Memory Management

* How to Use Memory?
In terms of how to use memory/buffer, components could be divided into three categories.
- In place read-only
- In place read and write
- transform
For transform components, they accept the data in the input buffer, do some kind of transformation on the data, then put the new data to the new requested buffer and free the old one.

* Two Kinds of Buffer Factory
- Discrete buffer factory: fixed size data buffers
- Circular buffer factory: variable size data buffers

* Discrete Buffer
- Reference count for zero-copy reuse
In the cases of sharing discrete data buffers, one solution is to make copies of buffer data. But it wastes memory. Reference passing is a better way and a reference count of a buffer is equal to the number of filters which have access to this buffer. The buffer memory is only freed when its reference count falls to zero. In order to support reference count, all the downstream components must be in place read-only ones. The buffer's reference count would be automatically added one or N (for multi-output stream connection) when it is written to a stream connection; but each component must release buffer when it has finished using it and passed its pointer to the stream connection.

* Circular buffer
- One exclusive write and multiple reads
One write pointer and multiple read pointers are maintained for this kind of buffer. The amount of empty space in the buffer is equal to the distance from the write pointer to the closet read pointer. In other words, the slowest reader controls the amount of available space in the buffer. And in this way, the mutual exclusion is realized. For those components which underflow in the circular buffer, they are responsible for combining the new data with the old ones; or they could not update the reader pointer and try again later.