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docs: Update the VM swap documentation.

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It was mostly up-to-date technically; a few details have been
amended where things have changed, and the "personal" stuff
dropped.
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Augustin Cavalier
2026-03-26 14:54:05 -04:00
parent 030b7c1426
commit 222c477cf0
1 changed files with 47 additions and 62 deletions
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docs/develop/kernel/vm/swap_file_support.rst
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Swap file
#######################
This section describes how to use swap file in Haiku and how the swap system
:hrev: hrev59537
This section describes how to use swap files in Haiku, and how the swap system
works.
How to use a swap file?
=======================
Like BeOS, Haiku uses "/var/swap" as default swap file. It is created
during the boot process and its size is twice the size of physical memory by
default. You can change its size through the VirtualMemory preference
Like BeOS, Haiku uses "/var/swap" as default swap file. It is created
during the boot process and its size is the size of physical memory by
default, unless there's under 1GB of physical memory, in which case it's
double that. You can change its size through the VirtualMemory preference
application and your settings will take effect after restarting the system.
The default swap file "/var/swap" may not satisfy your need. Haiku allows
adding/removing a swap file dynamically. (This is *NOT* implemented yet, since
I do not know how to add bin commands "swapon" and "swapoff" in the system.
It needs to be done in the future.)
The default swap file "/var/swap" may not satisfy your need. Haiku internally
supports adding/removing a swap file dynamically, but this functionality
isn't exposed to userspace yet.
How swap system works?
======================
How does the swap system work?
==============================
The virtual memory subsystem of Haiku is very similar to that of FreeBSD,
therefore our swap system implementation is borrowed from FreeBSD.
The virtual memory subsystem of Haiku is similar to that of FreeBSD,
therefore our swap system implementation was inspired by FreeBSD's.
A swap system has two main functions: (1) maintain a map between anonymous
pages and swap space, so we can page in/out when needed. (2) manage the
pages and swap space, so we can page in/out when needed. (2) manage the
allocation/deallocation of swap space. Let's see how these are implemented in
Haiku.
In order to maintain a map between pages and swap space, we need to record
the pages' swap address somewhere. Here we use swap blocks. A "swap_block"
structure contains swap address information for 32 (value of SWAP_BLOCK_PAGES)
consecutive pages from a same cache. So whenever we look for a page in swap
files, we should get the swap block for it. But how to get the swap block?
Here we use hash table. All swap blocks in the system are arranged into a global
hash table. The hash table uses a cache's address and page index in this cache
as hash key.
Here is an example. Suppose a page has been paged out to swap space and now
its cache wants to page it in. It works as follows: look up the swap hash table
using address of the cache and page index as hash key, if successful, we get
the swap block containing the this page's swap address. Then search the swap
block to get the exact swap address of this page. After that, we can read the
page from swap file using vfs functions.
I draw a picture and hope it could help you understand the above words.
structure contains swap address information for `SWAP_BLOCK_PAGES` (32)
consecutive pages from a same cache. So whenever we look for a page in swap
files, we should get the swap block for it. But how to get the swap block?
Here we use a hash table. All swap blocks in the system are arranged into a global
hash table. The hash table uses a cache's address and page index in this cache
as the hash key.
.. code-block:: text
@@ -52,9 +45,9 @@ I draw a picture and hope it could help you understand the above words.
sSwapHashTable |__________|___NULL___|___NULL___|___________|____NULL____|
| |
| |
___V___ ___V___
___V___ ___V___
swap_block /----|__0__| /--------|__5__|
| |__3__|--------\ | /---|__6__|
| |__3__|--------\ | /---|__6__|
| |_..._| | | | |_..._|
| |__2__|----\ | | | |__20_|--------------->
| | | | |
@@ -63,40 +56,32 @@ I draw a picture and hope it could help you understand the above words.
|_0__|_1__|_2__|_3__|_4__|_5__|_6__|_7__|_8__|_9__|____|__
The swap system also manages allocation/deallocation of swap space. In our
implementation, each swap file is divided into page-sized slots(called "swap
pages") and a swap file can be seen as an array of many swap pages(see the
above picture). Swap page is the unit for swap space allocation/deallocation
Here is an example. Suppose a page has been written out to swap space and now
its cache wants to read it in. It works as follows: look up the swap hash table
using address of the cache and page index as hash key, if successful, we get
the swap block containing the this page's swap address. Then search the swap
block to get the exact swap address of this page. After that, we can read the
page from swap file using vfs functions.
The swap system also manages allocation/deallocation of swap space. In our
implementation, each swap file is divided into page-sized slots (called "swap
pages") and a swap file can be seen as an array of many swap pages (see the
above diagram). Swap page is the unit for swap space allocation/deallocation
and we use swap page index (slot index) as swap space address instead of offset.
All the swap pages in the system are given a unified address and we leave one
page gap between two swap files. (e.g. there are 3 swap files in the system,
each has 100 swap pages, the address range(to be exact, page index) for each
All the swap pages in the system are given a unified address and we leave one
page gap between two swap files. (e.g. there are 3 swap files in the system,
each has 100 swap pages, the address range (to be exact, page index) for each
swap file is: 0-99, 101-200, 202-301) Why leave a page gap between swap files?
Because in this way, we can easily tell if two adjacent pages are in a same
swap file. (See the code in VMAnonymousCache::Read()).
Because in this way, we can easily tell if two adjacent pages are in the same
swap file. (See the code in `VMAnonymousCache::Read()`).
The efficiency of the FreeBSD swap system lies in a special data structure:
radix bitmap(i.e. bitmap using radix tree for hinting.) It can operate well no
matter how much fragmentation there is and no matter how large a bitmap is
used. I have ported the radix bitmap structure to Haiku, so our swap system
will have a good performance. More information on radix bitmap, please look
at the source code.
radix bitmap (i.e. bitmap using radix tree for hinting.) It can operate well no
matter how much fragmentation there is and no matter how large a bitmap is
used. FreeBSD's radix bitmap structure was ported to Haiku and used for
the same purpose.
Swap space allocation takes place when we swap anonymous pages out.
In order to make the allocation less probable to fail, anonymous cache will
reserve swap space when it is initialized. If there is not enough swap space
left, physical memory will be reserved. Swap space deallocation happens when
available swap space is low. The page daemon will scan a number of pages and
if the scanned page has swap space assigned, its swap space will be freed.
Acknowledgement
---------------
Special thanks to my mentor Ingo. He is a knowledged person and always
gives me encouragement. Without his consistent and illuminating instructions,
this project would not have reached its present status.
If you find bugs or have suggestions for swap system, you can contact me
via [email protected]. Thanks in advance.
Zhao Shuai - [email protected] - 2008-08-21
Swap space allocation takes place when we write anonymous pages out. Since
all VMAnonymousCaches reserve their commitments from a global "memory and
swap" pool, this allocation should rarely (if ever) fail. Swap space
deallocation is triggered by the page daemon.