Request regarding CHAN_INPUT_MAX

[Damien Miller]

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