[HECnet] Emulated XQ polling timer setting and data overrun

[Johnny Billquist]

On 2014-06-05 19:23, Paul_Koning at Dell.com wrote:

On Jun 5, 2014, at 12:47 PM, Johnny Billquist <bqt at softjar.se> wrote:

...
It don't. Believe me, I've seen this, and investigated it years ago.
When you have a real PDP-11 running on half-duplex 10Mb/s talking to something on a 1GB/s full duplex, the PDP-11 simply can't keep up.

Makes sense.   The issue isn   t the 10 Mb/s Ethernet.   The switch deals with that part.   The issue is that a PDP-11 isn   t fast enough to keep up with a 10 Mb/s Ethernet going flat out.   If I remember right, a Unibus is slower than Ethernet, and while a Q22 bus is slightly faster and could theoretically keep up, a practical system cannot.

It's several things. The Unibus is definitely slower than the ethernet if I remember right. The Qbus, while faster, is also slower than ethernet.
So there is definitely a bottleneck at that level.

However, there is also an issue in the switch. If one system is pumping out packets on a 1Gb/s port, and the switch is forwarding them to a 10Mb/s port, the switch needs to buffer, and might need to buffer a lot. There are limitations at that level as well, and I would not be surprised if that also can come into play here.

Thridly, even given the limitations above, we then also have the software on the PDP-11, which also needs to set up new buffers to receive packet into, and the system itself will not be able to keep up here. So the ethernet controller is probably running out of buffers to DMA data into as well.

	Johnny