Cores and hyperthreading

Question

I'm writing an extremely optimized and CPU-intensive multithreaded code in C which performs a task in a more or less limited time space. During this time it does not venture out of its L1 cache except to load initial values and to store final results. So essentially this is a parallelized code which scales linearly for every core added. This is what happens on non-HT cores.

On my 2-core i5 with HT (which the BIOS does not allow to be disabled - this is an impractical solution anyway) I get an annoyingly dismal improvement when going from one core to two. My hypothesis is that the first thread runs alone on a core and the second shares the core with the first.

There are functions in the Windows API to retrieve info about available cores and HTs. But how do I make use of this information to ensure that there is only one thread on one hyperthread per core?

Answer 1

This article might be able to help: http://msdn.microsoft.com/en-us/magazine/cc300701.aspx#S11

See the "CPU Affinity" section and the "Detecting Hyper-Threading" section.

Answer 2

The OS will be using the HT logical cores whether or not you are, and the upshot of that is that the cache is effectively halved in size. You can pin a thread to a logical core, but I suspect it won't help you. Your problem is the mere presence of HT. You do need to turn it off.