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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 571
540
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
An example of the latter situation occurs regularly in program development en-
vironments. Large systems often consist of some number of header files containing
macros, type definitions, and variable declarations that are used by the actual code
files. When a header file is changed, all the code files that include it must be re-
compiled. The program
make
is commonly used to manage development. When
make
is invoked, it starts the compilation of only those code files that must be re-
compiled on account of changes to the header or code files. Object files that are
still valid are not regenerated.
The original version of
make
did its work sequentially, but newer versions de-
signed for multiprocessors can start up all the compilations at once.
If 10 compila-
tions are needed, it does not make sense to schedule 9 of them to run immediately
and leave the last one until much later since the user will not perceive the work as
completed until the last one has finished. In this case it makes sense to regard the
threads doing the compilations as a group and to take that into account when
scheduling them.
Moroever sometimes it is useful to schedule threads that communicate exten-
sively, say in a producer-consumer fashion, not just at the same time, but also close
together in space. For instance, they may benefit from sharing caches. Likewise, in
NUMA architectures, it may help if they access memory that is close by.
Time Sharing
Let us first address the case of scheduling independent threads; later we will
consider how to schedule related threads. The simplest scheduling algorithm for
dealing with unrelated threads is to have a single systemwide data structure for
ready threads, possibly just a list, but more likely a set of lists for threads at dif-
ferent priorities as depicted in Fig. 8-12(a). Here the 16 CPUs are all currently
busy, and a prioritized set of 14 threads are waiting to run. The first CPU to finish
its current work (or have its thread block) is CPU 4, which then locks the schedul-
ing queues and selects the highest-priority thread,
A
, as shown in Fig. 8-12(b).
Next, CPU 12 goes idle and chooses thread
B
, as illustrated in Fig. 8-12(c). As
long as the threads are completely unrelated, doing scheduling this way is a rea-
sonable choice and it is very simple to implement efficiently.
Having a single scheduling data structure used by all CPUs timeshares the
CPUs, much as they would be in a uniprocessor system.
It also provides automatic
load balancing because it can never happen that one CPU is idle while others are
overloaded. Two disadvantages of this approach are the potential contention for the
scheduling data structure as the number of CPUs grows and the usual overhead in
doing a context switch when a thread blocks for I/O.
It is also possible that a context switch happens when a thread’s quantum ex-
pires. On a multiprocessor, that has certain properties not present on a uniproc-
essor.
Suppose that the thread happens to hold a spin lock when its quantum ex-
pires. Other CPUs waiting on the spin lock just waste their time spinning until that
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