Rendering Speed Issue [example comp]

Hello there..

For a few years now I know a speed issue, which is very prominent to me when working with Optical Flow tools and a total frame window beyond 3 or 5. I'm aware that caching might be slower, at a point, when all memory is already in use by Fusion, but this is pervasive on all compositions here in different flavours.

What I did to recreate this in a smaller scope is this composition file:
http://scientific-media.de/_exchange/bmdf/Fusion_SpeedLoss_v02.comp

On a system with 32GB RAM it was sufficient to render ~100 frames starting at frame 10. The output folder will be created right next to the comp-file and no external footage is needed.

The Issue:

The render performance is slowing down significantly, when there is no more free memory (starting @ image 43) to use right away, so it might be related to that or certain tools, that the speed (timestamp difference between written images) changes over time.

Although there is no other process running, which might need larger resources too, there are also some images, which took much more time, than others (by mod-date). This can be seen by the high bars, which appear more frequently, the longer the rendering goes. This is not GUI related or at least this was rendered by the render manager.
It is interesting that the average render time per frame - shown by Fusion - is increasing mainly by these few very "slow" images, since the overall speed in the graph is not changing that much, after all memory is consumed.

It would be interesting to know, if this is a general issue and if there are things to circumvent that. Caching certain nodes i.e. wasn't helping me.

Regards,
Rainer

Hey Rainer,

I ran the Comp on my Machine Ryzen 3970x RT 3080ti 128GB Ram (~102 Available to Fusion), FU 18b5. I got a pretty consistent 8sec seconds per frame all the way through to 75 frames then slowed down to about 11.5 spf at about 105 frames and seems to have plateaued. My RAM Cache was filled up by 24 frames and my GPU Cache got to about 9/12 GB and remained pretty much there. I have been having a few issues with similar behaviour, mainly when running Resolve at the Same time or with a heavy comp. All sorts of weird behaviour where my viewers will just go black, no errors, so I just save and restart Fusion then it usually works fine. For me I usually see problems when the GPU fills up, I turned of the GPU processing but I lost quite a bit of speed doing so just have to deal with it I suppose.

The viewer turning black is probably a new fusion 18 bug. I’ve had that too.

Regarding the rendering issues. It’s gotten much better in 17 and again better in 18 but I can also confirm that Fusion will get noticeably slower over time because it keeps too much data in RAM and VRAM.

I have started rendering to the local render node as I find this is about 10-15% faster and tends to delay the onset of these problems. V18 does feel faster to me, I think its using the GPU a lot more, just be nice if they could sort out some of these issues as they really are effecting the usability of V18.

Nevertheless, it's not comprehensible to me, why a fusion comp I can playback in realtime take about 1.25sec/frame to render in 1080. It's nearly pure fusion shape animation, only one icon (.png, 1000x1000px) in the scene.
Within 1000 frames fusion occupies more then 30GB of Ram while rendering png image sequence as output via a saver node.
That is ridiculous.