Question about Gflops with 3930k

[novadaemon]

Running IBT on my 3930k using ddr3 1600 ram @ Standard preset I get the following average numbers:

4 threads 112 gflops
6 threads 154 gflops
8 threads 154 gflops
12 threads 125 gflops
All 125 gflops
Auto 154 Gflops

The 6 thread number would be consistent with the numbers I should see if I disabled HT, yet leaving HT on is perfectly fine if I run 6 threads and I even get the same number running 8 threads. 12 threads is interesting because it looks like there's a conflict with 12 threads running with HT on.

Does anyone have an explanation for my results?

 

[ferzerp]

HT only works because most cpus are often waiting. Linpack is coded such that the cpu is never waiting, so adding more threads than the core count just creates thread contention and lower scores.

8 Threads isn't enough to cause much of an effect it looks like, but 12 is. This is how linpack (which IBT is running) works on all HT Intel procs. Peak performance is when thread count matches core count.