msync.2 (2010 09)

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msync(2) msync(2)

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

completed. 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

synchronization request. This will serve to more evenly distribute I/O activity across the mapped ﬁle,

while reducing the number of write operations per

msync().

RETURN VALUE

msync() returns the following values:

0 Successful completion.

-1 Failure. errno is set to indicate the error.

ERRORS

The

msync() function will fail if:

[EAGAIN] A transient error occurred while reading from or writing to the ﬁle system.

[EBUSY] Some or all of the addresses in the range starting at addr and continuing for

len bytes are locked, and

MS_INVALIDATE

is speciﬁed.

[EINVAL] The addr argument is not a multiple of the page size as returned by

sysconf(_SC_PAGE_SIZE)

[EINVAL] The address range speciﬁed by addr and len was not created by a successful

call to

mmap().

[EINVAL] The value of ﬂags is invalid.

[EIO] An I/O error occurred while reading from or writing to the ﬁle system.

[ENOMEM] Some or all the addresses in the range [addr, addr+len] are invalid for the

address space of the process. Or, pages that are not mapped are speciﬁed.

APPLICATION USAGE

The

msync() function should be used by programs that require a memory object to be in a known state.

For example, in building transaction facilities.

Normal system activity can cause pages to be written to disk. Therefore, there are no guarantees that

msync() is the only control over when pages are or are not written to disk.

AUTHOR

msync() was developed by HP, AT&T, and OSF.