[FFmpeg-devel] [PATCH] Runtime detection for the number of processors/cores

[Michael Niedermayer]

On Wed, May 21, 2008 at 05:45:46PM -0400, Alexander Strange wrote:
> 
> On May 21, 2008, at 4:07 PM, Roman Shaposhnik wrote:
> 
> > On Wed, 2008-05-21 at 11:24 +0100, M?ns Rullg?rd wrote:
> >> Philipp Meinen wrote:
> >>> Hello FFmpeg Team
> >>>
> >>> The attached patch is an attempt to allow runtime detection for the
> >>> number of online processors/cores instead of having to specify the
> >>> number of threads. The idea is to type:
> >>>    ffmpeg -threads 0 ....
> >>> to use as many threads as processors/cores are online.
> >>>
> >>> I guess cmdutils.c/h is not the right file to place the new
> >>> detection function. To which file should this function belong?
> >>>
> >>> Comments welcome :)
> >>
> >> This is the wrong way to go about it.  The number of processors in  
> >> the
> >> machine is not interesting, the number of processors we're running on
> >> is.  On Linux, this information can be found from  
> >> sched_get_affinity().
> >> Other systems have other methods.
> >
> > This is a reasonable generic statement, but the nice thing about
> > FFmpeg's approach to multithreading is that it sort of self-schedules.
> > IOW, even if you create more (far more!) threads than you have actual
> > execution units (could be CPUs, could be cores or could even be
> > virtual threads -- think Hype-R-Threading) the only overhead is
> > at the start-up phase. Thus, using _sysconf(_SC_NPROCESSORS_ONLN)
> > seems quite reasonable of a compromise.
> >
> > On a related note: I still don't quite understand why MPEG based
> > codecs don't allow for higher degree of parallelism. Unlike DV codec
> > which I've just tested on a nice Maramba box (2 CPUs, 16 cores,
> > 128 execution units) and it scaled pretty much linearly to the
> > # of cores, the MPEG based ones just refused to go higher than 4.
> 
> They only scale to the number of slices in a frame, and then only to  
> the first 8 of those, I think. 

-#define MAX_THREADS 8
+#define MAX_THREADS 128

(yes feedback is welcome ...)

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The educated differ from the uneducated as much as the living from the
dead. -- Aristotle 
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