What if not enough VRAM to meet DCS demand?

[imacken]

With my new 3090, I am seeing VRAM demands of over 12Gb on maps like The Channel. I was just wondering, what happens when DCS asks for more RAM than is available on a GPU card? What does it do when told there isn't enough? Is system RAM used as some kind of buffer and swapping goes on?

Just curious.

[DimSim]

When the VRAM limit is exceeded, the GPU driver will move some resources from the GPU to CPU memory leaving enough VRAM for that frame. If there was not enough VRAM even for a single frame, it can swap memory between CPU and GPU memory multiple times per frame killing performance.

 

While a GPU may have 12GB and CPU may have 32GB or such, no games or applications attempt to use all of that at the same moment, if they did, performance would be terrible, instead, resources move from disk to RAM to VRAM in ways that are hopefully intelligent and intentional. When unintentional, the operating system will move some resources around, but that tends to happen at undesirable times causing stutters and jerky frame rate. You'll hear the term 'paging' for that.

 

Since you have one of the best video cards and systems money can buy, I expect you won't experience this much. If you do, turn some settings down. Some games, or game mods, do not prepare their texture resources well and can use excessive amounts of memory. So don't expect to run all games at max settings and resolutions even with the best hardware.

[imacken]

Thanks for that. I'm not experiencing any issues myself, I was just curious to know what happens when the application demands more VRAM than is available, Does it look at the amount of VRAM first and then allocate and compromise, or does it make the allocation first and then the system decides how to apply that allocation?

If the latter, then one would think that a considerable decrease in performance would occur say with a 8gb card when 12Gb was needed.

[DimSim]

It can do either (check available, or just use). Depends on the API (DirectX 11, 12, OpenGL + Extensions, Vulkan). Some APIs or extensions expose available VRAM others don't. Often games just target a minimum or recommended spec, then allow the user to turn settings up or down if they experience issues.

 

Windows PCs and GPU drivers (from intel, nVidia, AMD) are generally graceful about moving memory around best they can. VRAM can be consumed rapidly with very high resolution displays, high res textures or pre-loading lots of models and textures (called graphical resources).

 

Both GPU and CPU can 'thrash' if they exceed usable RAM. Thrashing means rapidly and repeatedly moving memory to where it is needed because what is needed can't fit (typically in what is called a cache). When that happens, the user will see a sudden and significant performance decrease. Users should never have to worry about any of this, but a general appreciation of what's going on can help them isolate issues and make better guesses at what settings to change based on observed behavior.

 

In general, more RAM (System or VRAM) is better, but it won't make things faster. Instead, having not enough will make things slower.

[imacken]

Thanks for that detailed explanation. I appreciate it.

[imacken]

I never understand why these posts get moved to what I would consider to be inappropriate sub-folders. I mean, why is this moved to a 'Bugs' area when there is no bug? It was just an enquiry about how VRAM gets allocated in applications like DCS.

[twistking]

I never understand why these posts get moved to what I would consider to be inappropriate sub-folders. I mean, why is this moved to a 'Bugs' area when there is no bug? It was just an enquiry about how VRAM gets allocated in applications like DCS.

i would argue that dcs vram mangement is a bug in itself, therefore you were discussing a bug ;)