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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 257
226
MEMORY MANAGEMENT
CHAP. 3
Determining the best page size requires balancing several competing factors.
As a result, there is no overall optimum.
To start with, two factors argue for a
small page size.
A randomly chosen text, data, or stack segment will not fill an
integral number of pages.
On the average, half of the final page will be empty.
The extra space in that page is wasted. This wastage is called
internal fragmenta-
tion
. With
n
segments in memory and a page size of
p
bytes,
np
/2 bytes will be
wasted on internal fragmentation. This reasoning argues for a small page size.
Another argument for a small page size becomes apparent if we think about a
program consisting of eight sequential phases of 4 KB each. With a 32-KB page
size, the program must be allocated 32 KB all the time.
With a 16-KB page size, it
needs only 16 KB.
With a page size of 4 KB or smaller, it requires only 4 KB at
any instant. In general, a large page size will cause more wasted space to be in
memory than a small page size.
On the other hand, small pages mean that programs will need many pages, and
thus a large page table.
A 32-KB program needs only four 8-KB pages, but 64
512-byte pages.
Transfers to and from the disk are generally a page at a time, with
most of the time being for the seek and rotational delay, so that transferring a small
page takes almost as much time as transferring a large page.
It might take 64
×
10
msec to load 64 512-byte pages, but only 4
×
12 msec to load four 8-KB pages.
Also, small pages use up much valuable space in the
TLB
.
Say your program
uses 1 MB of memory with a working set of 64 KB. With 4-KB pages, the pro-
gram would occupy at least 16 entries in the TLB. With 2-MB pages, a single TLB
entry would be sufficient (in theory, it may be that you want to separate data and
instructions). As TLB entries are scarce, and critical for performance, it pays to use
large pages wherever possible. To balance all these trade-offs, operating systems
sometimes use different page sizes for different parts of the system. For instance,
large pages for the kernel and smaller ones for user processes.
On some machines, the page table must be loaded (by the operating system)
into hardware registers every time the CPU switches from one process to another.
On these machines, having a small page size means that the time required to load
the page registers gets longer as the page size gets smaller. Furthermore, the space
occupied by the page table increases as the page size decreases.
This last point can be analyzed mathematically. Let the average process size be
s
bytes and the page size be
p
bytes. Furthermore, assume that each page entry re-
quires
e
bytes. The approximate number of pages needed per process is then
s
/
p
,
occupying
se
/
p
bytes of page table space. The wasted memory in the last page of
the process due to internal fragmentation is
p
/2. Thus, the total overhead due to
the page table and the internal fragmentation loss is given by the sum of these two
terms:
overhead
=
se
/
p
+
p
/2
The first term (page table size) is large when the page size is small. The second
term (internal fragmentation) is large when the page size is large. The optimum
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