What is the optimal size for GTT memory?

[chrisr]

Hi,

I have a number of AGP radeons and have been digging into their memory settings lately, since 2.6.35.x now allows full access to all VRAM. One of my PCs has a RV350 card with 256MB of VRAM, and is currently using 128MB for AGP. This 128MB seems to correspond to the GTT memory, which the radeon drivers says it uses for a "ring" (?! As in "ring buffer"?) However, the BIOS would allow me to increase this memory from 128MB to 256MB. So my question is: would it be worth setting the GTT memory to 256MB, please? Is more GTT memory always better? Or would I be trading the increased GTT memory for less of something else?

A quick study of the memory space suggests that this motherboard does have enough room for 256MB of GTT memory without overlapping the VRAM:

radeon 0000:01:00.0: GTT: 128M 0xC8000000 - 0xCFFFFFFF
[drm] Generation 2 PCI interface, using max accessible memory
radeon 0000:01:00.0: VRAM: 256M 0xE0000000 - 0xEFFFFFFF (256M used)

Thanks in advance for any advice.

[marek]

The more GTT memory, the better. The TTM memory manager may go crazy if it doesn't have enough GTT memory.

[chrisr]

Which is the theoretical upper limit?

The more GTT memory, the better. The TTM memory manager may go crazy if it doesn't have enough GTT memory.

So what's the limit beyond which it's pointless to try and go beyond, please? The settings offered in the BIOS? (An obvious hard limit .) The amount of VRAM on the graphics card? Or a certain percentage of the amount of physical RAM in the PC itself?

[marek]

My two r500 PCIe GPUs have VRAM=GTT=256MB (512MB in total) and VRAM=GTT=512MB (1GB in total), respectively. It depends on what 3D apps you want to use. I guess 64MB was ok in 2002, but todays apps are much more demanding. 256MB seems enough to me.

[agd5f]

GTT memory is system memory that can be accessed by the GPU. On AGP cards it's handled by the northbridge and the size is set by the AGP aperture size in the bios. On PCI/PCIE cards the GPU uses an on-chip gart (graphics address remapping table) mechanism and the size is configured by the driver; currently it's set to 256 MB, but could be set larger. Note that this is just the aperture size (i.e., the max amount of system memory that can be addressed by the GPU at one time). Pages are mapped into the aperture dynamically as needed, so the ram is not "wasted" when it's not in use by the GPU.

The max size of the AGP aperture is limited by the northbridge; for PCI/PCIE, it's limited by the on chip gart mechanism.

For some buffers it's advantageous to store the content in system memory rather than VRAM, in other cases, system memory can be used for spill-over when VRAM is exhausted.

[RealNC]

Note though that in the spill-over case, when it happens, the results aren't really useful. A 3D game would become pretty much unenjoyable. Because of this, and at least on Windows, games do not do this and won't even allow it.

I suppose there are workloads that can use/benefit from GTT memory. I've no idea what they are

[agd5f]

Note though that in the spill-over case, when it happens, the results aren't really useful. A 3D game would become pretty much unenjoyable. Because of this, and at least on Windows, games do not do this and won't even allow it.

I suppose there are workloads that can use/benefit from GTT memory. I've no idea what they are

GTT memory is used more than you think. In most cases it's up to the driver rather than the app where resources are stored and the driver will put them where they make sense depending on the conditions at hand.

[pvautrin]

Sorry but does this mean that the advice "the more GTT, the better" also applies to AGP?
Some websites actually recommend to set the AGP aperture lower for cards with a lot of VRAM.

I have desktop with HD3650 AGP (512MB VRAM) and Athlon64 3200+ w/ 1GB RAM

[agd5f]

It's less of a factor with lots of vram, but still important.

[chrisr]

So in that case...

GTT memory is system memory that can be accessed by the GPU.... Pages are mapped into the aperture dynamically as needed, so the ram is not "wasted" when it's not in use by the GPU.

256MB of GTT memory it is... .

Thanks.