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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 787
756
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
When two users are running the same program, such as the editor, it would be
possible, but inefficient, to keep two copies of the editor’s program text in memory
at once. Instead, Linux systems support
shared text segments
.
In
Fig. 10-12(a)
and Fig. 10-12(c) we see two processes,
A
and
B
, that have the same text segment.
In Fig. 10-12(b) we see a possible layout of physical memory, in which both proc-
esses share the same piece of text. The mapping is done by the virtual-memory
hardware.
Data and stack segments are never shared except after a fork, and then only
those pages that are not modified. If either one needs to grow and there is no room
adjacent to it to grow into, there is no problem since adjacent virtual pages do not
have to map onto adjacent physical pages.
On some computers, the hardware supports separate address spaces for instruc-
tions and data. When this feature is available, Linux can use it. For example, on a
computer with 32-bit addresses, if this feature is available, there would be 2
32
bits
of address space for instructions and an additional 2
32
bits of address space for the
data and stack segments to share.
A jump or branch to 0 goes to address 0 of text
space, whereas a move from 0 uses address 0 in data space. This feature doubles
the address space available.
In addition to dynamically allocating more memory, processes in Linux can ac-
cess file data through
memory-mapped files
.
This feature makes it possible to
map a file onto a portion of a process’ address space so that the file can be read and
written as if it were a byte array in memory. Mapping a file in makes random ac-
cess to it much easier than using I/O system calls such as
read
and
write
. Shared
libraries are accessed by mapping them in using this mechanism.
In Fig. 10-13 we
see a file that is mapped into two processes at the same time, at different virtual ad-
dresses.
An additional advantage of mapping a file in is that two or more processes can
map in the same file at the same time. Writes to the file by any one of them are
then instantly visible to the others.
In fact, by mapping in a scratch file (which will
be discarded after all the processes exit), this mechanism provides a high-band-
width way for multiple processes to share memory.
In the most extreme case, two
(or more) processes could map in a file that covers the entire address space, giving
a form of sharing that is partway between separate processes and threads. Here the
address space is shared (like threads), but each process maintains its own open files
and signals, for example, which is not like threads. In practice, however, making
two address spaces exactly correspond is never done.
10.4.2 Memory Management System Calls in Linux
POSIX does not specify any system calls for memory management. This topic
was considered too machine dependent for standardization.
Instead, the problem
was swept under the rug by saying that programs needing dynamic memory man-
agement can use the
malloc
library procedure (defined by the ANSI C standard).
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