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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 971
940
CASE STUDY 2: WINDOWS 8
CHAP. 11
X
X
X
X
State
Cnt
WS
PT
Other
Next
Clean
Dirty
Clean
Active
Clean
Dirty
Active
Dirty
Free
Free
Zeroed
Active
Active
Zeroed
13
12
11
20
10
8
4
7
6
5
4
3
6
2
1
14
0
14
Standby
Modified
Free
Zeroed
Page tables
Page-frame number database
Zeroed
List headers
9
Figure 11-33.
Some of the major fields in the page-frame database for a valid
page.
faulted back to it, so the valid pages in its page table and any of its pages on the
modified or standby lists go on the free list (3).
Any pagefile space in use by the
process is also freed.
Working
Sets
Modified
page
list
Standby
page
list
Free
page
list
Zeroed
page
list
Bad memory
page
list
Modified
page
writer
(4)
Dealloc
(5)
Zero
page
thread
(7)
Page evicted from all working sets (1)
Process exit (3)
Soft page fault (2)
Zero page needed (8)
Page referenced (6)
Figure 11-34.
The various page lists and the transitions between them.
Other transitions are caused by other system threads.
Every 4 seconds the bal-
ance set manager thread runs and looks for processes all of whose threads have
been idle for a certain number of seconds.
If it finds any such processes, their
Page 972
SEC. 11.5
MEMORY MANAGEMENT
941
kernel stacks are unpinned from physical memory and their pages are moved to the
standby or modified lists, also shown as (1).
Two other system threads, the
mapped page writer
and the
modified page
writer
, wake up periodically to see if there are enough clean pages.
If not, they
take pages from the top of the modified list, write them back to disk, and then
move them to the standby list (4).
The former handles writes to mapped files and
the latter handles writes to the pagefiles. The result of these writes is to transform
modified (dirty) pages into standby (clean) pages.
The reason for having two threads is that a mapped file might have to grow as
a result of the write, and growing it requires access to on-disk data structures to al-
locate a free disk block.
If there is no room in memory to bring them in when a
page has to be written, a deadlock could result.
The other thread can solve the
problem by writing out pages to a paging file.
The other transitions in Fig. 11-34 are as follows. If a process unmaps a page,
the page is no longer associated with a process and can go on the free list (5), ex-
cept for the case that it is shared.
When a page fault requires a page frame to hold
the page about to be read in, the page frame is taken from the free list (6), if pos-
sible. It does not matter that the page may still contain confidential information
because it is about to be overwritten in its entirety.
The situation is different when a stack grows. In that case, an empty page
frame is needed and the security rules require the page to contain all zeros.
For
this reason, another kernel system thread, the
ZeroPage thread
, runs at the lowest
priority (see Fig. 11-26), erasing pages that are on the free list and putting them on
the zeroed page list (7).
Whenever the CPU is idle and there are free pages, they
might as well be zeroed since a zeroed page is potentially more useful than a free
page and it costs nothing to zero the page when the CPU is idle.
The existence of all these lists leads to some subtle policy choices. For ex-
ample, suppose that a page has to be brought in from disk and the free list is empty.
The system is now forced to choose between taking a clean page from the standby
list (which might otherwise have been faulted back in later) or an empty page from
the zeroed page list (throwing away the work done in zeroing it).
Which is better?
The memory manager has to decide how aggressively the system threads
should move pages from the modified list to the standby list.
Having clean pages
around is better than having dirty pages around (since clean ones can be reused in-
stantly), but an aggressive cleaning policy means more disk I/O and there is some
chance that a newly cleaned page may be faulted back into a working set and dirt-
ied again anyway.
In general, Windows resolves these kinds of trade-offs through
algorithms, heuristics, guesswork, historical precedent, rules of thumb, and
administrator-controlled parameter settings.
Modern Windows introduced an additional abstraction layer at the bottom of
the memory manager, called the
store manager
.
This layer makes decisions about
how to optimize the I/O operations to the available backing stores.
Persistent stor-
age systems include auxiliary flash memory and SSDs in addition to rotating disks.
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