SMT Technology and Performance

When we talked about the architectural peculiarities of Core i7 processors we stressed that one of their key features that may affect the overall performance was SMT (Simultaneous Multi-Threading) technology support. Thanks to this technology each processor core may process two computational threads at the same time, thus loading the execution units more effectively.

However, as we remember from our experience with Pentium 4 processors that supported a similar technology called Hyper-Threading, it may also have a negative effect in some cases. It usually happens for two reasons. The first reason why the performance may drop is the silly work of the operating system task manager that may not distinguish between the physical and virtual CPU cores and assign a pair of threads to the same physical core despite the fact that other cores are not utilized at the time. The second reason has to do with the fact that some of the internal processor buffers are shared equally between the threads when SMT is enabled. Therefore, the performance of a core processing only one thread may sometimes be lower with enabled SMT.

To evaluate how enabling SMT in Core i7 affects the performance we tested Core i7-965 Extreme Edition in popular applications with SMT technology enabled and disabled. (Turbo Boost was disabled).

We can’t make any definite conclusions about SMT technology here. Quite offer it may really have a negative effect on performance. I believe that Intel put the corresponding option of their Intel DX58SO Smackover BIOS Setup onto the very first screen for a good reason…

It is very easy to figure out when you can actually benefit from SMT support. The applications with easily paralleled workload will be working faster. In this case the performance may improve by impressive 25-35%. However, when the applications, such as games for instance, create a limited number of threads, Core i7 will work slower with enabled SMT. Although, this will be a fairly small performance drop, no more than 4-5%.