Hyperthreading makes my code run slower?

Question

Some multithreaded code I just wrote appears to run slower under hyperthreaded CPUs - i.e. disabling hyperthreading makes it run FASTER. Is this normal?

Answer 1

This depends entirely on use case. A subjective term like normal has a lot of leeway! There are use cases where Hyper-Threading (HT) makes sense, and cases where it will have a performance impact.

One such case of performance decrease is for applications making heavy use of AVX instructions. The AVX instructions are carried out in the vector processing unit(VPU), of which there is one per core in Intel Xeon processors. Additional threads will block when trying to access the VPU if it is not available, leading to no performance improvement with the use of HT.

If you have say, 4 cores with HT, allowing you to run 8 threads, you will only actually be able to run 4 VPU instructions at a time - so your other 4 threads will be blocked as they complete. The additional overhead of the blocking and scheduling will usually net you a lower throughput than if you were running 4 threads on 4 cores, with HT disabled.

Likewise, running just 4 threads on the 8 cores, the OS scheduler can schedule the threads to run on any physical core - so there may still be a chance where one thread blocks waiting for another to complete. Some newer applications and job schedulers can now coordinate with the OS to "pin" threads on physical cores, allowing HT to be enabled, but not to oversubscribe the amount of threads that are running on a core. Over time this will probably get better, but does require awareness on the developer's part.

For more general purpose use cases, like a generic server handling many types of workloads, the advantage of HT running additional threads in a single core it usually a performance gain.