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| author | dillon <dillon@FreeBSD.org> | 1999-12-12 03:19:33 +0000 |
|---|---|---|
| committer | dillon <dillon@FreeBSD.org> | 1999-12-12 03:19:33 +0000 |
| commit | b66fb2c64801a0ee59e638561bfd8d3fe36b647c (patch) | |
| tree | d1cf00b34925743e2181910ae5e72af2d03be373 /lib/libc | |
| parent | aeee88b81a6982928d45c0d80c325cd8372bbab0 (diff) | |
| download | FreeBSD-src-b66fb2c64801a0ee59e638561bfd8d3fe36b647c.zip FreeBSD-src-b66fb2c64801a0ee59e638561bfd8d3fe36b647c.tar.gz | |
Add MAP_NOSYNC feature to mmap(), and MADV_NOSYNC and MADV_AUTOSYNC to
madvise().
This feature prevents the update daemon from gratuitously flushing
dirty pages associated with a mapped file-backed region of memory. The
system pager will still page the memory as necessary and the VM system
will still be fully coherent with the filesystem. Modifications made
by other means to the same area of memory, for example by write(), are
unaffected. The feature works on a page-granularity basis.
MAP_NOSYNC allows one to use mmap() to share memory between processes
without incuring any significant filesystem overhead, putting it in
the same performance category as SysV Shared memory and anonymous memory.
Reviewed by: julian, alc, dg
Diffstat (limited to 'lib/libc')
| -rw-r--r-- | lib/libc/sys/madvise.2 | 27 | ||||
| -rw-r--r-- | lib/libc/sys/mmap.2 | 29 |
2 files changed, 56 insertions, 0 deletions
diff --git a/lib/libc/sys/madvise.2 b/lib/libc/sys/madvise.2 index 415af68..9a82782 100644 --- a/lib/libc/sys/madvise.2 +++ b/lib/libc/sys/madvise.2 @@ -58,6 +58,8 @@ The known behaviors are given in #define MADV_WILLNEED 3 /* will need these pages */ #define MADV_DONTNEED 4 /* don't need these pages */ #define MADV_FREE 5 /* data is now unimportant */ +#define MADV_NOSYNC 6 /* no explicit commit to physical backing store */ +#define MADV_AUTOSYNC 7 /* default commit method to physical backing store */ .Ed .Pp .Bl -tag -width MADV_SEQUENTIAL @@ -96,6 +98,31 @@ call. References made to that address space range will not make the VM system page the information back in from backing store until the page is modified again. +.It Dv MADV_NOSYNC +Request that the system not flush the data associated with this map to +physical backing store unless it needs to. Typically this prevents the +filesystem update daemon from gratuitously writing pages dirtied +by the VM system to physical disk. Note that VM/filesystem coherency is +always maintained, this feature simply ensures that the mapped data is +only flush when it needs to be, usually by the system pager. +.Pp +This feature is typically used when you want to use a file-backed shared +memory area to communicate between processes (IPC) and do not particularly +need the data being stored in that area to be physically written to disk. +With this feature you get the equivalent performance with mmap that you +would expect to get with SysV shared memory calls, but in a more controllable +and less restrictive manner. However, note that this feature is not portable +across UNIX platforms (though some may do the right thing by default). +For more information see the MAP_NOSYNC section of +.Xr mmap 2 +.It Dv MADV_AUTOSYNC +Undoes the effects of MADV_NOSYNC for any future pages dirtied within the +address range. The effect on pages already dirtied is indeterminate - they +may or may not be reverted. You can guarentee reversion by using the +.Xr msync 2 +or +.Xr fsync 2 +system calls. .El .Sh RETURN VALUES Upon successful completion, diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2 index 6e3619e..e165d32 100644 --- a/lib/libc/sys/mmap.2 +++ b/lib/libc/sys/mmap.2 @@ -150,6 +150,35 @@ stack top is the starting address returned by the call, plus .Fa len bytes. The bottom of the stack at maximum growth is the starting address returned by the call. +.It Dv MAP_NOSYNC +Causes data dirtied via this VM map to be flushed to physical media +only when necessary (usually by the pager) rather then gratuitously. +Typically this prevents the update daemons from flushing pages dirtied +through such maps and thus allows efficient sharing of memory across +unassociated processes using a file-backed shared memory map. Without +this option any VM pages you dirty may be flushed to disk every so often +(every 30-60 seconds usually) which can create performance problems if you +do not need that to occur (such as when you are using shared file-backed +mmap regions for IPC purposes). Note that VM/filesystem coherency is +maintained whether you use MAP_NOSYNC or not. This option is not portable +across UNIX platforms (yet), though some may implement the same behavior +by default. +.Pp +The +.Xr fsync 2 +function will flush all dirty data and metadata associated with a file, +including dirty NOSYNC VM data, to physical media. The +.Xr sync 1 +command and +.Xr sync 2 +system call generally do not flush dirty NOSYNC VM data. +The +.Xr msync 2 +system call is obsolete since +.Os BSD +implements a coherent filesystem buffer cache. However, it may be +used to associate dirty VM pages with filesystem buffers and thus cause +them to be flushed to physical media sooner rather then later. .El .Pp The |
