> For example doing very heavy computation on 8 threads of a 4-core CPU with 8 hardware threads, can actually reduce performance, because all threads will be co

> For example doing very heavy computation on 8 threads of a 4-core CPU with 8 hardware threads, can actually reduce performance, because all threads will be contending for the same functional blocks of the CPU. We have exactly this scenario on a server we're running (non-virtualised) and, if it didn't involve rebooting the machine, I'd already have run a test to benchmark whether we get better performance with or without hyperthreading. Unfortunately our test environments aren't similar enough to be representative in terms of testing yet, but this is going to change in the next few weeks so I'll finally be able to run my test without needing to take down the production box.