Original URL: http://www.theregister.co.uk/2006/04/18/amd_reverse_smt/

AMD said to be researching 'reverse multi-threading' tech

Multiple cores to appear as one?

By Tony Smith

Posted in Hardware, 18th April 2006 10:27 GMT

AMD is working on a way to make a multi-core processor appear to the host operating system as a single-core chip, it has been claimed. If true, the move turns on its head the drive to develop multi-threaded apps the better to take advantage of multiple cores.

The technology is aimed at the next architecture after K8, according to a purported company mole cited by French-language site x86 Secret. It's well known that two CPUs - whether two separate processors or two cores on the same die - don't generate, clock for clock, double the performance of a single CPU. However, by making the CPU once again appear as a single logical processor, AMD is claimed to believe it may be able to double the single-chip performance with a two-core chip or provide quadruple the performance with a quad-core processor.

It's the very antithesis of the push for greater levels of parallelism - performing more operations on data simultaneously, in other words - in computer processors. Intel's HyperThreading, for example, was developed to make use of under-utilised processor components to fool the CPU into believing it had two processors at its disposal, not one. Adding more cores just makes these virtual cores real, and retaining the technology allows two cores to appear as four.

Of course, better out-of-order execution techniques render HyperThreading - Intel's version of the simultaneous multi-threading (SMT) technique - less important. Put simply, they ensure there are fewer parts of the pipeline going unused at any given time, so there's less performance to be gained by throwing extra threads at the processor. Indeed, Intel's briefings on its next-generation architecture, due to debut in Q3 as the 'Conroe' chip, play down HyperThreading and talk up out-of-order execution.

But Conroe, so far as Intel is admitting, still appears as two CPUs to the host OS. So is there anything to be gained by making it appear as just one processor? Well, operating systems already do a good job of scheduling hundreds of threads on a single-core CPU let alone a dual- or quad-core part, and AMD may have found that OS is so good at this that it can make up for the apparent reduction in parallelism, particularly in cases where one thread predominates, in a game, for instance.

However, by the time the technology ships - if it proves real, and ever becomes more than a lab experiment - the software industry will have had several years focusing on multi-threaded apps, and it may not want to go back. ®