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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 253
222
MEMORY MANAGEMENT
CHAP. 3
Second chance is a modification to FIFO that checks if a page is in use before
removing it.
If it is, the page is spared.
This modification greatly improves the
performance. Clock is simply a different implementation of second chance.
It has
the same performance properties, but takes a little less time to execute the algo-
rithm.
LRU is an excellent algorithm, but it cannot be implemented without special
hardware. If this hardware is not available, it cannot be used.
NFU is a crude at-
tempt to approximate LRU. It is not very good. However, aging is a much better
approximation to LRU and can be implemented efficiently.
It is a good choice.
The last two algorithms use the working set. The working set algorithm gives
reasonable performance, but it is somewhat expensive to implement. WSClock is a
variant that not only gives good performance but is also efficient to implement.
All in all, the two best algorithms are aging and WSClock. They are based on
LRU and the working set, respectively.
Both give good paging performance and
can be implemented efficiently.
A few other good algorithms exist, but these two
are probably the most important in practice.
3.5 DESIGN ISSUES FOR PAGING SYSTEMS
In the previous sections we have explained how paging works and have given a
few of the basic page replacement algorithms.
But knowing the bare mechanics is
not enough.
To design a system and make it work well you have to know a lot
more. It is like the difference between knowing how to move the rook, knight,
bishop, and other pieces in chess, and being a good player.
In the following sec-
tions, we will look at other issues that operating system designers must consider
carefully in order to get good performance from a paging system.
3.5.1 Local versus Global Allocation Policies
In the preceding sections we have discussed several algorithms for choosing a
page to replace when a fault occurs.
A major issue associated with this choice
(which we have carefully swept under the rug until now) is how memory should be
allocated among the competing runnable processes.
Take a look at Fig. 3-22(a). In this figure, three processes,
A
,
B
, and
C
, make
up the set of runnable processes. Suppose
A
gets a page fault. Should the page re-
placement algorithm try to find the least recently used page considering only the
six pages currently allocated to
A
, or should it consider all the pages in memory?
If it looks only at
A
’s pages, the page with the lowest age value is
A5
, so we get the
situation of Fig. 3-22(b).
On the other hand, if the page with the lowest age value is removed without
regard to whose page it is, page
B3
will be chosen and we will get the situation of
Fig. 3-22(c).
The algorithm of Fig. 3-22(b) is said to be a
local
page replacement
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