HP-UX Reference (11i v2 07/12) - 2 System Calls (vol 5)

m
msync(2) msync(2)
NAME
msync() - synchronize the memory of a mapped file with physical storage
SYNOPSIS
#include <sys/mman.h>
int msync(void *addr, size_t len, int flags);
DESCRIPTION
The msync() function writes all modified copies of pages over the range [addr, addr+len] to the underly-
ing hardware, or invalidates any copies so that further references to the pages will be obtained by the sys-
tem from their permanent storage locations.
The flags argument is one of the following:
MS_ASYNC perform asynchronous writes
MS_SYNC perform synchronous writes
MS_INVALIDATE invalidate mappings
If flags is MS_ASYNC or MS_SYNC, the function synchronizes the file contents to match the current con-
tents of the memory region.
• All write references to the memory region made prior to the call are visible by subsequent read
operations on the file.
• It is unspecified whether writes to the same portion of the file prior to the call are visible by read
references to the memory region.
• It is unspecified whether unmodified pages in the specified range are also written to the underlying
hardware.
If flags is MS_ASYNC, the function may return immediately once all write operations are scheduled; if flags
is
MS_SYNC, the function does not return until all write operations are completed.
If flags is MS_INVALIDATE
, the function synchronizes the contents of the memory region to match the
current file contents.
• All writes to the mapped portion of the file made prior to the call are visible by subsequent read
references to the mapped memory region.
• It is unspecified whether write references prior to the call, by any process, to memory regions
mapped to the same portion of the file using
MAP_SHARED , are visible by read references to the
region.
addr and len specify the region to be synchronized. If these are not the address and length of a region
created by a previous successful call to mmap(), msync() returns an error. The behavior of
msync()
upon a region created with the MAP_ANONYMOUS
or MAP_PRIVATE flags is undefined.
If
msync() causes any write to the file, then the file’s st_ctime and st_mtime fields are marked for update.
Performance Considerations
The following performance considerations only apply when using the
MS_INVALIDATE
option with
msync(). These performance constraints do not apply when either MS_ASYNC or MS_SYNC are
exclusively used with msync().
Direct read/write references to portions of a mapped memory region currently undergoing an msync()
operation (with MS_INVALIDATE specified), may be blocked until all scheduled write operations are com-
pleted. This is especially true when performing an msync() operation across a relatively large address
range that requires many individual write operations to be scheduled out to the underlying hardware. HP-
UX will schedule a separate write operation for each contiguous group of modified pages on disk. As more
write operations are queued out to the device, the overall suspension time of direct read/write references to
the same portions of the memory region will generally increase.
The suspension times of direct read/write references can be reduced by issuing msync() requests over
smaller portions of the memory region, but issuing them more frequently than a corresponding larger syn-
chronization request. This will serve to more evenly distribute I/O activity across the mapped file, while
reducing the number of write operations per msync().
HP-UX 11i Version 2: December 2007 Update − 1 − Hewlett-Packard Company 231