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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
Showing 796 out of 1137
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 796
SEC. 10.4
MEMORY MANAGEMENT IN LINUX
765
may be needed soon, in the hope it will be there when needed).
Text segments and
mapped files are paged to their respective files on disk. Everything else is paged to
either the paging partition (if present) or one of the fixed-length paging files, called
the
swap area
.
Paging files can be added and removed dynamically and each one
has a priority. Paging to a separate partition, accessed as a raw device, is more ef-
ficient than paging to a file for several reasons. First, the mapping between file
blocks and disk blocks is not needed (saves disk I/O reading indirect blocks).
Sec-
ond, the physical writes can be of any size, not just the file block size. Third, a
page is always written contiguously to disk; with a paging file, it may or may not
be.
Pages are not allocated on the paging device or partition until they are needed.
Each device and file starts with a bitmap telling which pages are free.
When a
page without backing store has to be tossed out of memory, the highest-priority
paging partition or file that still has space is chosen and a page allocated on it. Nor-
mally, the paging partition, if present, has higher priority than any paging file. The
page table is updated to reflect that the page is no longer present in memory (e.g.,
the page-not-present bit is set) and the disk location is written into the page-table
entry.
The Page Replacement Algorithm
Page replacement works as follows. Linux tries to keep some pages free so that
they can be claimed as needed.
Of course, this pool must be continually replen-
ished. The
PFRA
(
Page Frame Reclaiming Algorithm
) algorithm is how this
happens.
First of all, Linux distinguishes between four different types of pages:
unre-
claimable
,
swappable
,
syncable
, and
discardable
.
Unreclaimable pages, which in-
clude reserved or locked pages, kernel mode stacks, and the like, may not be paged
out. Swappable pages must be written back to the swap area or the paging disk par-
tition before the page can be reclaimed. Syncable pages must be written back to
disk if they have been marked as dirty. Finally, discardable pages can be reclaimed
immediately.
At boot time,
init
starts up a page daemon,
kswapd
, for each memory node, and
configures them to run periodically. Each time
kswapd
awakens, it checks to see if
there are enough free pages available, by comparing the low and high watermarks
with the current memory usage for each memory zone.
If there is enough memory,
it goes back to sleep, although it can be awakened early if more pages are suddenly
needed. If the available memory for any of the zones ever falls below a threshold,
kswapd
initiates the page frame reclaiming algorithm. During each run, only a cer-
tain target number of pages is reclaimed, typically a maximum of 32. This number
is limited to control the I/O pressure (the number of disk writes created during the
PFRA operations). Both the number of reclaimed pages and the total number of
scanned pages are configurable parameters.
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