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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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 128
SEC. 2.1
PROCESSES
97
For the sake of accuracy, it should be pointed out that the probabilistic model
just described is only an approximation.
It implicitly assumes that all
n
processes
are independent, meaning that it is quite acceptable for a system with five proc-
esses in memory to have three running and two waiting. But with a single CPU, we
cannot have three processes running at once, so a process becoming ready while
the CPU is busy will have to wait. Thus the processes are not independent.
A more
accurate model can be constructed using queueing theory, but the point we are
making—multiprogramming lets processes use the CPU when it would otherwise
become idle—is, of course, still valid, even if the true curves of Fig. 2-6 are slight-
ly different from those shown in the figure.
Even though the model of Fig. 2-6 is simple-minded, it can nevertheless be
used to make specific, although approximate, predictions about CPU performance.
Suppose, for example, that a computer has 8 GB of memory, with the operating
system and its tables taking up 2 GB and each user program also taking up 2 GB.
These sizes allow three user programs to be in memory at once. With an 80% aver-
age I/O wait, we have a CPU utilization (ignoring operating system overhead) of
1
−
0. 8
3
or about 49%. Adding another 8 GB of memory allows the system to go
from three-way multiprogramming to seven-way multiprogramming, thus raising
the CPU utilization to 79%.
In other words, the additional 8 GB will raise the
throughput by 30%.
Adding yet another 8 GB would increase CPU utilization only from 79% to
91%, thus raising the throughput by only another 12%. Using this model, the com-
puter’s owner might decide that the first addition was a good investment but that
the second was not.
2.2 THREADS
In traditional operating systems, each process has an address space and a single
thread of control.
In fact, that is almost the definition of a process.
Nevertheless,
in many situations, it is desirable to have multiple threads of control in the same
address space running in quasi-parallel, as though they were (almost) separate
processes (except for the shared address space).
In the following sections we will
discuss these situations and their implications.
2.2.1 Thread Usage
Why would anyone want to have a kind of process within a process?
It turns
out there are several reasons for having these miniprocesses, called
threads
. Let
us now examine some of them. The main reason for having threads is that in many
applications, multiple activities are going on at once. Some of these may block
from time to time.
By decomposing such an application into multiple sequential
threads that run in quasi-parallel, the programming model becomes simpler.
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