Справочник Пользователя для IBM REDP-4285-00
4285ch01.fm
Draft Document for Review May 4, 2007 11:35 am
14
Linux Performance and Tuning Guidelines
Page frame allocation
A page is a group of contiguous linear addresses in physical memory (page frame) or virtual
memory. The Linux kernel handles memory with this page unit. A page is usually 4K bytes in
size. When a process requests a certain amount of pages, if there are available pages, the
Linux kernel can allocate them to the process immediately. Otherwise pages have to be taken
from some other process or page cache. The kernel knows how many memory pages are
available and where they are located.
memory. The Linux kernel handles memory with this page unit. A page is usually 4K bytes in
size. When a process requests a certain amount of pages, if there are available pages, the
Linux kernel can allocate them to the process immediately. Otherwise pages have to be taken
from some other process or page cache. The kernel knows how many memory pages are
available and where they are located.
Buddy system
The Linux kernel maintains its free pages by using the mechanism called
buddy system
. The
buddy system maintains free pages and tries to allocate pages for page allocation requests. It
tries to keep the memory area contiguous. If small pages are scattered without consideration,
it may cause memory fragmentation and it’s more difficult to allocate large portion of pages
into a contiguous area. It may lead to inefficient memory use and performance decline.
Figure 1-13 illustrates how the buddy system allocates pages.
tries to keep the memory area contiguous. If small pages are scattered without consideration,
it may cause memory fragmentation and it’s more difficult to allocate large portion of pages
into a contiguous area. It may lead to inefficient memory use and performance decline.
Figure 1-13 illustrates how the buddy system allocates pages.
Figure 1-13 Buddy System
When the attempt of pages allocation failed, the page reclaiming will be activated. Refer to
“Page frame reclaiming” on page 14.
“Page frame reclaiming” on page 14.
You can find information on the buddy system through /proc/buddyinfo. For detail, please
refer to “Memory used in a zone” on page 47.
refer to “Memory used in a zone” on page 47.
Page frame reclaiming
If pages are not available when a process requests to map a certain amount of pages, the
Linux kernel tries to get pages for the new request by releasing certain pages which are used
before but not used anymore and still marked as active pages based on certain principals and
allocating the memory to new process. This process is called
Linux kernel tries to get pages for the new request by releasing certain pages which are used
before but not used anymore and still marked as active pages based on certain principals and
allocating the memory to new process. This process is called
page reclaiming
.
kswapd
kernel
thread and try_to_free_page() kernel function are responsible for page reclaiming.
While kswapd is usually sleeping in task interruptible state, it is called by the buddy system
when free pages in a zone fall short of a certain threshold. It then tries to find the candidate
pages to be gotten out of active pages based on the Least Recently Used (
when free pages in a zone fall short of a certain threshold. It then tries to find the candidate
pages to be gotten out of active pages based on the Least Recently Used (
LRU
) principal.
This is relatively simple. The pages least recently used should be released first. The active list
and the inactive list are used to maintain the candidate pages. kswapd scans part of the
active list and check how recently the pages were used then the pages not used recently is
put into inactive list. You can take a look at how much memory is considered as active and
inactive using vmstat -a command. For detail refer to 2.3.2, “vmstat”.
and the inactive list are used to maintain the candidate pages. kswapd scans part of the
active list and check how recently the pages were used then the pages not used recently is
put into inactive list. You can take a look at how much memory is considered as active and
inactive using vmstat -a command. For detail refer to 2.3.2, “vmstat”.
kswapd also follows another principal. The pages are used mainly for two purpose;
page
cache
and
process address space
. The page cache is pages mapped to a file on disk. The
pages belonging to a process address space is used for heap and stack (called anonymous
memory because it‘s not mapped to any files, and has no name) (refer to 1.1.8, “Process
memory because it‘s not mapped to any files, and has no name) (refer to 1.1.8, “Process
Used
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for 2pages
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for 2 pages
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Used
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Used
Release
2 pages
Used
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8 pages
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