If you typically follow GPU performance as it related to gaming but have become curious about Bitcoin mining,you’ve probably noticed and been surprised by the fact that AMD GPUs are the uncontested performance leaders in the market. This is in stark contrast to the PC graphics business,where AMD’s HD 7000 series has been playing a defensive game against Nvidia’s GK104 / GeForce 600 family of products. In Bitcoin mining,the situation is almost completely reversed — the Radeon 7970 is capable of 550MHash/second,while the GTX 680 is roughly 1/5 as fast.

There’s an article at the Bitcoin Wiki that attempts to explain the difference,but the original piece was written in 2010-2011 and hasn’t been updated since. It refers to Fermi and AMD’s VLIW architectures and implies that AMD’s better performance is due to having far more shader cores than the equivalent Nvidia cards. This isn’t quite accurate,and it doesn’t explain why the GTX 680 out of the water.

The first reason AMD cards outperform their Nvidia counterparts in BTC mining (and the current Bitcoin entry does cover this) is because the SHA-256 algorithm utilizes a 32-bit integer right rotate operation. This means that the integer value is shifted (explanation here),but the missing bits are then re-attached to the value.In a right rotation,bits that fall off the right are reattached at the left. AMD GPUs can do this operation in a single step. Prior to the launch of the GTX Titan,Nvidia GPUs required three steps — two shifts and an add.

We say “prior to Titan, ” because one of the features Nvidia introduced with Compute Capability 3.5 (only supported on the GTX Titan and the Tesla K20/K20X) is a funnel shifter. The funnel shifter can combine operations,shrinking the 3-cycle penalty Nvidia significantly. We’ll look at how much performance improves momentarily,because this isn’t GK110′s only improvement over GK104. GK110 is also capable of up to 64 32-bit integer shifts per SMX (Titan has 14 SMX’s). GK104,in contrast,could only handle 32 integer shifts per SMX,and had just eight SMX blocks.

Source: www.extremetech.com