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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 968
SEC. 11.5
MEMORY MANAGEMENT
937
The memory manager can allocate pages as needed using either the free list or
the standby list.
Before allocating a page and copying it in from disk, the memory
manager always checks the standby and modified lists to see if it already has the
page in memory. The prepaging scheme in Windows thus converts future hard
faults into soft faults by reading in the pages that are expected to be needed and
pushing them onto the standby list.
The memory manager itself does a small
amount of ordinary prepaging by accessing groups of consecutive pages rather than
single pages.
The additional pages are immediately put on the standby list.
This is
not generally wasteful because the overhead in the memory manager is very much
dominated by the cost of doing a single I/O.
Reading a cluster of pages rather than
a single page is negligibly more expensive.
The page-table entries in Fig. 11-31 refer to physical page numbers, not virtual
page numbers.
To update page-table (and page-directory) entries, the kernel needs
to use virtual addresses.
Windows maps the page tables and page directories for
the current process into kernel virtual address space using self-map entries in the
page directory, as shown in Fig. 11-32. By making page-directory entries point at
the page directory (the self-map), there are virtual addresses that can be used to
refer to page-directory entries (a) as well as page table entries (b).
The self-map
occupies the same 8 MB of kernel virtual addresses for every process (on the x86).
For simplicity the figure shows the x86 self-map for 32-bit
PTEs
(
Page-Table
Entries
). Windows actually uses 64-bit PTEs so the system can makes use of
more than 4 GB of physical memory.
With 32-bit PTEs, the self-map uses only
one
PDE
(
Page-Directory Entry
) in the page directory, and thus occupies only 4
MB of addresses rather than 8 MB.
The Page Replacement Algorithm
When the number of free physical memory pages starts to get low, the memory
manager starts working to make more physical pages available by removing them
from user-mode processes as well as the system process, which represents kernel-
mode use of pages.
The goal is to have the most important virtual pages present in
memory and the others on disk.
The trick is in determining what
important
means.
In Windows this is answered by making heavy use of the working-set concept.
Each process (
not
each thread) has a working set.
This set consists of the map-
ped-in pages that are in memory and thus can be referenced without a page fault.
The size and composition of the working set fluctuates as the process’ threads run,
of course.
Each process’ working set is described by two parameters: the minimum size
and the maximum size.
These are not hard bounds, so a process may have fewer
pages in memory than its minimum or (under certain circumstances) more than its
maximum. Every process starts with the same minimum and maximum, but these
bounds can change over time, or can be determined by the job object for processes
contained in a job.
The default initial minimum is in the range 20–50 pages and
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