Request regarding CHAN_INPUT_MAX

[Chris Rapier]

On 4/10/26 15:32, Chris Rapier wrote:
> 
> 
> On 4/9/26 19:05, Damien Miller wrote:
>> On Thu, 9 Apr 2026, Chris Rapier wrote:
>>
>>> Hi all,
>>>
>>> Is there any way that the limit of CHAN_INPUT_MAX could be increased?
>>>
>>> In channels.h it's set to 16MB.
>>> #define CHAN_INPUT_MAX  (16*1024*1024)
>>>
>>> Which is fine for most implementations of SSH but it's causing 
>>> performance
>>> degradation when OpenSSH is sending data to an HPN-SSH instance. I'm 
>>> currently
>>> dealing with this by limiting the advertised window to less than 16MB 
>>> but that
>>> limits throughput in this situations. While I'd love it if this was 
>>> increased
>>> to 128MB an increase to 32MB or more would help out a lot of my users 
>>> (people
>>> often install HPN-SSH on the server side as it provides a throughput 
>>> boost to
>>> any client sending data). If HPN-SSH advertises more than that to these
>>> clients the connection might fail or enter a pathological state in 
>>> terms of
>>> performance.
>>
>> That's the pathological state here? INPUT_MAX sets the maximum size of 
>> the
>> buffer for data that has been locally read by not yet put on the wire.
>> > If the server has advertised a larger channel window then I'd assume 
>> that
>> this buffer should pretty much always be empty as it will be drained to
>> the wire by channel_output_poll_input_open() in maxpacket increments.
> 
> I can spend some more time nailing this down if you are potentially open 
> to this change.

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

For example running an advertised window restricted window to SSH with 
CHAN_INPUT_MAX at 512KB at 50ms delay (hpnssh localhost -p3232 
-caes256-gcm at openssh.com "cat /dev/zero" > /dev/null) hets me about 5Gbps.

If I run the same against SSH with a 16MB CHAN_INPUT_MAX I get 2.3Gbps.

It's really interesting whats happening. Not expecting you to change 
anything on your end I'm just really curious now.

Chris