View Full Version : Multi-threading with a DP G5

11-08-2003, 11:16 AM
Anyone else experience slowdowns in render time when multi-threading is turned up higher than 1 thread? I am rendering a fairly simple scene that only takes about 45 secs to render when set to one thread, but when set to 2, 4, or 8 threads, it slows down considerably (twice the time at 2 threads, and even more when 4 or 8 threads are used)...I was under the impression that this would speed up my render times??? This is my first dual processor machine, so I have never really experimented with threading before. Any help with this would be appreciated. comments/suggestions


512 MB RAM
ATI 9600 64 MB RAM

11-08-2003, 01:02 PM
I've heard this mentioned about G5's, but in any case 512 ram is really skimpy, that may be part of the problem - apparently the render speeds are dramatically better with 2 gb ram, maybe then it can handle more threads?

11-08-2003, 05:23 PM
thanks for the info....i guess i was hoping that there was some little trick to getting it to run right....but i am sure i would have caved and bought the extra RAM sooner or later anyways...thanks again

11-08-2003, 05:39 PM
It can also depend on what you're rendering, Hypervoxels for example seem to render slower with more than one thread.

I don't think things with LW on the G5 will be sorted out until LW 8 and OS 10.3.1 or 2 come out.

11-09-2003, 06:12 AM
Has NT ever commented on better dual support?

11-09-2003, 07:27 AM
Er...on PC HyperVoxels get multithreaded great, better than most geometry-only scenes :confused:

Anyway, what LW version are you using?
In 6.x for example neither 'Image World' nor 'LW TextureEnvironment' are multithreaded, causing a HEAVY slowdown when using it in conjunction with radiosity (since it traces the backdrop very often). This has been resolved in 7.x (which i unfortunately still don't own, but veeeery soon :D)

Other plugins might also use routines that conflict when multithreading, especially environment handlers as mentioned.

If it's really too few RAM perhaps try reducing segment memory, but that'd be kind of a double negative makes positive :D

11-09-2003, 10:03 AM
i am currently using 7.5...i have talked to other people who seem to not have any trouble with multithreading on it at all....but they all seem to have one thing in common, more RAM....so it looks to me right now that that is the most likely cause of the slowdown....maybe i'll try a couple of other scenes and see if i can get any better results....

wouldn't decreasing the segment memory only slow it down further because it would have to make more than one pass??.....just a question...i am sure that most people on here know a lot more about all of this than i do....

11-09-2003, 12:34 PM
Originally posted by Lynx3d
Er...on PC HyperVoxels get multithreaded great, better than most geometry-only scenes :confused:

On the Mac, set to surface or sprite it will slow down with more than 1 thread -

11-09-2003, 01:33 PM
Oh...possible, i rarely use surface HVs and Sprites usually render quickly enough for me anyway :D

I don't know if reducing segment memory helps, just tells LW to render it in segments of which each takes less RAM obviously. It's like rendering two (or more) smaller images instead of one large.
Hoverever if it's currently at say 32MB reducing it to 16 would rarely help fitting the rest of the scene into the ram if it takes >500MB...

Does the HD work during rendering with multiple threads?
Also, did you use large image maps or shadow maps?

11-09-2003, 02:07 PM
I set up a different scene today, pretty simple one actually, just to see if i could figure out some of this stuff out....i was able to get some results that were much better than the other scene:

1 thread: 42.6 sec/frame
2 threads: 33.2 sec/frame
4 threads: 33.7sec/frame
8 threads: 35.4 sec/frame

i am not quite sure why it slowed down at 4 and 8 threads, but at least i was happy that it rendered better when set to more than one thread. the cpu usage seemed to be better and more equal also.

there must have been something in the other scene that was slowing it down. anyways, whatever it was, it looks like it was just my mistake. thanks for all the comments.

11-09-2003, 02:36 PM
I said it in a different thread, but I'm doing a scene at the moment. With 1 or 2 threads it takes around 10 seconds to render.

With 4 threads it takes 6 seconds, almost half the time.

Dual 2GHz G5
2.5 gigs of RAM

By the way, I think 2.5GB of RAM is a good amount for the G5. Lightwave 7.5c occupies a maximum of 2GB of RAM. The OS can take the other 500MB.

11-10-2003, 09:47 AM
I'm seeing a similar scenario with my new Mac too... a scene I'm working on renders in 4+min (on a DP G5 with 2 gig RAM) with 4 threads and 41 seconds with 1 thread.

Since I got the new machine, I've seen a lot of my complex scenes do this, though both processor bars on the CPU monitor jump up...so I've no complaints. Render times are still fantastic compared to my last DP G4, so Im still pleased with the machine.

11-10-2003, 02:28 PM
Yeah, that is what seems to be the case with some of my scenes.....like I said, some render faster with more threads...some don't.....I guess that is just how it goes though.

And like you said though, the render times are still fantastic no matter what, so I don't really have anything to complain about. I was just wondering if there was anythign i was doing wrong...but that doesn't seem to be the case.