aio_physmem_pct(5)aio_physmem_pct(5)NAMEaio_physmem_pct - percentage of physical memory lockable for request
call-back POSIX asynchronous I/O operations
The minimum value allowed is The maximum value allowed is
Specify a positive integer value.
This parameter places a limit on how much system memory can be locked
by the total number of POSIX asynchronous I/O operations that are in
progress at any given time.
It is important to be aware that an operation remains on the active
queue and memory is not released, even if the operation is complete,
until it is properly terminated by an call for that operation.
The value of represents a percentage of system memory, and the limit it
controls will adjust with Online Addition or Deletion of physical mem‐
ory (OL*), as appropriate.
Asynchronous I/O operations that use a request-and-callback mechanism
for I/O must be able to lock the memory they are using. The request-
and-callback mechanism is used only if the device drivers involved sup‐
port it. Memory is locked only while the I/O transfer is in progress.
imposes a system-wide limit on lockable physical memory. A per-process
lockable-memory limit can also be self-imposed by using the system call
within the application program (see setrlimit(2)).
Who Is Expected to Change This Tunable?
System administrators that run applications requiring heavy usage of
POSIX AIO to file systems.
Restrictions on Changing
This tunable is dynamic. Any changes to the value of this tunable will
take effect immediately without requiring system reboot.
The amount of memory that can be locked under the limit imposed by can‐
not exceed the total system wide lockable memory limit imposed by
The limit dictated by this tunable, is checked individually for each
incoming request for file system or devices that support the
request/callback mechanism. When tuning to a lower value, the new
limit will be immediately enforced for all new requests, and the tuning
will succeed even if the current usage count is higher. The usage will
then gradually adjust to the new limit.
When Should the Value of This Tunable Be Raised?
On a large server it is better to increase to higher values (up to 50).
What Other Tunables Should Be Changed at the Same Time as This One?
The total amount of memory that can be locked at any given time for any
reason, not just for asynchronous I/O, is controlled by the system-wide
limit Other system activity, including explicit memory locking with the
and/or interfaces can also affect the amount of lockable memory at any
There is no kernel parameter named but there is a parameter named that
affects it. The value of is determined by subtracting the value of
from the amount of system memory available after system startup. Dur‐
ing startup, the system displays on the system console the amount of
its lockable memory (along with available memory and physical memory).
These values can be retrieved while the system is running with command.
All HP-UX kernel tunable parameters are release specific. This parame‐
ter may be removed or have its meaning changed in future releases of
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 val‐
ues. For information about the effects of installation on tunable val‐
ues, consult the documentation for the kernel software being installed.
For information about optional kernel software that was factory
installed on your system, see at
was developed by HP.
SEE ALSOkctune(1M), sam(1M), gettune(2), settune(2), aio(5), aio_return(2),
dmesg(1M), mlock(2), plock(2), getrlimit(2), setrlimit(2), unlock‐
Tunable Kernel Parameters aio_physmem_pct(5)