HP-UX Reference (11i v3 07/02) - 5 Miscellaneous Topics (vol 9)

a
aio_physmem_pct(5) aio_physmem_pct(5)
(Tunable Kernel Parameters)
NAME
aio_physmem_pct - percentage of physical memory lockable for request call-back POSIX asynchronous I/O
operations
VALUES
Failsafe
10
Default
10
Allowed values
The minimum value allowed is 5. The maximum value allowed is
50.
Specify a positive integer value.
DESCRIPTION
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 opera-
tion.
The value of aio_physmem_pct
represents a percentage of system memory, and the limit it controls
will adjust with Online Addition or Deletion of physical memory (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
support it. Memory is locked only while the I/O transfer is in progress.
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 pro-
gram (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 aio_physmem_pct
cannot exceed
the total system wide lockable memory limit imposed by
unlockable_mem
.
The limit dictated by this tunable,
aio_physmem_pct
, 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
aio_physmem_pct 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 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.
48 Hewlett-Packard Company − 1 − HP-UX 11i Version 3: February 2007