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

1 files changed, 27 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,