Does TCP benefit from fixed writes?

[cmazakas]

I'm working on a small HTTP/TCP server and I was wondering, do fixed writes provide any benefit over the normal ones?

I'm reading the docs and they mention fixed writes are useful in conjunction with O_DIRECT which doesn't apply. Do we still get benefits from using fixed buffers with network sends?

[axboe]

No benefit for that right now, registered buffers (and the associated read/write fixed) is purely an optimization regular files with O_DIRECT. It does make a considerable difference there. For buffered writes, it doesn't matter, and for socket it also has no effect.

[cmazakas]

Ha, thank you for the fast and informative reply.

I was reading the docs and this was my intuition. Thank you for confirming.

[redbaron]

Does it apply to zero copy send to socket? I've read somewhere that ZC operations should use fixed buffers

[isilence]

Unlike normal sends / socket write, IORING_OP_SEND_ZC benefits from using fixed buffers, i.e. no per IO page pinning and lookups