Page 49 - The Art of Designing Embedded Systems

36 THE ART OF DESIGNING EMBEDDED SYSTEMS

COCOMO Data
Barry Boehm codified this concept in his Constructive Cost Model
(COCOMO). He found that
Effort to create a project = C x KLOC‘.

(KLOC means “thousands of lines of code.”)
Though the exact values of C and M vary depending on a number of
factors (e.g., real-time code is harder than that for the user interface), both
are always greater than 1.
A bit of algebra shows that, since M > 1, effort grows much faster
than the size of the program.
For real-time projects managed with the very best practices, C is typ-
ically 3.6 and M around 1.2. In embedded systems, which combine the
worst problems of real time with hardware dependencies, these coeffi-
cients are higher. Toss in the typical poor software practices of the em-
bedded industries and the M exponent can climb well above 1.5.
Suppose C = 1 and M = 1.4. At the risk of oversimplifying Boehm’s
model, we can still get an idea of the nonlinear growth of complexity with
program size as follows:
Lines of Effort Comments
code
10,000 25.1
20,000 66.3 Double size of code; effort goes up by 2.64
100,000 63 1 Size grows by factor of 10; effort grows by 25

So, in doubling the size of the program we incur 32% additional
overhead.
The human analogy of this phenomenon is the one so colorfully il-
lustrated by Fred Brooks in his The Mythical Man-Month (a must read for
all software folks). As projects grow, adding people has a diminishing re-
turn. One reason is the increased number of communications channels.
Two people can only talk to each other; there’s only a single comm path.
Three workers have three communications paths; four have six. In fact, the
growth of links is exponential: given n workers, there are (n2 - n)/2 links
between team members.
In other words, add one worker and suddenly he’s interfacing in n2
ways with the others. Pretty soon memos and meetings eat up the entire
work day.
The solution is clear: break teams into smaller, autonomous, and in-
dependent units to reduce these communications links.