copy_on_write(5) File Formats Manual copy_on_write(5)
NAME
copy_on_write - dynamically enable or disable copy-on-write semantics for privately mapped files, private segments of child process after
fork(), and private segments locked by mlock()
VALUES
Failsafe
Default
Allowed values
or
DESCRIPTION
This tunable dynamically enables or disables the copy-on-write behavior in private and system calls. When the tunable is disabled, copy-
on-access behavior is followed; this is the legacy behavior (HP-UX 11i v3 and previous releases) where private segments are copied upon
access. Disabling this tunable will preserve that old behavior.
Following is a summary of what happens when the tunable is enabled:
o For a process' private data (data segment, stack, heap, RSE) is copied only when the child process writes for the first time.
o For if mapping is created for a file for which exists, a separate copy of the page is created for only when it first writes to
the page. As long as reads, it shares the page with mapping. That is, updates made by shared mapping will be visible to private
mapping until private mapping writes.
o For locking privately mapped segments does not result in copying those segments. However, segments are locked in memory and only
when the content of the page changes, a copy is made and the lock is transferred to the new segment.
Who Is Expected to Change This Tunable?
System administrators who wish to change the default copy-on-write behavior.
Restrictions on Changing
Changes to this tunable take effect immediately.
When Should the Tunable Be Turned Off?
If an application depends on copy-on-access behavior, where a process accessing a privately mapped file for the first time gets a copy of
the page and the contents of the page does not change unless private mapping writes to it, the tunable should be turned off to get copy-on-
access behavior.
What Are the Side Effects of Turning the Tunable Off?
Some workloads might see increased memory consumption since private segments are copied when accessed for the first time.
When Should the Tunable Be Turned On?
If an application wants industry standard semantics, which allows users to share data via private mapping of a file for which shared map-
ping exists, the tunable should be enabled. Turning the tunable on can also reduce memory consumption on some workloads since private seg-
ments are only copied upon write.
What Are the Side Effects of Turning the Tunable On?
Applications that privately map a file for which shared mapping exists can now see the updates performed by shared mapping.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of
HP-UX.
Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation,
some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tun-
able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was
factory installed on your system, see at
AUTHOR
was developed by HP.
SEE ALSO
fork(2), mlock(2), mmap(2).
Tunable Kernel Parameters copy_on_write(5)