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HP-UX Reference > A
aio_physmem_pct(5)Tunable Kernel ParametersHP-UX 11i Version 3: February 2007 |
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NAMEaio_physmem_pct — percentage of physical memory lockable for request call-back POSIX asynchronous I/O operations DESCRIPTIONThis 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. 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 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 ChangingThis 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. WARNINGSAll 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 http://docs.hp.com. SEE ALSOkctune(1M), sam(1M), gettune(2), settune(2), aio(5), aio_return(2), dmesg(1M), mlock(2), plock(2), getrlimit(2), setrlimit(2), unlockable_mem(5). |
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