by Anton Shilov
06/22/2013 | 09:24 AM
It looks like there is a way to boost performance of AMD microprocessors and accelerated processing units in applications that rely on old-school x87 processing. Performance is there, just push the button. But does this additional performance have any value in real life?
Advanced Micro Devices has received a fair amount of critics after its long-awaited Bulldozer micro-architecture and then its Piledriver successor failed to meet performance expectations. While with optimized software AMD’s multi-core central processing units do show decent speed, they still cannot fight against Intel Corp.’s premium offerings in demanding applications. As it turns out, not only software makers need to tweak their apps. With some BIOS tunings, Stilt, a performance enthusiast from Finland, has managed to significantly boost performance results of AMD Piledriver-based microprocessor in SuperPi benchmark.
Apparently, AMD microprocessors with Bulldozer and Piledriver cores come with certain registers disabled and a certain block called NRAC enabled. In case registers are enabled and the module is disabled, performance in SuperPi benchmark rises by 14% - 18% on AMD A10-6800K APU.
“I was doing some low level testing for other purposes, I found something that did not make any sense to me. [...] I roughly know what it is and what it does, but still some questions remain: Why does this ’feature’ exist in the first place and why it is activated on all 15h [AMD Bulldozer and similar] family parts. I would normally assume it is a workaround for some errata, however no bulletin exists for this one either. Also this feature does not exist in any documentation, or it does but only AMD has access to the required level. I find it hard to believe that it would be a design issue as the affected instructions work fine (but slowly) and it existed since early Zambezi revisions and, currently is still present in Richland and probably beyond (within family 15h),“ wrote Stilt on XtremeSystems forum (the message was reposted on Hwbot.org).
According to performance tests conducted by Ilya Gavrichenkov, CPU performance analyst at X-bit labs, the downloadable tweak, which is available for download (not recommended by X-bit labs), does improve performance in SuperPi, which relies on x87 computing and many other things which influence performance in single-threaded applications (e.g. memory, cache size, cache latencies, etc.). However, it looks like the tweak not only does not have any effect on performance in modern applications in general since they rely on SSE/AVX processing, but it also does not work in multi-threaded apps in particular. In fact there are so few programs nowadays that depend on poor FPU/x87 performance that they simply do not matter.
It is obvious that the performance tweak works in SuperPi, but there are no evidences that it can boost performance in other applications. On the other hand, it is not obvious that the latest processors from AMD do not have any other undocumented features that improve their performance. Perhaps, with further tweaking it is possible to speed up many other apps, not only SuperPi. Unfortunately, a way to lift performance of recent AMD CPUs in all programs with a BIOS tweak has not been found yet.
AMD did not comment on the news-story.