[FFmpeg-devel] [PATCH 17/42] avcodec/refstruct: Add RefStruct pool API

[Andreas Rheinhardt]

Anton Khirnov:
> Quoting Andreas Rheinhardt (2023-09-19 21:57:09)
>> Very similar to the AVBufferPool API, but with some differences:
>> 1. Reusing an already existing entry does not incur an allocation
>> at all any more (the AVBufferPool API needs to allocate an AVBufferRef).
>> 2. The tasks done while holding the lock are smaller; e.g.
>> allocating new entries is now performed without holding the lock.
>> The same goes for freeing.
>> 3. The entries are freed as soon as possible (the AVBufferPool API
>> frees them in two batches: The first in av_buffer_pool_uninit() and
>> the second immediately before the pool is freed when the last
>> outstanding entry is returned to the pool).
>> 4. The API is designed for objects and not naked buffers and
>> therefore has a reset callback. This is called whenever an object
>> is returned to the pool.
>> 5. Just like with the RefStruct API, custom allocators are not
>> supported.
>>
>> (If desired, the FFRefStructPool struct itself could be made
>> reference counted via the RefStruct API; an FFRefStructPool
>> would then be freed via ff_refstruct_unref().)
> 
> Considering that it's intended to be used from multiple threads, that
> seems like the better option. Though I have not seen the following
> patches yet, so maybe the advantage is not as big as I'd think.
> 

It is implemented in patch #24.

>> +/**
>> + * If this flag is not set, every object in the pool will be zeroed before
>> + * the init callback is called or before it is turned over to the user
>> + * for the first time if no init callback has been provided.
>> + */
>> +#define FF_REFSTRUCT_POOL_FLAG_NO_ZEROING         FF_REFSTRUCT_FLAG_NO_ZEROING
> 
> Do you think using the same namespace really improves things? It does
> not seem so to me.
> 

Actually, the namespaces are separate. The pool API uses
FF_REFSTRUCT_POOL_FLAG_*, the ordinary RefStruct API uses
FF_REFSTRUCT_FLAG_*. That some pool flags are simply mapped to non-pool
flags is an implementation detail and does not join the namespaces.

- Andreas