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I'm experimenting with fluid sims, so I've been baking and rendering the whole day. One thing I've noticed is that the I/O seems to be really slow. Like really slow. I'm only rendering at 30 samples per pixel and denoising, and when rendering an animation, it takes a couple of seconds to render the frame, then it hangs, I'm assuming reading the cache from the disk, and resumes the render. Guys I'm not kidding, the reading from the disk is twice the time it takes to render the frame. I'm on a SSD and every baked frame in the cache, between the actual sim and the noise sim (I'm working with fire at the moment) is around 35mb, so it should be very quick to load... And yet. Also the timeline performance is afwul, scrubbing on the timeline is basically impossibile if the cache is active, I have to remove it and the reload it if I want it to be workable.

I only have 16GB of RAM, so I know I won't be able to do gigantic sims, but at the moment blender is only using 1.2GB of RAM during rendering.

Is there a setting I'm missing? IS there a way to I don't know, load more of the cache in RAM or something to speed things up?

Edit: Blend file https://we.tl/t-RCntPkgsnu it's really just a test scene

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  • $\begingroup$ show us your blend file would help a lot. $\endgroup$
    – Chris
    Feb 3, 2021 at 2:22
  • $\begingroup$ @Chris done we.tl/t-RCntPkgsnu $\endgroup$
    – Paul
    Feb 3, 2021 at 5:00
  • $\begingroup$ Have you tried using a ramdisk? 🤔 $\endgroup$ Dec 13, 2021 at 21:18

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ok, although this is not an answer to your question, but i think it is important that you understand what is going on in fluid/smoke simulations.

First there is the domain, in which the whole simulation is going on. So every piece of smoke of fluid must be inside of this. It "normally" makes no sense to move or animate the domain at all (from my point of view). So why are you animating the domain?

Inside the domain you can use inflows or geometry, which is the "real" smoke or fluid which then will be animated inside the domain. You can imagine the domain as something, which is the world of fluids and all fluids can only live inside of that.

So my first advice: do not animate or move the domain at all. I never made it and maybe i am wrong - but i don't think it is a good idea to move it at all. Move the inflows or your fluid inside the domain.

Maybe just changing this will solve your lagging problems. If not, just update your .blend file without moving the domain or explain me, what do you want to achieve by moving the domain?

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  • $\begingroup$ I move the domain together with the emitter because this way I can have a smaller domain with a manageable resolution, instead of a huge domain with a high resolution in order to keep everything smooth while still achieving a movement on the flame. I know right now it has a few issues with the flame touching the walls of the domain, but I'm just testing stuff out. I don't know if it's bad practice. I have another problem now though, I opened blender again today and I loaded up the cached sim and I don't know why it won't match the geometry anymore. I didn't move the geo at all. :\ $\endgroup$
    – Paul
    Feb 3, 2021 at 13:41
  • $\begingroup$ did you ever try to make the domain just a big as you need it? so that all your animation is inside, but width, height and length just as long as you need it? $\endgroup$
    – Chris
    Feb 3, 2021 at 13:44
  • $\begingroup$ I actually haven't. Tried it now, with adaptive domain on, the performance it's slightly better. I noticed that with just the basic bake, performance is ok (15fps, not full 24 but better than 1fps), but when I add the noise bake, it goes down to 3. It also varies with the size of the domain, when it shrinks it goes faster but that's kind of understandable. Maybe I'm just overestimating my pc? but still I wonder if there's a way for it to go faster. $\endgroup$
    – Paul
    Feb 3, 2021 at 14:18
  • $\begingroup$ nearly all fluid and smoke simulations in good quality need really much time. This is normal. Of course with faster gpu's/computers it is a bit faster - but not that much faster. They still bake and render real slow. $\endgroup$
    – Chris
    Feb 3, 2021 at 14:21
  • $\begingroup$ Plus domain has an adaptative option, so you really don't need to move the domain arround theoretically. $\endgroup$
    – Lauloque
    Dec 13, 2021 at 21:27
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AFAIK Blender has no tools for profiling nor benchmarking exactly what happens and for how long so it's hard to tell what's going on. Are you sure it's the fluid cache provoking the long loading times and not something else like the textures loading?

Have you chosen a fast I/O file format as output? EXR or Targa are most usually the best options, others are low quality like JPG, or way slower like PNG

Also, have you enabled Persistent Data?

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