Memory Bandwidth - Conventions

Conventions

Perhaps surprisingly, there are at least three different conventions for counting the quantity of data transferred in the numerator of bytes/second, as discussed in more detail in.

1. bcopy convention: counts the amount of data copied from one location in memory to another location per unit time. For example, copying 1 million bytes from one location in memory to another location in memory in one second would be counted as 1 million bytes per second.
2. STREAM convention: sums the amount of data that the application code explicitly reads plus the amount of data that the application code explicitly writes. Using the previous 1 million byte copy example, the STREAM bandwidth would be counted as 1 million bytes read plus 1 million bytes written in one second, for a total of 2 million bytes per second.
3. hardware convention: counts the actual amount of data read or written by the hardware, whether the data motion was explicitly requested by the user code or not. Using the same 1 million byte copy example, the hardware bandwidth on computer systems with a write allocate cache policy would include an additional 1 million bytes of traffic because the hardware reads the target array from memory into cache before performing the stores. This gives a total of 3 million bytes per second actually transferred by the hardware.

The bcopy convention is self-consistent, but is not easily extended to cover cases with more complex access patterns, for example three reads and one write. The STREAM convention is most directly tied to the user code, but may not count all the data traffic that the hardware is actually required to perform. The hardware convention is most directly tied to the hardware, but may not represent the minimum amount of data traffic required to implement the user's code. For example, some computer systems have the ability to avoid write allocate traffic using special instructions, leading to the possibility of misleading comparisons of bandwidth based on different amounts of data traffic performed.