Re: [9fans] Nagle algorithm

[forsyth@caldo.demon.co.uk]

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i think it's fundamentally misconceived and should at least never be
on by default.  dhog's description was ``[it] attempts to buffer up
multiple writes and send them as a single TCP packet -- where the
writes are small''.  what that actually means is that if the writes
are small it typically delays sending out anything at all (subject to
certain specific conditions) until a given interval has elapsed or
its idea of enough data has arrived, and that interval can be large.  in other
words, give it a few bytes you wish to send now and it demands more,
delaying if that demand is not met.  it's not just RPC that's affected.

i claim it's not the TCP/IP subsystem's responsibility to delay
sending something so that it can buffer up writes to make larger
packets.  that's for stdio or bio (or local OS equivalent).  (an
interesting variant was in a version of the Unix streams subsystem
where there was a buffering module that could be pushed onto a tcp/ip
stream.)  it's fine for TCP/IP to adapt to the network and adjust the
rate at which bytes are shuffled in and out, and adapt retransmission
efforts to apparent error rates, and since TCP/IP doesn't preserve
delimiters coalesce new data (from writes large or small) into already
queued data to make larger output packets.  those are all within its
responsibility, i think, and any delays that result are arguably
inherent in the network.

if applications are stuttering bytes needlessly, deal with it there.


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To: 9fans@cse.psu.edu
Subject: Re: [9fans] Nagle algorithm
Date: Thu, 22 Nov 2001 09:57:11 GMT
Message-ID: <87itc31syv.fsf@becket.becket.net>

andrew@mbmnz.co.nz (Andrew Simmons) writes:

> That makes sense, thanks. Not a problem for my application, but it's good
> to be aware of a potential problem.

(It should be noted that the same thing proved true of X applications;
Nagle isn't just bad for Plan 9 RPC's; it's bad for *any* RPC
application.)