Author Topic: Terragen GI cache blending and Multithreading  (Read 552 times)

Offline Oshyan

  • Planetside Staff
  • *
  • Posts: 11796
  • Holy snagging ducks!
Re: Terragen GI cache blending and Multithreading
« Reply #15 on: December 02, 2017, 08:33:04 PM »
That is incredibly strange! My first thought when you mentioned flipping the render upside down was that the hardest work for the renderer to do was at the bottom of the image, but by the time the render process gets down there, it has fewer areas of work for render buckets than the amount of render work you have left to do. In other words you end up with a few render buckets - assigned to a single CPU thread each - that take proportionally longer, and thus the total render time is longer. This does happen on occasion and I have seen similar situations where flipping the camera fixes it because what then happen is these difficult buckets start rendering earlier on and can be crunching away while the rest of the image renders, and they have more time to finish before the rest are done, basically. So you put the hardest to render stuff at the beginning of the render process.

But... your situation isn't that way at all! Somehow and for some reason the *beginning* of the render is slower (lower CPU utilization) than later. Which is really... odd. This is definitely worth looking into further, and thank you for all the in-depth testing and reporting back.

- Oshyan

Offline D.A. Bentley

  • Member
  • *
  • Posts: 89
    • D.A. Bentley
Re: Terragen GI cache blending and Multithreading
« Reply #16 on: December 05, 2017, 01:45:46 AM »
Another interesting comparison was I rendered the same exact scene twice; once with Hyper-Threading on, and a second time with it off.  When Hyper-Threading was off my CPU utilization (according to windows 10) was 100% the entire render, but ironically it took about 3-4 hours longer to finish compared to the Hyper-Threading On render which was showing underutilization at times

Render with HT on took about 19 hours, and with HT Off it took 23 hours.

Not sure what to conclude from that, but that's what happened.


Offline Oshyan

  • Planetside Staff
  • *
  • Posts: 11796
  • Holy snagging ducks!
Re: Terragen GI cache blending and Multithreading
« Reply #17 on: December 06, 2017, 05:01:40 AM »
That's quite a difference. HT should add at most 20% of additional performance, and that's under ideal conditions. Typical is more like 10-15%. So that's on the edge of possibility. ;)

Remind me, are you Mac or Windows? And what OS version are you running?

- Oshyan

Offline D.A. Bentley

  • Member
  • *
  • Posts: 89
    • D.A. Bentley
Re: Terragen GI cache blending and Multithreading
« Reply #18 on: December 11, 2017, 05:42:20 AM »
Windows 10 Pro x64

Offline Oshyan

  • Planetside Staff
  • *
  • Posts: 11796
  • Holy snagging ducks!
Re: Terragen GI cache blending and Multithreading
« Reply #19 on: December 12, 2017, 02:58:19 AM »
OK, good to know. Let us know the results of any other tests. Is the scene file you're testing with the same as the one you sent us?

- Oshyan

 

anything