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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 269
238
MEMORY MANAGEMENT
CHAP. 3
However, this simple model has a problem: processes can increase in size after
starting. Although the program text is usually fixed, the data area can sometimes
grow, and the stack can always grow. Consequently, it may be better to reserve sep-
arate swap areas for the text, data, and stack and allow each of these areas to con-
sist of more than one chunk on the disk.
The other extreme is to allocate nothing in advance and allocate disk space for
each page when it is swapped out and deallocate it when it is swapped back in.
In
this way, processes in memory do not tie up any swap space. The disadvantage is
that a disk address is needed in memory to keep track of each page on disk.
In
other words, there must be a table per process telling for each page on disk where
it is. The two alternatives are shown in Fig. 3-28.
0
4
3
6
6
4
3
0
7
5
2
1
Pages
Page
table
Main memory
Disk
Swap area
(a)
0
4
3
6
6
4
3
0
5
1
7
2
Pages
Page
table
Main memory
Disk
Swap area
(b)
Disk
map
Figure 3-28.
(a) Paging to a static swap area. (b) Backing up pages dynamically.
In Fig. 3-28(a), a page table with eight pages is shown. Pages 0, 3, 4, and 6 are
in main memory.
Pages 1, 2, 5, and 7 are on disk. The swap area on disk is as large
as the process virtual address space (eight pages), with each page having a fixed lo-
cation to which it is written when it is evicted from main memory. Calculating this
address requires knowing only where the process’ paging area begins, since pages
are stored in it contiguously in order of their virtual page number. A page that is in
memory always has a shadow copy on disk, but this copy may be out of date if the
page has been modified since being loaded. The shaded pages in memory indicate
pages not present in memory.
The shaded pages on the disk are (in principle)
superseded by the copies in memory, although if a memory page has to be swapped
back to disk and it has not been modified since it was loaded, the (shaded) disk
copy will be used.
In Fig. 3-28(b), pages do not have fixed addresses on disk. When a page is
swapped out, an empty disk page is chosen on the fly and the disk map (which has
Page 270
SEC. 3.6
IMPLEMENTATION ISSUES
239
room for one disk address per virtual page) is updated accordingly. A page in
memory has no copy on disk. The pages’ entries in the disk map contain an invalid
disk address or a bit marking them as not in use.
Having a fixed swap partition is not always possible. For example, no disk par-
titions may be available. In this case, one or more large, preallocated files within
the normal file system can be used.
Windows uses this approach. However, an
optimization can be used here to reduce the amount of disk space needed. Since the
program text of every process came from some (executable) file in the file system,
the executable file can be used as the swap area. Better yet, since the program text
is generally read only, when memory is tight and program pages have to be evicted
from memory, they are just discarded and read in again from the executable file
when needed. Shared libraries can also work this way.
3.6.6 Separation of Policy and Mechanism
An important tool for managing the complexity of any system is to split policy
from mechanism. This principle can be applied to memory management by having
most of the memory manager run as a user-level process. Such a separation was
first done in Mach (Young et al., 1987) on which the discussion below is based.
A simple example of how policy and mechanism can be separated is shown in
Fig. 3-29.
Here the memory management system is divided into three parts:
1. A low-level MMU handler.
2.
A page fault handler that is part of the kernel.
3.
An external pager running in user space.
All the details of how the MMU works are encapsulated in the MMU handler,
which is machine-dependent code and has to be rewritten for each new platform
the operating system is ported to. The page-fault handler is machine-independent
code and contains most of the mechanism for paging. The policy is largely deter-
mined by the external pager, which runs as a user process.
When a process starts up, the external pager is notified in order to set up the
process’ page map and allocate the necessary backing store on the disk if need be.
As the process runs, it may map new objects into its address space, so the external
pager is once again notified.
Once the process starts running, it may get a page fault. The fault handler fig-
ures out which virtual page is needed and sends a message to the external pager,
telling it the problem. The external pager then reads the needed page in from the
disk and copies it to a portion of its own address space. Then it tells the fault hand-
ler where the page is.
The fault handler then unmaps the page from the external
pager’s address space and asks the MMU handler to put it into the user’s address
space at the right place. Then the user process can be restarted.
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