[mythtv-users] ffmpeg and multiple CPUs. Was: Should I be switching to 0.21?

[lemongecko at gmail.com]

On 4/14/08, Tom Dexter <digitalaudiorock at gmail.com> wrote:
> However...and this is the interesting part...if I use ffmpeg but
> switch the number of CPUs to 1 instead of 2 the performance of the two
> is virtually identical.  It seems that, at least with my
> hyper-threaded 3 Ghz P4, ffmpeg works much better with a single core.

Telling ffmpeg you have one CPU was the right choice. We do a lot of
FEA analysis at work, and we did plenty of benchmarking on HT CPUs
when they came out. We saw results similar to yours, though completely
disabling HT in the BIOS *improved* our performance.

That's because hyperthreaded CPUs still only have one floating point
unit. They DO have two schedulers, which decreases latency and makes
them excellent processors for workstation use (and is probably a good
thing for a Myth box in general). But when it comes down to sheer
number crunching like video decoding, HT can get in the way. Keeping
two pipelines full imposes some performance penalties.