Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 548
Since its inception, the computer industry has been driven by an endless quest
for more and more computing power. The ENIAC could perform 300 operations
per second, easily 1000 times faster than any calculator before it, yet people were
not satisfied with it.
We now have machines millions of times faster than the
ENIAC and still there is a demand for yet more horsepower. Astronomers are try-
ing to make sense of the universe, biologists are trying to understand the implica-
tions of the human genome, and aeronautical engineers are interested in building
safer and more efficient aircraft, and all want more CPU cycles. However much
computing power there is, it is never enough.
In the past, the solution was always to make the clock run faster. Unfortunate-
ly, we have begun to hit some fundamental limits on clock speed.
According to
Einstein’s special theory of relativity, no electrical signal can propagate faster than
the speed of light, which is about 30 cm/nsec in vacuum and about 20 cm/nsec in
copper wire or optical fiber. This means that in a computer with a 10-GHz clock,
the signals cannot travel more than 2 cm in total. For a 100-GHz computer the total
path length is at most 2 mm.
A 1-THz (1000-GHz) computer will have to be smal-
ler than 100 microns, just to let the signal get from one end to the other and back
once within a single clock cycle.
Making computers this small may be possible, but then we hit another funda-
mental problem: heat dissipation. The faster the computer runs, the more heat it
generates, and the smaller the computer, the harder it is to get rid of this heat. Al-
ready on high-end x86 systems, the CPU cooler is bigger than the CPU itself. All

Page 549
in all, going from 1 MHz to 1 GHz simply required incrementally better engineer-
ing of the chip manufacturing process. Going from 1 GHz to 1 THz is going to re-
quire a radically different approach.
One approach to greater speed is through massively parallel computers. These
machines consist of many CPUs, each of which runs at ‘‘normal’’ speed (whatever
that may mean in a given year), but which collectively have far more computing
power than a single CPU.
Systems with tens of thousands of CPUs are now com-
mercially available. Systems with 1 million CPUs are already being built in the lab
(Furber et al., 2013).
While there are other potential approaches to greater speed,
such as biological computers, in this chapter we will focus on systems with multi-
ple conventional CPUs.
Highly parallel computers are frequently used for heavy-duty number crunch-
ing. Problems such as predicting the weather, modeling airflow around an aircraft
wing, simulating the world economy, or understanding drug-receptor interactions
in the brain are all computationally intensive. Their solutions require long runs on
many CPUs at once.
The multiple processor systems discussed in this chapter are
widely used for these and similar problems in science and engineering, among
other areas.
Another relevant development is the incredibly rapid growth of the Internet.
was originally designed as a prototype for a fault-tolerant military control system,
then became popular among academic computer scientists, and long ago acquired
many new uses. One of these is linking up thousands of computers all over the
world to work together on large scientific problems.
In a sense, a system consist-
ing of 1000 computers spread all over the world is no different than one consisting
of 1000 computers in a single room, although the delay and other technical charac-
teristics are different. We will also consider these systems in this chapter.
Putting 1 million unrelated computers in a room is easy to do provided that
you have enough money and a sufficiently large room. Spreading 1 million unrelat-
ed computers around the world is even easier since it finesses the second problem.
The trouble comes in when you want them to communicate with one another to
work together on a single problem.
As a consequence, a great deal of work has
been done on interconnection technology, and different interconnect technologies
have led to qualitatively different kinds of systems and different software organiza-
All communication between electronic (or optical) components ultimately
comes down to sending messages—well-defined bit strings—between them. The
differences are in the time scale, distance scale, and logical organization involved.
At one extreme are the shared-memory multiprocessors, in which somewhere be-
tween two and about 1000 CPUs communicate via a shared memory.
In this
model, every CPU has equal access to the entire physical memory, and can read
and write individual words using
instructions. Accessing a mem-
ory word usually takes 1–10 nsec.
As we shall see, it is now common to put more
than one processing core on a single CPU chip, with the cores sharing access to

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