dz interrupt rate

[Mark Plotnick]

After a recent session of unibus card musical chairs on our 4.2BSD 780
(which involved removing a dr11w and one of two 3com ethernet boards
and replacing an lp11-like device with a different one), we noticed
that system cpu time increased a lot whenever there was sustained
output on our dz's.  Simply catting files on 2 9600 baud lines
simultaneously, for instance, would increase system cpu time to about
50% and make the interrupt rate as reported by vmstat go up to about
1800.  (about 1 interrupt per character output).  We didn't think this
was normal, since 4.2 is supposed to use some pseudo-dma method to cut
down the interrupt rate.

We don't have any other 780 running 4.2, but on a 4.2 750 (with 1 dz
and 1 3com board on the unibus), vmstat shows about 5 interrupts per
second when doing 9600 baud output on a line (the 780 would show around
1000).

DEC's diagnostics indicated the uba was bad, but the FE came to the
conclusion that this was due to the remaining 3com board, so we put in an
interlan, but this didn't reduce the interrupt rate.  The FE thinks all
the NPG wires are correct, and pulling the ethernet and lp
boards out doesn't change things.  What are we doing (or thinking) wrong?

Here are the devices on the 780:

uba0 at tr3
dz0 at uba0 csr 160100 vec 300, ipl 15
dz1 at uba0 csr 160110 vec 310, ipl 15
dz2 at uba0 csr 160120 vec 320, ipl 15
dz3 at uba0 csr 160130 vec 330, ipl 15
dz4 at uba0 csr 160140 vec 340, ipl 15
dz5 at uba0 csr 160150 vec 350, ipl 15
dz6 at uba0 csr 160160 vec 360, ipl 15
dz7 at uba0 csr 160170 vec 370, ipl 15
il0 at uba0 csr 164000 vec 400, ipl 15
lp0 at uba0 csr 177514 vec 200, ipl 14
dkitdr0 at uba0 csr 167770 vec 420, ipl 15 [ dr11c for datakit ]
(the massbusses are occupied by 2 emulex Sc7000's, a TU-78,
and an RH780, if that matters).

	Mark Plotnick
	allegra!mp