Tunable Kernel Parameters
NAME [Toc] [Back]
aio_physmem_pct - percentage of physical memory lockable for request
call-back POSIX asynchronous I/O operations
VALUES [Toc] [Back]
Default [Toc] [Back]
Allowed values [Toc] [Back]
The minimum value allowed is 5. The maximum value allowed is 50.
Specify a positive integer value.
DESCRIPTION [Toc] [Back]
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
aio_return() call for that operation.
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 requestand-callback
mechanism is used only if the device drivers involved
support it. Memory is locked only while the I/O transfer is in
aio_physmem_pct imposes a system-wide limit on lockable physical
memory. A per-process lockable-memory limit can also be self-imposed
by using the setrlimit() system call within the application program
Who Is Expected to Change This Tunable?
System administrators that run applications requiring heavy usage of
POSIX AIO to file systems.
Restrictions on Changing [Toc] [Back]
This tunable is static. Any changes to the value of this tunable will
require system reboot before taking effect.
The amount of memory that can be locked under the limit imposed by
aio_physmem_pct cannot exceed the total system wide lockable memory
limit imposed by unlockable_mem.
When Should the Value of This Tunable Be Raised?
On a large server it is better to increase aio_physmem_pct to higher
values (up to 50).
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: Sep 2004
Tunable Kernel Parameters
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 lockable_mem. Other system activity, including
explicit memory locking with the plock() and/or mlock() interfaces can
also affect the amount of lockable memory at any given time.
There is no kernel parameter named lockable_mem, but there is a
parameter named unlockable_mem that affects it. The value of
lockable_mem is determined by subtracting the value of unlockable_mem
from the amount of system memory available after system startup.
During 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 /sbin/dmesg command.
WARNINGS [Toc] [Back]
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 tunable
values, consult the documentation for the kernel software being
installed. For information about optional kernel software that was
factory installed on your system, see HP-UX Release Notes at
AUTHOR [Toc] [Back]
aio_physmem_pct was developed by HP.
SEE ALSO [Toc] [Back]
kctune(1M), sam(1M), gettune(2), settune(2), aio(5), aio_return(2),
dmesg(1M), mlock(2), plock(2), getrlimit(2), setrlimit(2),
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: Sep 2004 [ Back ]