[Simh] EXT :Re: RSTS and slow DECnet operation in SIMH

[Hittner, David T (IS)]

Many of the older OS systems create rather small buffers for Ethernet packets, and do not handle the fast real-time receive rates of 100Mbps and 1000Mb Ethernet in the simulators very well.

This was due to:
	1) the OS expectation of being able to service the (maximum, never seen) Ethernet 10Mbps/sec buffered packets within the time frame allowed
	2) the OS expectation that packets not buffered (dropped) would eventually be retransmitted at some point by delivery protocols, which have an implied ACK/NACK delay
	3) Limited physical memory was available for Ethernet packet buffering.

Simulator speeds (for PDPs and VAXen) and modern network speeds are so fast compared to the original hardware that frequently the OS Ethernet buffers are just too small for efficient network packet processing. OS-side inefficiency can be addressed by patching the OS with larger Ethernet buffers. Emulator-side inefficiency can be fixed by "limiting" the Ethernet emulation speed by throttling it down to a speed more appropriate to the OS buffer processing rates. But this may cause extra dropped packets, which will likely be "fixed" by protocol retransmission.

The goal of early networking was to send and receive packets, not network efficiency. It wasn't until near-line-rate 10Mbps Ethernet and 100Mbps Fast Ethernet that network drivers started to think about efficiency by creating larger buffers, and network efficiency wasn't really fully implemented until 1G Ethernet became supported by (some) OSs, because it forces much larger buffers and better processing algorithms.

Later model network cards started implementing Early Receive and Early Transmit Interrupts, so that the card could "help" the OS understand that the buffers were almost exhausted and needed to be processed immediately .

C-Kermit was able to solve a lot of the synchronous packet transfer inefficiency by creating asynchronous "sliding window" transmission acknowledgements for both serial and network communications, which replaced the synchronous send-ACK/NACK cycle with asynchronous send1-send2-send-3/ACK-1/send-4/NACK-2/send-2(retransmit)/ACK-3/send-5, etc. sliding window scheme. It was astounding how much faster the sliding window algorithm was than the original Kermit. Note that some network protocols also have a similar asynchronous ACK/NACK processes.

Dave

-----Original Message-----
From: Simh [mailto:simh-bounces at trailing-edge.com] On Behalf Of Paul Koning
Sent: Monday, May 02, 2016 1:38 PM
To: SIMH
Subject: EXT :Re: [Simh] RSTS and slow DECnet operation in SIMH


> On Apr 19, 2016, at 2:46 PM, Paul Koning <paulkoning at comcast.net> wrote:
> 
> 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.

Followup: I created a patch that implements the "wake up when the queue goes not-full".  Or more precisely, it wakes up the process whenever an ack is received; that covers the probem case and probably doesn't create many other wakeups since the program is unlikely to be sleeping otherwise.

The attached patch script does the job.  This is for RSTS V10.1.  I will take a look at RSTS 9.6; the patch is unlikely to apply there (offsets probably don't match) but the concept will apply there too.  I don't have other DECnet/E versions, let alone source listings which is what's needed to create the patch.

With this patch, you can run at full emulation speed, with the default queue limit (5).  In fact, I would recommend setting that limit; if you make the queue limit significantly larger, the patch doesn't help and things are still slow.  I suspect that comes from overrunning the queue limits at the receiving end.  (Note that DECnet/E leaves the flow control choice to the application, and most use "no" flow control, i.e., on/off only which isn't effective if the sender can overrun the buffer pool of the receiver.)

To apply the patch, give it to ONLPAT and select the monitor SIL (just <CR> will give  you the installed one).  Or you can do it with the PATCH option at boot time, in that case enter the information manually.  The manual will spell this out some more, I expect.

I have no idea if this issue can appear on real PDP-11 systems.  Possibly, if you have a fast CPU, a fast network (Ethernet) and enough latency to make the issue visible (more than a few milliseconds but way under a second).  In any case, it's unlikely to hurt, and it clearly helps a great deal in emulated systems.

	paul