Error: System is out of GPU and shared host memory

As the error message implies, you are out of memory. When trying to render this scene on my computer, also with 64GB of RAM, it fills it all up plus an extra 30GB of SWAP (Pagefile) before I decided to close Blender. That means your scene takes up 90GB or more when trying to render, this easily exceeds the 8GB of VRAM + 64GB of RAM and probably the pagefile you have, and that's why you get that error.

As for what's causing it, I did some testing and this is what I observed:
The cause for this issue is the materials used by objects. Specifically the StarlightAtmosphere node group from the Physical Starlight and Atmosphere add-on. The node group is rather complex, and as such it takes up a bit of RAM. However, this isn't the issue, the amount of RAM is probably measured in tens of megabytes at the most. The issue is that you've re-used the node group a lot. You've given the node group to every material in the scene meaning that it now has to be stored for every material, which means more RAM is taken up. But once again, this probably isn't the issue, it's probably only going to take up hundreds of megabytes at the most. The issue is that you gave this node group to the "Grasswald" objects, the ones scattered using a particle system onto your landscape. The node group takes information about object position and distance from the camera and such that needs to be re-computed every time the Grasswald objects are scattered. Which is 472,024 times. And these calculations have to be stored in RAM. So although the node group only takes up a couple megabytes at most, it has to be duplicated 472,024 times, which means it takes up lots of RAM and results in the issue you're experiencing.

Removing the StarlightAtmosphere node group from the Grasswald objects resolves the issue with running out of memory. As such, I do not believe this is a bug and this report can probably be closed? I will wait for a developer to decide on this.

In theory Cycles or parts of Blender could be optimized to reduce the RAM usage, but I believe that falls under the catagory of "accidental features request" which we usually respond to with this message:

Thanks for the report, but the issue reported here is a request for modified/improved behavior and not a bug in current behavior. Closing as this bug tracker is only for bugs and errors.

For user requests and feedback, please use other channels: https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests

For more information on why this isn't considered a bug, visit: https://wiki.blender.org/wiki/Reference/Not_a_bug

Thank you @Alaska (Alaska) for your clear analysis. I can see how this would end up creating a unique problem that's not exactly a bug. However, I also really appreciate you going through the process of figuring this out so I can try to clean it up.

In T90440#1201991, @Alaska (Alaska) wrote:

Error: System is out of GPU and shared host memory

As the error message implies, you are out of memory. When trying yo render these scene on my computer, also with 64GB of RAM, it fills it all up plus an extra 30GB of SWAP (Pagefile) before I decided to close Blender. That means your scene takes up 90GB or more when trying to render, this easily exceeds the 8GB of VRAM + 64GB of RAM and probably the pagefile you have, and that's why you get that error.

As for what's causing it, I did some testing and this is what I observed:
The cause for this issue is the materials used by objects. Specifically the StarlightAtmosphere node group from the Physical Starlight and Atmosphere add-on. The node group is rather complex, and as such it takes up a bit of RAM. However, this isn't the issue, the amount of RAM is probably measured in tens of megabytes at the most. The issue is that you've re-used the node group a lot. You've given the node group to every material in the scene meaning that it now has to be stored for every material, which means more RAM is taken up. But once again, this probably isn't the issue, it's probably only going to take up hundreds of megabytes at the most. The issue is that you gave this node group to the "Grasswald" objects, the ones scattered using a particle system onto your landscape. The node group takes information about object position and distance from the camera and such that needs to be re-computed every time the Grasswald objects are scattered. Which is 472,024 times. And these calculations have to be stored in RAM. So although the node group only takes up a couple megabytes at most, it has to be duplicated 472,024 times, which means it takes up lots of RAM and results in the issue you're experiencing.

Removing the StarlightAtmosphere node group from the Grasswald objects resolves the issue with running out of memory. As such, I do not believe this is a bug and this report can probably be closed? I will wait for a developer to decide on this.

In theory Cycles or parts of Blender could be optimized to reduce the RAM usage, but I believe that falls under the catagory of "accidental features request" which we usually respond to with this message:

Thanks for the report, but the issue reported here is a request for modified/improved behavior and not a bug in current behavior. Closing as this bug tracker is only for bugs and errors.

For user requests and feedback, please use other channels: https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests

For more information on why this isn't considered a bug, visit: https://wiki.blender.org/wiki/Reference/Not_a_bug