When does Bit-Rate bottleneck VRAM?

[Gottagame9000]

For instance i've heard that 128bit (on graphics cards) is only good with a VRAM of 1-2Gb, 256bit= 2-3 Gb etc. ?

[thekeemo]

it just affects speed but not much else

[AustinKZombie]

Its the interface technology the more bits it can do per second.

[Deletive]

Depends on the architecture.

[Guest]

That's not exactly bit-rate, it's a memory bus we're talking about here. Also, it's not as simple as you might think. Older GPUs have had a pretty wide memory bus already (GTX 480 = 384bit, GT8800 256bit). It only makes sense to directly compare the memory bus of GPUs from the same architecture, a broad statement like you posted doesn't work.

[MegaDave91]

General rule of thumb is 125 bit per 1gb.

[Guest]

General rule of thumb is 125 bit per 1gb.

 

GTX 480 = 1536MB of memory on a 384bit bus

 

GT8800 = 512MB of memory on a 256bit bus

 

It's not that simple.

[D2ultima]

General rule of thumb is 125 bit per 1gb.

That rule of thumb makes no sense...

 

OP, the end-result is memory bandwidth. If you have not enough memory bandwidth to fill an overly large buffer, you have a problem.

 

Technically, I would not put more than 2GB of vRAM on a 128-bit memory bus, as you'd need a very fast memory clock to make good use of more than that. 256-bit memory buses can deal with 4GB and over if their memory clocks are fast enough. If your card has a 192-bit memory bus (or higher) and 5000MHz memory clock (or higher), then you probably won't be bottlenecked at all.

 

It's very difficult to determine an actual "bottleneck" for vRAM really... but the full story is only told by memory bandwidth in total, not clock speed or memory bus alone.

 

To calculate memory bandwidth, multiply the effective clock speed by the memory bus then divide that by 8. The number should be in Megabytes per second.

Example: 6000MHz memory clock on GTX 680 * 256-bit memory bus / 8 (cuz 8 bits = 1 byte) = 192,000. Then divide by 1000 for Gigabytes per second, so 192GB/s. If you check nVidia's specs for this card, you'll see that's what they list the bandwidth as.

 

If you need more info, you can click the vRAM information guide link in my signature.