Alexander Bluhm <bluhm@openbsd.org> wrote:

> On Mon, May 13, 2024 at 06:41:55PM +0000, Job Snijders wrote:
> > Perhaps in the future - after more study & contemplation - we'll to
> > change this sysctl's default from 0 to 1?
> 
> The common network API is to enable TCP_NODELAY to turn off Nagle's
> algorithm.  If we change that default, userland software has to be
> written differenty for OpenBSD.

Correct.  Toggling this system-wide will affect a tremendous number of
applications which have not been studied (and are not being studied).
I guess the theory is if this toggle existed, some people would study
it.

I think they can study it today by making a new kernel.  The proposal
contains no pre-study of this.

The proposal mentions .1% of the network speaking applications which
manually turn it off.  It doesn't contain a discussion from the other
side.  That's weird.

> What is the default on FreeBSD and Linux?  If they have switched,
> mainstream developers will do it differently.  Then we could also
> adapt.

Noone has switched to turning it off by default.

> Although for most cases Nagle does not improve performance, it still
> depends on the protocol above TCP if that is the case.  I doubt
> that a global sysctl can handle this.

Nagle also has a large number of unmentioned side effects.  It reduces
receive interrupts on the other side.  It reduces transmit-complete
interrupts on this side.  By bundling data, it probably accidentally
prevents some latency inspection problems, etc etc etc