aio_physmem_pct(5)

VALUES

DESCRIPTION

This parameter places a limit on how much system memory can be locked by the combined total number of POSIX asynchronous I/O operations that are in progress at any given time. It is also 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 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. On a large server it is better to increase aio_physmem_pct to higher values (up to 50).

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 (see setrlimit(2)).

Remember too that 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.

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.

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.

AUTHOR

aio_physmem_pct was developed by HP.

SEE ALSO

aio(5), aio_return(2), dmesg(1M), mlock(2), plock(2), getrlimit(2), unlockable_mem(5).