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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 123
92
PROCESSES AND THREADS
CHAP. 2
per terminal. These processes wait for someone to log in.
If a login is successful,
the login process executes a shell to accept commands. These commands may start
up more processes, and so forth.
Thus, all the processes in the whole system be-
long to a single tree, with
init
at the root.
In contrast, Windows has no concept of a process hierarchy. All processes are
equal. The only hint of a process hierarchy is that when a process is created, the
parent is given a special token (called a
handle
) that it can use to control the child.
However, it is free to pass this token to some other process, thus invalidating the
hierarchy. Processes in UNIX cannot disinherit their children.
2.1.5 Process States
Although each process is an independent entity, with its own program counter
and internal state, processes often need to interact with other processes. One proc-
ess may generate some output that another process uses as input.
In the shell com-
mand
cat chapter1 chapter2 chapter3 | grep tree
the first process, running
cat
, concatenates and outputs three files. The second
process, running
grep
,selects all lines containing the word ‘‘tree.’’ Depending on
the relative speeds of the two processes (which depends on both the relative com-
plexity of the programs and how much CPU time each one has had), it may happen
that
grep
is ready to run, but there is no input waiting for it.
It must then block
until some input is available.
When a process blocks, it does so because logically it cannot continue, typi-
cally because it is waiting for input that is not yet available. It is also possible for a
process that is conceptually ready and able to run to be stopped because the operat-
ing system has decided to allocate the CPU to another process for a while. These
two conditions are completely different. In the first case, the suspension is inher-
ent in the problem (you cannot process the user’s command line until it has been
typed). In the second case, it is a technicality of the system (not enough CPUs to
give each process its own private processor).
In Fig. 2-2 we see a state diagram
showing the three states a process may be in:
1.
Running (actually using the CPU at that instant).
2.
Ready (runnable; temporarily stopped to let another process run).
3.
Blocked (unable to run until some external event happens).
Logically, the first two states are similar.
In both cases the process is willing to
run, only in the second one, there is temporarily no CPU available for it. The third
state is fundamentally different from the first two in that the process cannot run,
even if the CPU is idle and has nothing else to do.
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