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]<DOCUMENTATION>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

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Each session lasted about 10 minutes. The terms used below

are:

message -- a single message generated by the PDP-10 portion of X-NET

active command -- a command which involves, actually or virtually,

an interaction with the subject program (e.g., examine, deposit,

start, stop, set breakpoint, etc.)

bytes -- the total of all (8-bit) bytes, both sent and received,

plus any bytes due to receipt of asynchronous replies (e.g.,

breakpoint hit), during processing of the associated message or

active command.

wait -- total elapsed time from handing message to implementation

language (BCPL) network routines, to receipt of the reply from

these routines and through an inferior process in X-NET

The 35 active commands in the scenario are:

1 load program

8 start or proceed program

3 halt program

16 examine contents of memory word

1 deposit new contents in memory word

1 set breakpoint

1 remove breakpoint

1 word search

1 copy program onto disk file

2 network/process management (see user's manual)

The summary statistics are:.

BBN (local) SRI (distant)

AVG STD DEV AVG STD DEV

PER MESSAGE:

BYTES 154 295 164 295

WAIT 1.75 2.04 1.89 0.78 SEC

PER ACTIVE COMMAND:

MSGS 0.91 0.69 0.91 0.69

BYTES 150 331 150 331

WAIT 1.60 2.36 1.73 1.35 SEC

The standard deviation is relatively large partly because of a

small number of samples, but even more because the message size and

the command complexity are bimodal, as shown by the histograms

below. The load and word search commands transferred many bytes, as

did an examine (the first one while the program was halted;

subsequent examines were answerable from the cache; see user s

manual). Other commands needed little or no network transaction.

Those which needed none at all produced a no-delay mode in the

distribution of waiting time per active command.

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We conclude that the delay due to network communication is

tolerable. It is of the same order of magnitude as that often

experienced on moderately loaded time sharing systems. More

explicit measurements of delays seen by user programs in general are

in progress at BBN and elsewhere; it is beyond the scope of this RFC

to discuss these delays in detail, or to break down their causes

into process activation, queueing, IMP performance, etc. Instead,

we merely note that cross-network debugging is possible in a

practical sense.

                 PER MESSAGE                 PER ACTIVE COMMAND
0                .                            XXXXXXXXX
16               .                            .
32               XXXXXXXXXXXXXXXXXXXXXXXXXX   XXXXXXXXXXXXXXXXXXX
64               .                            XXX
128              .                            .
256              XX                           .
512              XXXX                         XX
1024             .                            XX

SIZE (BYTES), BBN (local data) = SRI (distant data) Left column

gives lower bound (inclusive) on logarithmic scale. Thus, two

messages had at least 256 bytes but less than 512 bytes. An

examination of network traffic per active command shows that it is

actually trimodal: some commands are answered from the cache,

incurring no network traffic; some, such as start or stop, require

only a few tens of bytes; and some commands, such as word search and

load, cause transfer of thousands of bytes.

                 PER MESSAGE                  PER ACTIVE COMMAND
0                .                            XXXXXXXXX
1/16             X                            .
1/8              .                            .
1/4              X                            .
1/2              XXXXXXXXXX                   XXXXXXXX
1                XXXXXXXXXXXXXX               XXXXXXXXXXX
2                XXXX                         XXXX
4                X                            X
8                X                            XX

WAITING TIME (SEC), BBN (local data)

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                 PER MESSAGE                  PER ACTIVE COMMAND
0                .                            XXXXXXXXX
1/16             .                            .
1/8              .                            .
1/4              X                            .
1/2              XXXXXX                       XXX
1                XXXXXXXXXXXX                 XXXXXXXXX
2                XXXXXXXXXXXXX                XXXXXXXXXXXX
4                .                            XX
8                .                            .

WAITING TIME (SEC), SRI (distant data)