many simultaneous ptrace users

[utzoo!duke!decvax!ucbvax!DYER at HARV-10]

Does anyone know whether performance suffers when many users
are "adbing" or "sdbing" simultaneously?  Our introductory assembly
language course (approx. 200 students; 30 simultaneous users on an 11/70)
is presently using "DDT", a home-grown, now unsupported version of DEC's DDT.
It is so old that it pre-dates V6 ptrace; it executes the student's PDP-11
code interpretively.

There is a lot of feeling that we should move to a more recent, more
maintainable debugger, but I have heard rumors that, due to contention
for the single "ipc" variable in the kernel, ptrace-based debuggers
are impractical when there is lots of simultaneous debugging.
At any point as many as 15 students might be running in the debugger,
and one would like to avoid severely pathological behavior even if
more chose to run.

My questions are:
	1.) Is this a practical (as well as a theoretical) concern?
	2.) Have there been any modifications to ptrace, using multiple
	    "ipc" variables, that might avoid problems?

Any comments or tales of your own experience would be appreciated.

Steve Dyer / Harvard Science Center / DYER at HARV-10