Posix fadvise and madvise: Difference between revisions
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==posix_fadvice== | |||
posix_fadvice() gives a non-binding hint about how the calling applications expect to access a (portion of a) file, in case that helps the OS's planning, mostly in terms of | posix_fadvice() gives a non-binding hint about how the calling applications expect to access a (portion of a) file, in case that helps the OS's planning, mostly in terms of | ||
| Line 23: | Line 25: | ||
* POSIX_FADV_SEQUENTIAL - "we expect to read this sequentially, if that helps" | * POSIX_FADV_SEQUENTIAL - "we expect to read this sequentially, if that helps" | ||
: on linux, | : on linux, amy double the regular readahead{{verify}} | ||
: may be swapped out after use{{verify}} | |||
* POSIX_FADV_RANDOM - "we don't expect to read this sequentially" | * POSIX_FADV_RANDOM - "we don't expect to read this sequentially" | ||
: may disable readahead{{verify}} | |||
: may not swap out as enthusiastically{{verify}} | |||
* POSIX_FADV_NOREUSE - "we expect to read this just once" | * POSIX_FADV_NOREUSE - "we expect to read this just once" | ||
| Line 37: | Line 42: | ||
Notes: | |||
* linux has its own paging daemon which may be clever enough in general, so not all advice is equally useful. | |||
* in theory, opening a file with O_DIRECT acts similarly to POSIX_FADV_DONTNEED | |||
See also: | See also: | ||
* http://linux.die.net/man/2/posix_fadvise | * http://linux.die.net/man/2/posix_fadvise | ||
==madvise== | |||
<!-- | |||
<syntaxhighlight lang="c"> | |||
int madvise(void addr[.length], size_t length, int advice); | |||
int posix_madvise(void addr[.length], size_t length, int advice); | |||
</syntaxhighlight> | |||
Where posix_fadvise is advice about how we access a file (which includes page cache details), madvise is advice about the page cache. | |||
https://stackoverflow.com/questions/20147881/fadvise-vs-madvise-can-i-use-both-together | |||
https://man7.org/linux/man-pages/man2/madvise.2.html | |||
--> | |||
Latest revision as of 16:37, 20 April 2024
posix_fadvice
posix_fadvice() gives a non-binding hint about how the calling applications expect to access a (portion of a) file, in case that helps the OS's planning, mostly in terms of
- how much readahead to do
- whether to keep the results in the page cache
The below is mostly selections from the man page:
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
- The advice applies to a (not necessarily existent) region
- starting at offset and extending
- for len bytes (or until the end of the file if len is 0)
- within the file referred to by fd
- POSIX_FADV_NORMAL - application has no advice (assumed default)
- on linux, allows readahead
- POSIX_FADV_SEQUENTIAL - "we expect to read this sequentially, if that helps"
- POSIX_FADV_RANDOM - "we don't expect to read this sequentially"
- POSIX_FADV_NOREUSE - "we expect to read this just once"
- "In kernels before 2.6.18, POSIX_FADV_NOREUSE had the same semantics as POSIX_FADV_WILLNEED. This was probably a bug; since kernel 2.6.18, this flag is a no-op."
- POSIX_FADV_WILLNEED - "we expect to read the same data again soon"
- POSIX_FADV_DONTNEED - "we expect to not read this data again soon"
- should be called after you are done with the data, not before you read it
- the point would often be not pushing more useful pages out of the page cache -- for larger files you probably want to do this regularly while reading it.
Notes:
- linux has its own paging daemon which may be clever enough in general, so not all advice is equally useful.
- in theory, opening a file with O_DIRECT acts similarly to POSIX_FADV_DONTNEED
See also: