Page 374 - DCAP103_Principle of operating system

Unit 12: Processes and Threads in Windows

Notes
Figure 12.13: The Various Page Lists and the Transitions between Them

Pages are moved between the working sets and the various lists by the working set manager
and other kernel daemon threads. Let us examine the transitions. When the working set manager
removes a page from a working set, the page goes on the bottom of the standby or modified
list, depending on its state of cleanliness. This transition is shown as (1). Pages on both lists are
still valid pages, so if a page fault occurs and one of these pages is needed, it is removed from
the list and faulted back into the working set without any disk I/O (2). When a process exits
its nonshared pages cannot be faulted back to it, so they go on the free list (3). These pages are
no longer associated with any process.
Other transitions are caused by other daemon threads. Every 4 seconds the swapper 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 kernel stacks are unpinned and their pages are moved to the
standby or modified lists, also shown as (1).
Two other daemon threads, the mapped page writer and the modified page writer, wake up
periodically to see if there are enough clean pages. If there are not, they take pages from the
top of the modified list, write them back to disk, and then move them to the standby list. The
former handles writes to mapped files and the latter handles writes to the paging files. The
result of these writes is to transform dirty pages into 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 allocate 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 the pages to a paging file, which never
grows. Nobody ever said Windows 2000 was simple.
The other transitions in Figure 12.13 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), except 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 possible. 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 daemon thread, the zero page thread, runs at the lowest
priority, erasing pages that are on the free list and putting them on the zeroed page list (7).

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