PDA

View Full Version : Multithreading Question



lwanmtr
02-19-2013, 06:21 PM
I've read some people having 12 cores but using 24 threads. Does this really improve render speed?

I have a MacPro with 8 cores...would I really see a big difference by setting to 16 threads?

Thanks

Celshader
02-19-2013, 06:46 PM
I've read some people having 12 cores but using 24 threads. Does this really improve render speed?

I have a MacPro with 8 cores...would I really see a big difference by setting to 16 threads?

Thanks

I do not have a MacPro, but I know of a good test for your machine. Try rendering the LW11.5 BenchmarkMarbles scene (http://forums.newtek.com/showthread.php?133251-11-5-s-BenchmarkMarbles-lws-share-your-machine-s-render-time-here) with 8 threads and with 16 threads, and see if a meaningful difference exists between the two render times.

lwanmtr
02-19-2013, 06:48 PM
Thanks, I'll give that a shot. I havent delved too deep into the 11.5 content, though 11.5 is now officially in my work pipeline.

JonW
02-19-2013, 07:32 PM
My Box, PC, but it's the same CPU family used in a Mac Pro.

W5580 x 2 CPUs in the box. Each CPU has 4 real cores & 4 hyper threading cores. If I turn off HT & only use the 8 real cores (2 CPUs x 4 cores), typically a render is about 25 - 30% slower. But there is one bonus, the computer is more responsive with single core work. Like Modeler & Layout when not rendering. The computer runs at about 13% efficiency (using 1 of the 8 cores).

If I have HT on so now there are a total of 2 CPUs x 4 real cores plus 2 CPUs x 4 HT cores, a total of 16 cores. The computer renders quicker. But single core work drops to 6 - 7% (using 1 of the 16 cores). I do notice a slow down on single core work.


Layout Threads, for my boxes I leave Multithreading on Auto. So whatever the total number of cores you are using on the computer, use the same number of threads in Layout. I did try using a higher number of threads but a render was a smidgen slower.


Just make sure if you have HT that the HT cores are being used when you want the fastest possible render. There has been the odd case in this forum where people have not realised that HT was off.

On the other hand if you are barely doing any rendering & all you programs can only use, say up to 4 cores for example, turn HT off.

lwanmtr
02-19-2013, 08:39 PM
Hmm... Not sure how you set HT on Mac OS.

jrandom
02-20-2013, 11:16 AM
Hyperthreading is enabled by default on a mac. In fact, I'm not even sure if there's a way to turn it off.

I was about to say that using 1 LW thread per hyperthread gives you the best performance, but then realized I've never actually measured this. I'll do a test when I get home, but I'll bet real donuts using all hyperthreads is faster. :)

(I've got the 12-core/24-hyperthread Mac Pro.)

jwiede
02-21-2013, 10:29 AM
I've done the test Celshader suggested, and using just 12 threads does indeed result in substantially longer render times than using 24 threads (on a 12c/24ht MacPro). Booting Windows on the same hw, and manually disabling hyper-threading showed similar proportionate increase in render times. I get slightly faster overall render times on MacOSX versus Win X64 normally anyway, but the percentage difference ht on vs off was about the same.

jrandom
02-21-2013, 10:49 AM
Oh good, that means I don't have to do that same test I said I would that I completely forgot about. :)

lwanmtr
02-21-2013, 02:12 PM
I started running a test, but then got distracted playing with FFX...lol