Re: [RFC net-next 4/4] tcp: defer regular ACK while processing socket backlog

On Thu, Sep 7, 2023 at 8:00 PM Jakub Kicinski [off-list ref] wrote:

On Thu, 7 Sep 2023 19:16:01 +0200 Eric Dumazet wrote:

quoted

quoted

Is it okay if I asked why quickack?
Is it related to delay-based CC?

Note the patch is also helping the 'regular' mode, without "quickack 1" .

This is CC related in any way, but some TCP tx zerocopy workload, sending
one chunk at a time, waiting for the TCP tx zerocopy completion in
order to proceed for the next chunk,
because the 'next chunk'  is re-using the memory.

The receiver application is not sending back a message (otherwise the
'delayed ack' would be piggybacked in the reply),
and it also does not know what size of the message was expected (so no
SO_RCVLOWAT or anything could be attempted)

For this kind of workload, it is crucial the last ACK is not delayed, at all.

Interesting. Some folks at Meta were recently looking into parsing RPCs
in the kernel to avoid unnecessary wakeups. Poor man's KCM using BPF
sockmaps. Passing message size hints from the sender would solve so
many problems..

Yes, RPC headers make things easier for sure.

(we internally have something similar named autolowat, where we parse
headers to set sk->sk_rcvlowat dynamically)