Mazzoli: How fast are Linux pipes anyway? [LWN.net]

Mazzoli: How fast are Linux pipes anyway?

Posted Jun 9, 2022 15:01 UTC (Thu) by willy (subscriber, #9762)
In reply to: Mazzoli: How fast are Linux pipes anyway? by NYKevin
Parent article: Mazzoli: How fast are Linux pipes anyway?

What does it mean to mmap() a pipe?

Let's suppose I have a pipefd and addr = mmap(offset=0, length=1M). Then I call read(4kB) on pipefd. Does the pipe shuffle down so that *addr now refers to what was at 4kB, or does it still have a reference to what was at 0 when I called mmap()?

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Mazzoli: How fast are Linux pipes anyway?

Posted Jun 10, 2022 17:32 UTC (Fri) by NYKevin (subscriber, #129325) [Link] (2 responses)

Whichever is easier. As long as it's consistent and well-documented, userspace can figure out the rest.

However, I should point out that, if the pipe does not shuffle down, then you need to add an API for telling userspace the current read/write offsets, so that userspace knows where to begin reading or writing. Regardless, you also want an API for setting (or at least advancing) those offsets, so that userspace can emulate read/write calls. Therefore, you might as well not bother with the shuffling-down logic and just provide full get/set support for the offsets.

Mazzoli: How fast are Linux pipes anyway?

Posted Jun 10, 2022 17:36 UTC (Fri) by willy (subscriber, #9762) [Link] (1 responses)

Haven't you just reinvented shared memory?

Mazzoli: How fast are Linux pipes anyway?

Posted Jun 10, 2022 17:55 UTC (Fri) by NYKevin (subscriber, #129325) [Link]

Well gee, I thought that's what vmsplice(2) was trying to do in the first place.

But I think the big difference is this: If you e.g. create a file in /dev/shm (or any tmpfs) and just keep writing more and more data to it, it'll get bigger and bigger indefinitely, so you have to periodically seek to zero at both ends (and/or truncate it). Pipes are effectively ring buffers, so they don't have this problem. Your consumer can just call read, and doesn't have to know anything about your fancy mmap nonsense.