[Simh] Cluster communications errors

[Johnny Billquist]

On 2018-07-18 23:36, Mark Pizzolato wrote:
> On Wednesday, July 18, 2018 at 2:32 PM, Paul Koning wrote:
>>> It might be better to hard set the Linux simh host system's port to
>>> 10Mbit on the switch.  That would help with the potential for
>>> overrunning the original DEC hardware...
>>
>> DEC hardware tends to handle line rate traffic; a lot of other Ethernet
>> hardware does not, especially not earlier models.  I remember arguing with the
>> DECnet/DOS folks that no, we would not modify the DECnet architecture to
>> handle the single buffer "design" of the 3c501.
>>
>> But if you have speed mismatches, you're likely to have congestion loss, unless
>> the bursts are less than the switch buffer quota.  Some switches have
>> thousands of buffers; other (inexpensive) ones have only a surprisingly small
>> number and can easily give you congestion loss.
> 
> Well, not all systems and hardware can actually handle back to back
> packets even at 10Mbits.   The XQ THROTTLING is based on the throttling that
> Johnny Billquist implemented in his bridge which was needed to allow his
> physical systems to be able to communicated with simulated systems without
> crazy packet loss...

It's probably worth pointing out that the reason I implemented that was 
not because of hardware problems, but because of software problems. 
DECnet can degenerate pretty badly when packets are lost. And if you 
shove packets fast enough at the interface, the interface will 
(obviously) eventually run out of buffers, at which point packets will 
be dropped.
This is especially noticeable in DECnet/RSX at least. I think I know how 
to improve that software, but I have not had enough time to actually try 
fixing it. And it is especially noticeable when doing file transfers 
over DECnet.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol