[x264-devel] Varying of PSNR as a function of thread count

[Loren Merritt]

On Mon, 20 Apr 2015, Harald Alvestrand wrote:

> (I also observed that the measured CPU time with --threads 1 on my
> system is 6.7 seconds, while all configurations with multiple threads
> come in above 8.1 seconds, with all configurations above 6 threads
> taking 11 CPU-seconds. Is this also as expected?)

This is expected *if* you have a hyperthreaded CPU.
Two hyperthreads on the same core counts as 200% CPU usage, but only gets
about 25% more work done per wallclock-second than a single thread.
Plus, most recent CPUs adjust their clockrate depending on how many
cores you're using in order to limit heat.
Unless you can eliminate these confounders (e.g. by turning them off in
the BIOS), you just can't compare "CPU-seconds" between two workloads
with different thread counts.

--Loren Merritt