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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 974 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 974
SEC. 11.6
CACHING IN WINDOWS
943
The Windows cache-manager facilities are shared among all the file systems.
Because the cache is virtually addressed according to individual files, the cache
manager is easily able to perform read-ahead on a per-file basis.
Requests to ac-
cess cached data come from each file system.
Virtual caching is convenient be-
cause the file systems do not have to first translate file offsets into physical block
numbers before requesting a cached file page.
Instead, the translation happens
later when the memory manager calls the file system to access the page on disk.
Besides management of the kernel virtual address and physical memory re-
sources used for caching, the cache manager also has to coordinate with file sys-
tems regarding issues like coherency of views, flushing to disk, and correct mainte-
nance of the end-of-file marks—particularly as files expand. One of the most dif-
ficult aspects of a file to manage between the file system, the cache manager, and
the memory manager is the offset of the last byte in the file, called the
ValidData-
Length
.
If a program writes past the end of the file, the blocks that were skipped
have to be filled with zeros, and for security reasons it is critical that the
Valid-
DataLength
recorded in the file metadata not allow access to uninitialized blocks,
so the zero blocks have to be written to disk before the metadata is updated with
the new length. While it is expected that if the system crashes, some of the blocks
in the file might not have been updated from memory, it is not acceptable that some
of the blocks might contain data previously belonging to other files.
Let us now examine how the cache manager works. When a file is referenced,
the cache manager maps a 256-KB chunk of kernel virtual address space onto the
file. If the file is larger than 256 KB, only a portion of the file is mapped at a time.
If the cache manager runs out of 256-KB chunks of virtual address space, it must
unmap an old file before mapping in a new one. Once a file is mapped, the cache
manager can satisfy requests for its blocks by just copying from kernel virtual ad-
dress space to the user buffer.
If the block to be copied is not in physical memory,
a page fault will occur and the memory manager will satisfy the fault in the usual
way. The cache manager is not even aware of whether the block was in memory or
not. The copy always succeeds.
The cache manager also works for pages that are mapped into virtual memory
and accessed with pointers rather than being copied between kernel and user-mode
buffers. When a thread accesses a virtual address mapped to a file and a page fault
occurs, the memory manager may in many cases be able to satisfy the access as a
soft fault. It does not need to access the disk, since it finds that the page is already
in physical memory because it is mapped by the cache manager.
11.7 INPUT/OUTPUT IN WINDOWS
The goals of the Windows I/O manager are to provide a fundamentally exten-
sive and flexible framework for efficiently handling a very wide variety of I/O de-
vices and services, support automatic device discovery and driver installation (plug
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