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GPU vs CPU


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Hi guys

 

I’m working on a GPU vs CPU rendering comparison which will be the subject of a podcast I’m working on with my colleagues. Every day I work as a support guy on a cloud render farm and I have my experiences with both CPU and GPU rendering (and I’ve seen experiences of our customers) but I’m very curious about your opinion.
 
If you use GPU render engines what are the reasons? The most common reason I’ve heard about is speed, but of course when you have access to more than one card or processor then I guess it’s rather speed\money.

 

What is interesting for me is are there any other reasons for using GPU render engines? Do they give a different, maybe cooler look, any features which CPU engines lack or type of subjects where they give better effects?

 

I’m very curious about your opinions and impressions when it comes to GPU rendering. What are the pros and cons?

By the way – when it comes to GPU unbiased engines – the GI and glossy reflections\materials highlights are computed as the same thing, right? For example, in CPU V-Ray GI and material reflections are visible on two separate layers\passes. 

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1 hour ago, BobGF said:

What is interesting for me is are there any other reasons for using GPU render engines? Do they give a different, maybe cooler look, any features which CPU engines lack or type of subjects where they give better effects?

 

Not that I'm aware of. If a GPU based engine has some special feature, it is most likely a feature at the software level, and not the hardware level.

 

1 hour ago, BobGF said:

 

I’m very curious about your opinions and impressions when it comes to GPU rendering. What are the pros and cons?

By the way – when it comes to GPU unbiased engines – the GI and glossy reflections\materials highlights are computed as the same thing, right? For example, in CPU V-Ray GI and material reflections are visible on two separate layers\passes. 

 

I've played around with the trial version of RedShift, and I have also been messing around with the GPU feature for Arnold. Arnolds GPU feature is still being developed and is lacking, so it's not anything that I would use right now for work. When switching from CPU to GPU, the Arnold sampling settings change drastically. When using CPU, you can control the sampling separately for diffuse, specular, volume, camera, etc. If you switch over to GPU, you can only control camera sampling.

 

I've thought about making the switch over to GPU rendering, but the cost right now is too great. A machine with 3 or 4 RTX GPUs is not going to be cheap.

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I use GPU render engines purely for the fact that they are insanely fast an none of my scenes were a problem with VRAM thus far, which is of course the main drawback of GPU render engines; the limited VRAM.

I used Octane for a couple of years but recently bought Redshift. I am super, super impressed by the speed of it. It practically flew through everything I've thrown at it so far, even with volumetrics, lots of tiny lights, etc.

 

I don't think any GPU render engine has any special feature that CPU render engines don't have, or at least I haven't heard of it yet. It's more the other way around. I'd say most people use CPU engines because they are more flexible and GPU because they are faster. If you want to render very, very big scenes with tons of geometry and lots and lots of textures you can't just use a GPU render.

 

 

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One of the major cons of GPU rendering is the limitation of the VRAM - that means your scene can only be rendered on GPU if the size of your scene with all textures and assets doesn't exceed your GPU's VRAM, otherwise you will get an errror. My VRAM for example is 8GB, but my scenes easily go over that very quickly. Especially if you are using a lot of Megascan textures or many different textures that are not procedurally generated within a node system the size gets pretty large. Redshift has worked around this by making use of loading textures from the harddrive in case the VRAM is full, but I heard that this makes the render almost as slow as CPU would be.

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5 minutes ago, HippoDasTamus said:

One of the major cons of GPU rendering is the limitation of the VRAM - that means your scene can only be rendered on GPU if the size of your scene with all textures and assets doesn't exceed your GPU's VRAM, otherwise you will get an errror. My VRAM for example is 8GB, but my scenes easily go over that very quickly. Especially if you are using a lot of Megascan textures or many different textures that are not procedurally generated within a node system the size gets pretty large. Redshift has worked around this by making use of loading textures from the harddrive in case the VRAM is full, but I heard that this makes the render almost as slow as CPU would be.

 

I overshot my VRAM once so far in Redshift. I don't remember what I did exactly, but it was... very very slow.

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Just now, DasFrodo said:

 

I overshot my VRAM once so far in Redshift. I don't remember what I did exactly, but it was... very very slow.

Yes, considering the speed of GPU comes largely from it's dense access to the scene data directly within it, that makes sense

 

I would say that GPU is not looking different than CPU, but in some cases CPU can look different than GPU; At least with my Arnold renderer the GPU sampling cannot be set manually for everything like in CPU. So with CPU I have access to tweaking not only AA, but also Diffuse, Transmission, Subsurface Scattering samples, whereas in the GPU I only have access to the AA Camera samples. I'm not sure if this still applies to all GPU engines, at least two years ago that was a problem with every GPU renderer.

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--------------------CPU render engines--------------------

PROS:

 

1) They have more buttons to push (functionality/flexibility wise), while "most" GPU render engines are playing catch up.

2) They can benefit from large pools of RAM (64 - 128 - 256 - 512 - 1024+) GB, while GPU render engines are limited by what they come up with (Quadro RTX 8000 have 48GB of memory, to add more you need to buy another GPU + NVlink, compared to a pair of RAM sticks).

 

CONS:

 

1) They are slower then GPU render engines.

   - Threadripper 3960X 24 cores ($1.399) is slower then RTX 2060 ($319).

   - Threadripper 3970X 32 cores ($1.899) is slower then RTX 2070 SUPER ($499).

   - Threadripper 3990X 64 cores ($3.449) is trading blows with RTX 2080Ti ($1.499) and slower then Titan RTX ($2.499).

Source: https://opendata.blender.org/

 

--------------------GPU render engines--------------------

 

PROS:

1) They are faster then CPU render engines.

2) With the introduction of RTX technology they are gaining unique functionalities that aren't available for CPU render engines (more dedicated hardware for specific task acceleration, RT cores and Tensor Cores).

3) Real time rendering ! (EEVEE, U-Render, Project Lavina, Redshift RT, OTOY Brigade).

 

CONS:

1) They are still playing catch up to what CPU render engines have access to (for now...).

 

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1 hour ago, Voidium said:

--------------------CPU render engines--------------------

PROS:

1) They have more buttons to push (functionality/flexibility wise), while "most" GPU render engines are playing catch up.

 

--------------------GPU render engines--------------------

CONS:

1) They are still playing catch up to what CPU render engines have access to (for now...).

 

 

Could you offer some specifics on the two points above?  

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41 minutes ago, MikeA said:

 

Could you offer some specifics on the two points above?  

CPU render engines tend to be more flexible on many, many things. Not only when it comes to scene sampling of specific effects, but also on the materials side since it's apparently WAY harder to implement certain features on a GPU than on a CPU. When Octane came out it did not have Subsurface Scattering, for example.

Having said that, many GPU renders are catching up nowadays. Redshift has a lot of settings for all kinds of optimization and scene setup. Octane on the other hand is just different, you basically only have ray depth for reflection, refraction and volumetrics and ONE global sample amount. This is why many artists that don't like the technical side of 3D work prefer Octane over other render engines. They more or less have one quality slider and that's it. Compare that to how the settings in V-Ray look and you know what's up.

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Ok - interesting. I don't know Octane - but I do know Redshift : )  That's my daily driver.  That has plenty of control as far as I'm concerned.... plenty... 

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The Pros of CPU:

Over all very robust and stable. 

RenderFarms for CPUs are easier to find and cheaper

Old Computers qualify much easier as rendernodes as you can upgrade RAM easily and for little money.

You can render out many different passes with just one rendering process. (Many GPU renderer are much more limited in this area)

It is easy to include or exclude Objects from lights, shadows or reflections.

easier to get unrealistic artificial looks.

Easy accessible as many 3D software Packages have a production ready CPU Renderer included.

Files easy to give away as render engine is included in Software. (if the GPU Renderengine is included it is the same of corse)

 

The Pros of GPU:

Fast Preview and interactive working

global ilumination always included.

much less to think about.

much easier to get realistic results.

easier to learn for 3D Beginners.

a lot of renderpower can be included into just one computer.

computers can be upgraded with GPUs very easily.

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4 minutes ago, MikeA said:

Ok - interesting. I don't know Octane - but I do know Redshift : )  That's my daily driver.  That has plenty of control as far as I'm concerned.... plenty... 

It does! But it doesn't come close to V-Ray, Arnold and others...

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