RFC 685 (rfc685) - Page 1 of 3

Response time in cross network debugging

Network Working Group                                Michael Beeler (BBN-TENEX)
Request for Comments: 685                                        April 16, 1975
NIC #32298


            Response Time in Cross-network Debugging



Cross-network debugging is a means whereby a programmer at one
computer on a network can debug a program which executes on another
computer. One form of cross-net debugging has been in use for some
years by programmers who maintain IMPs on the ARPA network. Another
form has been used by ARPA network users who employ TELNET or RSEXEC
to log into a distant computer and remotely run a debugger on that
machine. In both of these cases, the debugger is almost entirely
resident in the same machine as the subject program, and only a
remote means of access to that computer distinguishes the activity
from single-computer debugging.

In our case, we use a PDP-10 to perform complex debugging
manipulations. Simple manipulations, and complex actions which the
PDP-10 has partially digested into simple actions, are sent over the
ARPA network to a PDP-11. The portion of the debugger resident in
the PDP-11, where the subject program executes, can perform only
simple actions (examine, deposit, start, stop, set breakpoint,
etc.). This division of debugging computation between the two
machines is implemented and in use at BBN. A user s manual is
available (as (BBN]XNET.DOC) describing the
debugger s features and discussing some of the issues involved.

The purpose of this RFC is to describe our experience with
response times the debugger exhibits. Response time is a serious
problem in any elaboration to a debugger. Here we wish to discuss
the contribution of communicating over the ARPA network to response
time. The debugger (X-NET) keeps statistics during each debugging
session, and a debugger command prints them out. We used a
"standard scenario" to measure response times on two occasions. The
first was debugging a PDP-11 at BBN on the same IMP as the PDP-10.
The second was with a PDP-11 at SRI-ARC in California, with at least
nine IMPs intervening.

BBN (local) SRI (distant)
TENEX LOAD AVG
START 6.0 4.65
FINISH 3.9 6.6
TIME OF DAY 15:30 EST 11:00 EST
DAY 4/10/75
4/11/75