Request regarding CHAN_INPUT_MAX

[Chris Rapier]

I'll check that as soon as I can. Might not be until Monday though. Thanks
again,

Chris

On Fri, Apr 10, 2026, 20:31 Damien Miller <djm at mindrot.org> wrote:

> On Fri, 10 Apr 2026, Chris Rapier wrote:
>
> > I spent some more time nailing this down. It turns out that there seems
> to be
> > an inverse relationship between CHAN_INPUT_MAX and throughput when
> OpenSSH is
> > sending to HPN-SSH.
> >
> > If I don't clamp the advertised window on my side
> > CHAN_INPUT_MAX                Throughput
> > 16MB                  200Mbps
> > 12MB                  400Mbps
> > 8MB                   950Mbps
> > 4MB                   1.5Gbps
> > 1MB                   2.8Gbps
> > 512KB                 2.8Gbps
> >
> > So yeah. Please don't increase CHAN_INPUT_MAX! In fact, I'd really like
> to
> > understand this behaviour. Especially because the smaller window size
> actually
> > seems to make a big difference
>
> I think this is the pathological behaviour I've discussed with you
> previously
> where a near-full sshbuf that is experiencing lots of small writes
> undergoes
> expensive compaction operations for each of them.
>
> Maybe it's time to change how buf->max_size is handled. I had a patch for
> that; does https://github.com/djmdjm/openssh-wip/pull/44 help here?
>
> -d
>