The majority of the work done by a CPU is integer-based, but for certain applications, floating-point calculations are very important. This week, AMD has revealed some information on how it plans to balance the two workloads in its upcoming Bulldozer CPUs, and why it feels that the new cores will be more capable and more efficient than anything from the competition.
A new approach
The approach that AMD is taking has been called Flex FP. Whereas each integer unit has traditionally had its own 128-bit floating-point unit (FPU), two Bulldozer cores will share a single 256-bit FPU. Since there are no 256-bit commands - only 32- or 64-bit - developers will be able to use the new AVX instruction to process eight 32-bit or four 64-bit commands per clock.
Obviously AVX isn't expected to see very wide adoption right off the bat - a fact that the chip-maker knows - and this is where the flexibility really comes into play. As mentioned, the majority of the hard-work in a CPU is carried out by the integer units, meaning that one very large, power hungry FPU would be likely to sit idle for a lot of the time. On the other hand, lots of smaller FPUs would create a bottleneck whenever there were a lot of floating-point operations.
Learn to share
Flex FP can dynamically share its resources between the two cores as the workload dictates on a per-clock basis. This means that one core might be performing a 256-bit AVX operation in one cycle while the next cycle could see both cores carrying out simultaneous 128-bit FMUL operations. According to AMD's John Fruehe, this will provide better performance and greater efficiency, meaning that the CPUs will use less power than competing solutions.
The other change to the Bulldozer FPUs is a dedicated scheduler that's unlinked from the integer scheduler. This should again allow for more efficient use of resources with lower latencies.
Obviously these changes will only see a real benefit in server and workstation applications. However, if the performance increases are as significant as AMD claims, they could pose serious competition to Intel's next generation Xeon CPUs.
Bulldozer-based CPUs for the server, workstation and high-performance desktop markets are expected to arrive sometime in the second or third quarter of next year.
