vaaapidecode, shmsink, and allocator

[Víctor Jáquez]

On Wed, 11 Dec 2019 at 20:30, kmliu wrote:
> I'm running the following pipeline that decodes H.264 video from a file and
> then sends the raw video to a shmsink. I have another pipeline that picks up
> the raw video from a corresponding shmsrc.
> 
> gst-launch-1.0 --gst-debug=vaapidecode:6,shmsink:6 filesrc
> location=bbb_sunflower_1080p_30fps_normal.mp4 ! qtdemux ! vaapih264dec !
> shmsink wait-for-connection=false socket-path=/tmp/tmpsock sync=true
> 
> It works but I see more memory reads and writes than I expected. BTW, I run
> 16 instances of this pipeline to get GBs per second of reads and writes. So
> I started looking into the ALLOCATION query between vaapih264dec and
> shmsink. The shmsink seems to propose a custom allocator that allocates from
> its shared memory buffer. But in gst_vaapi_plugin_base_decide_allocation(),
> it simply keeps a reference to shmsink's allocator in other_srcpad_allocator
> and proceeds to use its own allocator instead. I think because of this,
> shmsink needs to do a memcpy as indicated by the following debug message.
> BTW, this message is kind of misleading. I think it means the memory in the
> buffer was not allocated by shmsink's own allocator and it's allocated by
> vaapivideoallocator0 instead.
> 
> shmsink gstshmsink.c:714:gst_shm_sink_render:<shmsink0> Memory in buffer
> 0x7f2aa8052480 was not allocated by <vaapivideoallocator0>, will memcpy
> 
> I don't understand vaapidecode enough to tell why it doesn't just agree to
> use shmsink's allocator. The only place other_srcpad_allocator is used is in
> gst_vaapidecode_push_decoded_frame(). It doesn't look like that particular
> codepath is taken though.

I haven't tested shmsink, but vaapi needs to use its own source pad allocator,
because it produces VASurfaces. Those surface have to be "downloaded" to another
memory area, and that's a memcpy for many use cases.

vmjl

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