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After this documentation was released in July 2003, I was approached by Prentice Hall and asked to write a book on the Linux VM under the Bruce Peren's Open Book Series.

The book is available and called simply "Understanding The Linux Virtual Memory Manager". There is a lot of additional material in the book that is not available here, including details on later 2.4 kernels, introductions to 2.6, a whole new chapter on the shared memory filesystem, coverage of TLB management, a lot more code commentary, countless other additions and clarifications and a CD with lots of cool stuff on it. This material (although now dated and lacking in comparison to the book) will remain available although I obviously encourge you to buy the book from your favourite book store :-) . As the book is under the Bruce Perens Open Book Series, it will be available 90 days after appearing on the book shelves which means it is not available right now. When it is available, it will be downloadable from http://www.phptr.com/perens so check there for more information.

To be fully clear, this webpage is not the actual book.
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12. Swap Management

Just as Linux uses free memory for purposes such as buffering data from disk, there eventually is a need to free up private or anonymous pages used by a process. These pages, unlike those backed by a file on disk, cannot be simply discarded to be read in later. Instead they have to be carefully copied to backing storage, sometimes called the swap area. This chapter details how Linux uses and manages its backing storage.

Strictly speaking, Linux does not swap as such as ``swapping'' refers to coping an entire process address space to disk and ``paging'' to copying out portions or pages. Linux actually implements paging as modern hardware supports it, but traditionally has called it swapping in discussions and documentation. To be consistent with the Linux usage of the word, we too will refer to it as swapping.

There are two principle reasons that the existence of swap space is desirable. First, it expands the amount of memory a process may use. Virtual memory and swap space allows a large process to run even if the process is only partially resident. As ``old'' pages may be swapped out, the amount of memory addressed may easily exceed RAM as demand paging will ensure the pages are reloaded if necessary.

The casual reader12.1 may think that with a sufficient amount of memory, swap is unnecessary but this brings us to the second reason. A significant number of the pages referenced by a process early in its life may only be used for initialisation and then never used again. It is better to swap out those pages and create more disk buffers than leave them resident and unused.

It is important to note that swap is not without its drawbacks and the most important one is the most obvious one; Disk is slow, very very slow. If processes are frequently addressing a large amount of memory, no amount of swap or expensive high-performance disks will make it run within a reasonable time, only more RAM will help. This is why it is very important that the correct page be swapped out as discussed in Chapter 11, but also that related pages be stored close together in the swap space so they are likely to be swapped in at the same time while reading ahead. We will start with how Linux describes a swap area.

This chapter begins with describing the structures Linux maintains about each active swap area in the system and how the swap area information is organised on disk. We then cover how Linux remembers how to find pages in the swap after they have been paged out and how swap slots are allocated. After that the Swap Cache is discussed which is important for shared pages. At that point, there is enough information to begin understanding how swap areas are activated and deactivated, how pages are paged in and paged out and finally how the swap area is read and written to.


... reader12.1
Not to mention the affluent reader.

next up previous contents index
Next: 12.1 Describing the Swap Up: understand-html Previous: 11.6 Page Replacement Policy   Contents   Index
Mel 2004-02-15

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