[gstreamer-bugs] [Bug 619806] New: [basetransform] caps functions should be faster

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=619806
  GStreamer | gstreamer (core) | git

           Summary: [basetransform] caps functions should be faster
    Classification: Desktop
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: otte at gnome.org
         QAContact: gstreamer-bugs at lists.sourceforge.net
      GNOME target: ---
     GNOME version: ---


This is fallout from bug 618853:

When running tests/benchmark/capsnego in a profile I noticed that a lot of time
was spent inside basetransform doing rather weird things with caps. I'll take
ffmpegcolorspace as an example of why this is very slow:

- In the getcaps function, peer caps are intersected with its template
This is absolutely not necessary, as the peer's getcaps function will return a
subset of the template caps. (With asserts enabled, it even checks this.) If
the peer is ffmpegcolorspace you'll mostly intersect the template caps with
itself: 32x32 = 1024 structure intersections.

- In the getcaps function, transformed caps are intersected with template caps
This is unnecessary again, as transform_caps must return a subset of the
template caps. It might make sense to keep a check with assertions enabled -
though that is quite expensive. Again, 1024 structure intersections with
ffmpegcolorspace.

- the algorithm for calling the transform function is slow
The current algorithm calls the transform function with every structure
separately and then merges the result. This is often very slow.
ffmpegcolorspace's transform function will expand the simple caps to its
template caps and return them appended to the original, so it'll return 1060
structures via 32 caps that'll get merged into a 32 structures large caps.

All of this is considerable overhead taking more than 5 times the performance
of the actual transform caps function.

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