I did some benchmark testing to see how SMT on/off affects rendering speed.
Just FYI:
AMD = SMT (Simultaneously Multi Threading)
Intel = Hyper Threading
So based on the render results from my benchmark tests my understanding of how SMT/Hyper Threading works might have been wrong.
First here are the test results using the TG4 Benchmark Scene:
Terragen version | SMT ON | SMT OFF | Diff
Terragen v4.4.67 | 3:37 | 4:11 | 34s
Terragen v4.4.46 | 3:23 | 3:58 | 35s
I attached some screen shots showing the results with SMT ON. Wasn't taking screen shots with SMT off because I didn't know I was going to post this.
Those results are based on just one scene though. Further testing with scenes with more water and more complicated clouds could produce different results, but I have some guesses on how SMT/Hyper Threading works but I don't know how to test (maybe a programmer could make a test).
But basically my gut is telling me even though each and every core is split into two threads when SMT is on, it may be that if only one thread on a core is being used (and at 100%), that core might still be getting 100% utilization. Windows 10 Performance Monitor might be just providing a visual representation that is not quite accurate. This is just a guess, but if a programmer could make a test that specifically targets specific threads, and leaves others unused this could determine the result I am looking for.
For example, if you could on my 24-core CPU target Thread 0,2,4,6,8,10,12,14,16,18,20,22 as the group of threads to use but leave the odd threads unused. I would expect if my theory is correct that the rendering results would be similar to if SMT was OFF which just a small performance decrees (not 50% less performance).
I also captured a Perf Monitor shot (attached) of when I was using RTP with the TG4 Benchmark scene. Notice how the first 32 threads are at 100%, and the rest are somewhere around 80%. When I had SMT off I saw the same representation but with 16 threads at 100% and the remaining 8 at about 80%
So for now I think I am going to leave SMT on, and if I ever encounter any scenes that have really long hangs on a few straggler buckets I'll try and negate that by rendering upside down.
-Derek