[Bug 672701] mpegvideoparse: high cpu usage breaks playback of mpeg2 video in mkv on dreambox

Mon May 21 05:30:12 PDT 2012

https://bugzilla.gnome.org/show_bug.cgi?id=672701
  GStreamer | gst-plugins-bad | 0.10.23

--- Comment #9 from Edward Hervey <bilboed at gmail.com> 2012-05-21 12:30:06 UTC ---
(In reply to comment #6)
> Review of attachment 213760 [details]:
> 
> Doesn't look wrong but do you also have some numbers (just curious :) )? And do
> the addition of new branches in the code really improve performance that much,
> especially as this doesn't look very friendly to branch prediction? Might make
> sense to use lookup table maybe?

  The checks are actually laid out in order of biggest probability, the branch
prediction is therefore the closest to reality.
  For a perfectly random sequence, there's a 254/256 (99.2%) probability that
the 3rd byte is > 1, followed by a 255/256 (99.6%) probabilty that the 2nd byte
is non null. I'd say the prediction is pretty well established :)

  The impact on L1 read miss is on the same order, since you might need to
"read back" (i.e. refetch) data which you don't have locally... but that only
happens with a 1/128 probability.

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