Some documentation on swap file support by Zhao Shuai.
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docs/develop/kernel/swap_file_support
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docs/develop/kernel/swap_file_support
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Haiku swap file support
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This article describes how to use swap file in Haiku and how the swap system
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works.
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1. How to use a swap file?
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Like BeOS, Haiku uses "/var/swap" as default swap file. It is created
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during the boot process and its size is twice the size of physical memory by
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default. You can change its size through the VirtualMemory preference
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application and your settings will take effect after restarting the system.
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The default swap file "/var/swap" may not satisfy your need. Haiku allows
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adding/removing a swap file dynamically. (This is *NOT* implemented yet, since
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I do not know how to add bin commands "swapon" and "swapoff" in the system.
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It needs to be done in the future.)
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2. How swap system works?
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The virtual memory subsystem of Haiku is very similar to that of FreeBSD,
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therefore our swap system implementation is borrowed from FreeBSD.
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A swap system has two main functions: (1) maintain a map between anonymous
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pages and swap space, so we can page in/out when needed. (2) manage the
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allocation/deallocation of swap space. Let's see how these are implemented in
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Haiku.
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In order to maintain a map between pages and swap space, we need to record
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the pages' swap address somewhere. Here we use swap blocks. A "swap_block"
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structure contains swap address information for 32 (value of SWAP_BLOCK_PAGES)
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consecutive pages from a same cache. So whenever we look for a page in swap
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files, we should get the swap block for it. But how to get the swap block?
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Here we use hash table. All swap blocks in the system are arranged into a global
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hash table. The hash table uses a cache's address and page index in this cache
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as hash key.
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Here is an example. Suppose a page has been paged out to swap space and now
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its cache wants to page it in. It works as follows: look up the swap hash table
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using address of the cache and page index as hash key, if successful, we get
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the swap block containing the this page's swap address. Then search the swap
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block to get the exact swap address of this page. After that, we can read the
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page from swap file using vfs functions.
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I draw a picture and hope it could help you understand the above words. If
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the pic becomes a mess on your computer, please set the tab width of your text
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editor to 4.
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___________________________________________________________
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sSwapHashTable |__________|___NULL___|___NULL___|___________|____NULL____|
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___V___ ___V___
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swap_block /----|__0__| /--------|__5__|
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| |__3__|--------\ | /---|__6__|
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| |_..._| | | | |_..._|
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| |__2__|----\ | | | |__20_|--------------->
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| | | | |
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| _____________V___V_________V____V_________________________
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swap_file `->|slot|slot|slot|slot|slot|slot|slot|slot|slot|slot|....|
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|_0__|_1__|_2__|_3__|_4__|_5__|_6__|_7__|_8__|_9__|____|__
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The swap system also manages allocation/deallocation of swap space. In our
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implementation, each swap file is divided into page-sized slots(called "swap
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pages") and a swap file can be seen as an array of many swap pages(see the
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above picture). Swap page is the unit for swap space allocation/deallocation
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and we use swap page index (slot index) as swap space address instead of offset.
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All the swap pages in the system are given a unified address and we leave one
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page gap between two swap files. (e.g. there are 3 swap files in the system,
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each has 100 swap pages, the address range(to be exact, page index) for each
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swap file is: 0-99, 101-200, 202-301) Why leave a page gap between swap files?
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Because in this way, we can easily tell if two adjacent pages are in a same
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swap file. (See the code in VMAnonymousCache::Read()).
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Each swap file has a linear bitmap, and all pages in the swap file are
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associated with a bit in the bitmap. When the page is used, the corresponding
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bit is set to 1 and when free, it is set to 0. The linear bitmap is different
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from FreeBSD. FreeBSD uses a radix bitmap and it's more efficient than linear
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bitmap especially when swap space is fragmented. The reason why I did not use
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a radix bitmap is I do not get a total understanding of radix tree structure
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currently. But I will replace the linear bitmap with radix bitmap in the
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future.
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Swap space allocation takes place when we swap anonymous pages out.
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In order to make the allocation less probable to fail, anonymous cache will
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reserve swap space when it is initialized. If there is not enough swap space
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left, physical memory will be reserved. Swap space deallocation happens when
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available swap space is low. The page daemon will scan a number of pages and
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if the scanned page has swap space assigned, its swap space will be freed.
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3. Acknowledgement
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Special thanks to my mentor Ingo. He is a knowledged person and always
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gives me encouragement. Without his consistent and illuminating instructions,
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this project would not have reached its present status.
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If you find bugs or have suggestions for swap system, you can contact me
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via upczhsh@163.com. Thanks in advance.
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Zhao Shuai
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upczhsh@163.com
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2008-08-21
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