[Bug 767350] New: jpegdec: Internal parser map/unmap the same buffer multiple time

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=767350

            Bug ID: 767350
           Summary: jpegdec: Internal parser map/unmap the same buffer
                    multiple time
    Classification: Platform
           Product: GStreamer
           Version: 1.8.2
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: nicolas at ndufresne.ca
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

The parse in jpegdec rely on gst_adapter_masked_scan_uint32() to find few 2
bytes sequences in the stream. The problem is that the adapter variant of this
function will, for every call, map/unmap the GstBuffer. If you combined with a
non-sysmem memory (like dmabuf) this will cause a huge performance hit. With
720p at 30 video, I get a full i7 core being used with (if I force parsing
obviously):

  v4l2src ! jpegdec ! fakesink

I didn't look how to fix it really, one could map the buffer in the adapter
before using that function. This would bring the performance to what the
byte_scanner version would give. Though, there is probably something even
smarter we could do.

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