produce a large amount of overhead.

07-30-2005, 10:48 AM
Ok, actually, I've straightened the problem out. I ended up reformatting and doing a clean install because I had a lot of instability with other programs etc. Somewhere in there the problem got fixed, and now LW is hauling. I think you really only get overhead on dual processor rigs, which is one of the big advantages to having two cores on one die. Anyway, even simple 30s/frame scenes are having render times halved, so I am now quite happy. Just making this post to try to keep from discouraging anyone who was pondering getting a dual core rig.

08-01-2005, 07:15 AM
Dual core and dual processor are almost the same thing. There are still (2) cores that need threaded instructions.

BTW, glad to hear everything's working good.

08-01-2005, 08:10 AM
Did you upgrade to the X2 from a single-core CPU, without re-installing windows?
In that case it's no surprise, because then you had not installed the multiprocessor kernel of Windows XP, but the uniprocessor one...you could've changed that without formatting though...

And as said dual-core is very similar to dual-cpu, since they still have seperate caches there is still an overhead to keep coherency, but it's a bit smaller due to their more direct connection (in case of AMD, does not apply to Intel's dual-core CPUs...their cores are simply connected via external FSB just like two Xeons)
But the software overhead for threading is the same in any case.