What fraction of work is iDCT?

Michel LESPINASSE walken at zoy.org
Sun Jun 24 17:22:45 CEST 2001

On Sat, Jun 23, 2001 at 12:39:01AM -0700, Jeffrey W. Baker wrote:
> I am unfamiliar with the workload needed to decode a DVD.  Can someone
> please characterize for me how frequently the iDCT need to be performed,
> and what fraction of the CPU load it normally represents?  I have an iDCT
> routine for AltiVec to replace the broken routine currently in the source
> tree.  This routine performs the transform at the rate of 377000
> transforms per second on an otheriwse idle 400 MHz 7410.  With the
> "classic" iDCT, the machine drops about every fifth frame.  How much
> should I expect this new routine to boost performance?

I'm not sure about vlc, but I know for libmpeg2 on x86 the work is
roughly divided like that:

30% motion compensation
25% IDCT
25% parsing of IDCT coefficients
20% everything else (mostly parsing)

of course if any component is less optimized it will take more
time. For example the c IDCT would easily be 3-4 times slower than the
mmx/sse IDCT so that would affect the percentages a lot. Also color
conversion and display will add more time on top of that.

By the way, if you have working IDCT of MC routines for altivec I'd be
interested to get a copy too :)

Michel "Walken" LESPINASSE
Of course I think I'm right. If I thought I was wrong, I'd change my mind.

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