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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 130
SEC. 2.2
THREADS
99
Threads can help here. Suppose that the word processor is written as a two-
threaded program.
One thread interacts with the user and the other handles refor-
matting in the background.
As soon as the sentence is deleted from page 1, the
interactive thread tells the reformatting thread to reformat the whole book. Mean-
while, the interactive thread continues to listen to the keyboard and mouse and re-
sponds to simple commands like scrolling page 1 while the other thread is comput-
ing madly in the background. With a little luck, the reformatting will be completed
before the user asks to see page 600, so it can be displayed instantly.
While we are at it, why not add a third thread?
Many word processors have a
feature of automatically saving the entire file to disk every few minutes to protect
the user against losing a day’s work in the event of a program crash, system crash,
or power failure. The third thread can handle the disk backups without interfering
with the other two. The situation with three threads is shown in Fig. 2-7.
Kernel
Keyboard
Disk
Four score and seven
years ago, our fathers
brought forth upon this
continent a new nation:
conceived in liberty,
and dedicated to the
proposition
that
all
men are created equal.
Now we are engaged
in a great civil war
testing whether that
nation, or any nation
so conceived and so
dedicated, can long
endure. We are met on
a great battlefield of
that war.
We have come to
dedicate a portion of
that field as a final
resting place for those
who here gave their
lives that this nation
might
live.
It
is
altogether fitting and
proper that we should
do this.
But, in a larger sense,
we cannot dedicate, we
cannot consecrate we
cannot
hallow
this
ground.
The
brave
men, living and dead,
who
struggled
here
have consecrated it, far
above our poor power
to add or detract. The
world will little note,
nor long remember,
what we say here, but
it can never forget
what they did here.
It is for us the living,
rather, to be dedicated
here to the unfinished
work which they who
fought here have thus
far so nobly advanced.
It is rather for us to be
here dedicated to the
great task remaining
before us, that from
these honored dead we
take increased devotion
to that cause for which
they gave the last full
measure of devotion,
that we here highly
resolve that these dead
shall not have died in
vain that this nation,
under God, shall have
a new birth of freedom
and that government of
the
people
by
the
people, for the people
Figure 2-7.
A word processor with three threads.
If the program were single-threaded, then whenever a disk backup started,
commands from the keyboard and mouse would be ignored until the backup was
finished. The user would surely perceive this as sluggish performance.
Alterna-
tively, keyboard and mouse events could interrupt the disk backup, allowing good
performance but leading to a complex interrupt-driven programming model. With
three threads, the programming model is much simpler.
The first thread just inter-
acts with the user. The second thread reformats the document when told to.
The
third thread writes the contents of RAM to disk periodically.
It should be clear that having three separate processes would not work here be-
cause all three threads need to operate on the document.
By having three threads
instead of three processes, they share a common memory and thus all have access
to the document being edited.
With three processes this would be impossible.
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