[New-bugs-announce] [issue40007] An attempt to make asyncio.transport.writelines (selector) use Scatter I/O

[tzickel]

New submission from tzickel <icebreak at yahoo.com>:

I have a code that tries to be smart and prepare data to be chunked efficiently before sending, so I was happy to read about:

https://docs.python.org/3/library/asyncio-protocol.html#asyncio.WriteTransport.writelines

Only to see that it simply does:

self.write(b''.join(lines))

So I've attempted to write an version that uses sendmsg (scatter I/O) instead (will be attached in PR).

What I've learnt is:
1. It's hard to benchmark (If someone has an good example on checking if it's worth it, feel free to add such).
2. sendmsg has an OS limit on how many items can be done in one call. If the user does not call writer.drain() it might have too many items in the buffer, in that case I concat them (might be an expensive operation ? but that should not be th enormal case).
3. socket.socket.sendmsg can accept any bytes like iterable, but os.writev can only accept sequences, is that a bug ?
4. This is for the selector stream socket for now.

----------
components: asyncio
messages: 364565
nosy: asvetlov, tzickel, yselivanov
priority: normal
pull_requests: 18416
severity: normal
status: open
title: An attempt to make asyncio.transport.writelines (selector) use Scatter I/O
type: enhancement
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40007>
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