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Report: NVIDIA GeForce GTX 550 Ti and Asus DirectCU TOP
by Damien Triolet
Published on April 27, 2011

Gains with the 192-bit bus
To get a handle on the gains given by the singular memory architecture introduced with the GeForce GTX 550 Ti (1 GB of memory on a 192-bit bus), we clocked the Sparkle GeForce GTS 450 Calibre at the same clocks as the GeForce GTX 550 Ti. This gave us a comparison between two solutions that differ only in terms of their memory bus, one 128-bit bus and one 192-bit bus, and allowed us to check if the additional bandwidth on the GeForce GTX 550 Ti was making much of a difference.

The gains vary a great deal from one game to another. There’s absolutely no gain at all in BattleForge for example, but in Metro 2033 we measured a 20% gain. The gains with antialiasing were also bigger than without, which makes sense as this mode is more demanding in terms of memory bandwidth and suffers less from the pixel fillrate limitation associated with Fermi architecture.

To recap, a bottleneck limits the amount of data that can be transmitted from the processing units to the ROPs and memory controllers. Only 2 simple (non HDR) pixels can be transmitted per SM and per cycle, which makes it difficult to exploit all the available memory bandwidth fully. Using antialiasing however doesn’t limit this throughput as the additional load comes at ROP level. This is why the major benetits of the GeForce GTX 550 Ti 192-bit memory bus come with antialiasing.

We are nevertheless a long way from the theoretical gain of 50%, partly because of the limitation mentioned above and partly because GPU processing power is a bigger limiting fact in many situations. In other words, the memory bandwith available on the GeForce GTX 550 Ti is somewhat disproportionate to the power of the GPU.

Given the resolutions tested, we do however reckon that the asymmetric organisation of the memory isn’t a disadvantage compared to the standard use of a 192-bit bus.

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