[Beowulf] Re: High Performance SSH/SCP
ben at psc.edu
Thu Feb 14 12:06:21 PST 2008
Mark Hahn wrote:
> the switch-to-none feature is a good one. I'm a bit skeptical about
> the buffering changes, since you can accomplish the same thing with
> sysctls. (has anyone ever experienced a real case where too-large
> sysctl net memory settings caused problems? obviously, attempting to do
> long-fat-pipe transfers to a heavily used web server might be a problem,
> since the latter wants tight controls on sock mem use.)
> the really cool thing would be if you could associate a default setting
> for socket buffers with a _route_. heck, a round-port combo.
> it seems crazy for apps to be messing with these issues.
The SSH protocol has its own, per channel, flow control on top of TCP to
multiplex multiple SSH channels over a single TCP stream.
The stock OpenSSH code has static window sizes, IIRC 64KiB in <= 4.6,
and 1-2MiB in 4.7
HPN-SSH dynamic windows does not adjust the TCP buffer size, but these
SSH channel windows so that they're not a limiting factor when you have
an auto-tuning kernel, or have manually adjusted TCP buffer size above
these static values.
This is much better described in our paper  and the presentation of
that paper .
A colleague just pointed me to this thread, I'll try to keep an eye on
it if there are any questions, or feel free to contact hpn-ssh at psc.edu
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