I have scenes that render very slow in newer Blender versions. They sometimes just use round about 10% of the cpu.
I couldnt find out what exactly is causing it, but i made some observations that might help find the problem.
the bigger the render the more obvious the effect gets ( i sometime have the impression that it is rendering fast for round about 1 second and the slows down.
Blender version 2.69 (2.69.0 - r60995) is rendering fast with 100% cpu usage.
as I mentioned I was not able to find the reason, not the single button, but i striped down one of the problematic scenes.
here it is.
the speed loss is quite drastic and the “bug” was not fixed in the last versions of blender. so i think this is either already known (also I couldnt find it) or it is quite relevant.
Hm. Don’t see that here… I changed the following things in your file:
a) Increased the number of diffuse samples.
The switch from one tile to the next happens so fast with only 1 sample, that any performance monitor might not be able to show accurate CPU loads.
b) Made sure all settings are truly identical.
For example: When opening the file in 2.69 it was set to a tile size of 64 x 64, but with 2.73a it came in with 32 x 32. Also: 2.73a has clamp indirect set to 1.0 - this setting did not even exist in 2.69.
Result:
100 % CPU load in both versions. Render times:
2.69 = 00:43.54
2.73a = 00:41.79
[QUOTE=IkariShinji;2807826]The switch from one tile to the next happens so fast with only 1 sample, that any performance monitor might not be able to show accurate CPU loads.[QUOTE]
I have a i7 980 6core. With 2.73a I stopped the rendering with one diffuse sample after 10 min. with something about over one hour left of rendertime.
In 2.69 with the same settings (I have 64pix buckets where ever i open it) it took a bit over 15 seconds to render the whole scene.
I have no idea what causes this but it is a huge problem.
It occurs in some scenes and not others.
maybe others could also try to test it?
thanks a lot
best regards
Jops
Ah! I opened your file without “Load UI”… I just tried again with “Load UI” and I see the same slowdown.
It seems to be the histogramm (T-key) panel on the left side of the Image Editor: If it’s open while you render, the CPU has to permanently calculate and update the data in there. Close the T-panel and see the tiles fly…
As for the changing tile size issue: I totally forgot about the auto tile size addon - sorry for the confusion!
…unless the refreshing of those displays is just a single-threaded process and the other cores have to wait for it to finish after each tile is rendered. Just guessing of course…
I just tested it to check if I experience the same. IkariShinji is right, it’s the T-key panel on the left. With it open I gave up rendering after a while, after closing it I rendered the image in 32.00 seconds (31.34 after switching to tile size 128 x 128).
