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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 961
930
CASE STUDY 2: WINDOWS 8
CHAP. 11
Pagefiles
An interesting trade-off occurs with assignment of backing store to committed
pages that are not being mapped to specific files. These pages use the
pagefile
.
The question is
how
and
when
to map the virtual page to a specific location in the
pagefile. A simple strategy would be to assign each virtual page to a page in one
of the paging files on disk at the time the virtual page was committed.
This would
guarantee that there was always a known place to write out each committed page
should it be necessary to evict it from memory.
Windows uses a
just-in-time
strategy.
Committed pages that are backed by the
pagefile are not assigned space in the pagefile until the time that they have to be
paged out.
No disk space is allocated for pages that are never paged out.
If the
total virtual memory is less than the available physical memory, a pagefile is not
needed at all.
This is convenient for embedded systems based on Windows. It is
also the way the system is booted, since pagefiles are not initialized until the first
user-mode process,
smss.exe
, begins running.
With a preallocation strategy the total virtual memory in the system used for
private data (stacks, heap, and copy-on-write code pages) is limited to the size of
the pagefiles. With just-in-time allocation the total virtual memory can be almost
as large as the combined size of the pagefiles and physical memory.
With disks so
large and cheap vs. physical memory, the savings in space is not as significant as
the increased performance that is possible.
With demand-paging, requests to read pages from disk need to be initiated
right away, as the thread that encountered the missing page cannot continue until
this
page-in
operation completes.
The possible optimizations for faulting pages in-
to memory involve attempting to prepage additional pages in the same I/O opera-
tion. However, operations that write modified pages to disk are not normally syn-
chronous with the execution of threads.
The just-in-time strategy for allocating
pagefile space takes advantage of this to boost the performance of writing modified
pages to the pagefile. Modified pages are grouped together and written in big
chunks. Since the allocation of space in the pagefile does not happen until the
pages are being written, the number of seeks required to write a batch of pages can
be optimized by allocating the pagefile pages to be near each other, or even making
them contiguous.
When pages stored in the pagefile are read into memory, they keep their alloca-
tion in the pagefile until the first time they are modified. If a page is never modi-
fied, it will go onto a special list of free physical pages, called the
standby list
,
where it can be reused without having to be written back to disk.
If it
is
modified,
the memory manager will free the pagefile page and the only copy of the page will
be in memory.
The memory manager implements this by marking the page as
read-only after it is loaded.
The first time a thread attempts to write the page the
memory manager will detect this situation and free the pagefile page, grant write
access to the page, and then have the thread try again.
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