Phenom TLB workaround benchmarked
Our friends at the Tech Report have a somewhat alarming article posted that you all should check out. By now, we suspect you’ve all heard about the ‘TLB bug’ affecting all current quad-core AMD Opteron (Barcelona) and Phenom (Agena) processors. If you haven’t heard of the bug, to put it simply, there is a problem with the translation lookaside buffer (TLB) and L3 cache that causes system instability under certain conditions. The conditions that cause a problem are rare, but the issue has prompted AMD to release a workaround patch that motherboard makers will incorporate into the system BIOS of their motherboards to alleviate the issue. The problem with the TLB workaround patch is that it adversely affects performance. To see just how much performance is affected, the Tech Report acquired the necessary BIOS files and re-tested a Phenom 9600 CPU purchased at retail.
In the article Scott Wasson says, “Across every test we ran, the difference between the Phenom 9600 with and without the TLB patch averages out to 19.8%. However, if we rule out the synthetic memory tests and consider only the application tests that difference drops to 13.9%.
The most troubling results here are the applications where we see large performance drops with the TLB erratum workaround active, including the Firefox web browser and the picCOLOR image analysis tool. If one happens to spend a lot of time running an application whose memory access patterns don't mix well with the TLB patch, the result could prove frustrating. The BIOS-based workaround for the TLB erratum may achieve its intended result—system stability—but it comes at a pretty steep price in terms of performance.”
What this all means is that all Phenom evaluations that included benchmark results, including ours, are somewhat misleading. Once all of the motherboard manufacturers update their BIOS files with the necessary workaround, performance will drop for all current Phenom processors. The good news (or bad news if you’ve already bought a Phenom) is that revised quad-core processors from AMD are on the way that will fix the TLB bug, in mid- to late-Q1 2008. Let’s hope for competition’s and AMD’s sake that happens sooner rather than later.
As an aside, we’ve contacted a number of motherboard manufactures requesting BIOS updates infused with the TLB bug workaround and will update the benchmarks in our Phenom evaluation as soon as possible.