On Fri, 17 Dec 2021, Michael Niedermayer wrote:
On Fri, Dec 17, 2021 at 01:04:19AM +0100, Marton Balint wrote:
On Thu, 16 Dec 2021, James Almer wrote:
Resending the first two patches only, since this is meant to
show the implementation of one of the several suggestions made
in the previous set that need to be discussed and hopefully
resolved in a call.
Can you push the full branch somewhere?
The proposals so far to extend the API to support either custom
labels for channels are, or some form of extra user information.
- Fixed array of bytes to hold a label. Simple solution, but
the labels will have a hard limit that can only be extended
with a major bump. This is what i implemented in this version.
- "char *name" per channel that the user may allocate and the
API will manage, duplicate and free. Simple solution, and the
name can be arbitrarily long, but inefficient (av_strdup() per
channel with a custom label on layout copy).
- "const char *name" per channel for compile time constants, or
that the user may allocate and free. Very efficient, but for
non compile time strings ensuring they outlive the layout can
be tricky.
- Refcounted AVChannelCustom with a dictionary. This can't be
done with AVBufferRef, so it would require some other form
of reference counting. And a dictionary may add quite a bit of
complexity to the API, as you can set anything on them.
Until we have proper refcounting API we can make the AVBufferRef in
AVChannelLayout a void *, and only allow channel_layout functions to
dereference it as an AVBufferRef. This would mean adding some extra helper
functions to channel layout, but overall it is not unsolvable.
The real question is that if you want to use refcounting and add helpers to
query / replace per-channel metadata, or you find the idea too heavy weight
and would like to stick to flat structs.
what is the advantage of refcounting for channel metadata ?
is it about the used memory, about the reduced need to copy ?
Basicly it is the ability to store per-channel metadata in avdictionary,
because otherwise it would have to be copyed, and avdictionary is very
ineffective at copying because of many mallocs.
what kind of metadata and what size do you expect ?
bytes, kilobytes, megabytes, gigabytes per channel ?
Usually, nothing, because most format don't have support for per-channel
metadata. In some cases it is going to be a couple of textual metadata
key-value pairs, such as language, label, group, speaker, positon, so 4-5
dynamically allocated string pairs, plus the AVDictionary itself,
multiplied by the number of channels in a layout.
what is the overhead for dynamic allocation and ref counting?
that is at which point does it even make sense ?
I don't have exact measurements. It is generally felt that copying
AVDictionary per-channel is a huge overhead for something as lightweight
as an audio frame which is a 2-4 kB per channel at most and only a couple
of allocs usually not dependant on the number of channels. That's why
refcounting was proposed.
Also some people simply don't want to store extendable channel metadata in
channel layout, and want to keep it simple.
Regards,
Marton
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