Are Unix/Linux pipes producer or consumer driven?

Pipes in Unix have a buffer, so even if the right side process (RSP) does not consume any data, the left side process (LSP) is able to produce a few kilobytes before blocking.

Then, if the buffer gets full, the LSP is eventually blocked. When the RSP reads data it frees part or all of the buffer space and the LSP resumes the operation.

If instead of 2 processes you have 3, the situation is more or less the same: a faster producer is blocked by a slower consumer. And obviously, a faster consumer is blocked by a slower producer if the pipe gets empty: just think of an interactive shell, waiting of the slowest producer of all: the user.

For example the following command:

$ yes | cat | more

Since more blocks when the screen is full, until the user presses a key, the cat process will fill its output buffer and stall, then the yes process will fill its buffer and also stall. Everything waiting for the user to continue, as it should be.

PS: As an interesting fact is: what happens when the more process ends? well, the right side of that pipe is closed, so the cat process will get a SIGPIPE signal (if it ever writes again in the pipe, and it will) and will die. The same will happen to the yes process. All processes die, as it should be.

A has a pipe to B, and B has a pipe to C. Each pipe has a buffer; B and C block if they try to read, and there isn't any input available (end-of-stream counts as input). A and B block if they have output to write, but the pipe's buffer is full.

All three processes run concurrently, using as much CPU as they can. The OS blocks them in the read/write system call as necessary if the pipe buffer is exhausted/full respectively.

So, they're driven by both the consumer and producer, that is, the rate is the min of both the consuming rate and producing rate. If the consumer is faster, the performance is driven by the producer, and vv.