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

[John Drescher]

On Mon, Apr 14, 2008 at 4:01 PM, Tom Dexter <digitalaudiorock at gmail.com> wrote:
>
> On Mon, Apr 14, 2008 at 1:42 PM, John Drescher <drescherjm 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.
>  >  >  >
>  >  You do realize hyperthereading can have a negative impact on
>  >  performance? I mean a single core P4 with hyperthreading may look like
>  >  two cores but it certianly is not. Hyperthreading works only because
>  >  the P4 had such a long pipeline that stalls occurred so frequently
>  >  (mispredicted branch ...) and in this case a second thread could be
>  >  executed where no ops would be inserted in the pipeline.
>  >
>  >  John
>  >
>  >
>
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>  >
>
>  Yes I'm aware of that.  When I first set up my frontend I tested both
>  ways and the performance is better with it enabled.
>
>  I just gave it another try and it still seems to be an improvement
>  having it enabled.
>
I would like to make 1 point though, any improvement will will be
application and data specific. So having HT enabled may increase
performance by 20% in some application and decrease it by 5% in a
second application. In general it will increase performance though.

John