[Simh] RSTS and slow DECnet operation in SIMH

[Paul Koning]

With help from Mark Pizzolato, I've been looking at why RSTS (DECnet/E) operates so slowly when it's dealing with one way transfers.  This is independent of protocol and datalink type; it shows up very clearly in NFT (any kind of file transfer or directory listing) and also in NET (Set Host).  The symptom is that data comes across in fairly short bursts, separated by about a second of pause.

This turns out to be an interaction between the DECnet/E queueing rules and the very fast operation of SIMH on modern hosts.  DECnet/E will queue up to some small number of NSP segments for any given connection, set by the executor parameter "data transmit queue max".  The default value is 4 or 5, but it can be set higher, and that helps some.

The trouble is this: if you have a one way data flow, for example NFT or FAL doing a copy, the sending program simply fires off a sequence of send-packet operations until it gets a "queue full" reject from the kernel.  At that point it delays, but the delay is one second since sleep operations have one second granularity.  The other end acks all that data quite promptly, but since the emulation runs so fast, the whole transmit queue can fill up before the ack from the other end arrives, so the queue full condition occurs, then a one second delay, then the process starts over.

This sort of thing doesn't happen on request/response exchanges; for example the NCP command LOOP NODE runs at top speed because traffic is going both ways.

I tried fiddling with the data queue limit to see if increasing it would help.  It seems to, but it's not sufficient.  What does work is a larger queue limit (32 looks good) combined with CPU throttling to slow things down a bit.  I used "set throttle 2000/1" (which produces a 1 ms delay every 2000 instructions, i.e., roughly 2 MIPS processing speed which is at the high end of what real PDP-11s can do).  Those two changes combined make file transfer run smoothly and fast.

Ideally DECnet/E should cancel the program sleep when the queue transitions from full to not-full, but that's not part of the existing logic (at least not unless the program explicitly asks for "link status notifications").  I could probably add that; the question is how large a change it is -- does it exceed what's feasible for a patch.  I may still do that, but at least for now the above should be helpful.

	paul