aio_req_per_thread(5)						File Formats Manual					     aio_req_per_thread(5)

NAME

       aio_req_per_thread - desirable ratio between number of pending AIO requests and servicing threads

VALUES

   Failsafe
   Default
   Allowed values
   Recommended values
DESCRIPTION

       The  implementation of POSIX AIO on HP-UX uses kernel threads to perform I/Os to filesystems that do not directly support true asynchronous
       I/O.  (This distinction is transparent to the user.)  The kernel threads are organized into worker-thread pools (called AIO  thread  pools)
       created	on  a  per-process basis.  Since a thread pool mechanism for I/Os introduces a variety of trade-offs concerning utilization of CPU
       time vs. I/O resources, four dynamic  tunables  are  available  to  customize  the  behavior  of  this  thread  pool:  aio_proc_threads(5),
       aio_proc_thread_pct(5),	aio_req_per_thread(5),	and  aio_monitor_run_sec(5).   Please see individual manpages for details on each of these
       tunables.

       The tunable specifies, on a per-process basis, the desirable ratio between the number of pending POSIX  AIO  requests  and  the	number	of
       threads in the AIO thread pool.

       The  number  of	threads  in  the AIO thread pool is bounded by the tunables and but the tunable determines how the AIO thread pool behaves
       within that bound.  determines how much the AIO thread pool grows as the number of outstanding AIO requests grows,  by  defining  how  many
       I/Os each thread will be responsible for.

   Who Is Expected to Change This Tunable?
       System administrators that run applications requiring heavy usage of POSIX AIO to filesystems.

   Restrictions on Changing
       This  tunable is dynamic.  Changes to to this tunable take effect immediately for new processes started after the change.  They also impact
       existing processes, but the speed with which the changes propagate to running processes is determined by the tunable

   When Should the Value of This Tunable Be Raised?
       should be raised for applications that want to limit the number of threads used by the POSIX AIO subsystem.  Applications would want to	do
       this  to  either free up more process threads for other work, or to limit the level of concurrency inside POSIX AIO, perhaps to reduce load
       on physically limited I/O devices.

   What Are the Side Effects of Raising the Value of This Tunable?
       By allowing less threads for POSIX AIO requests, concurrency is reduced and AIO I/O requests will have to wait longer for servicing.   This
       can  result in increased latency and reduced POSIX AIO performance on systems whose I/O stack could otherwise handle heavier loads.  On the
       other hand, less threads per request can result in less context switching, reducing the CPU utilization of POSIX AIO.

   When Should the Value of This Tunable Be Lowered?
       should be lowered when applications want to maximize the concurrency and performance of POSIX AIO requests.  This should be  done  when	an
       application does not need a large number of threads for other work.

   What Are the Side Effects of Lowering the Value of This Tunable?
       Lowering  this  tunable	results  in more threads being used by POSIX AIO to handle I/O requests, which could increase CPU usage and use up
       threads that applications might need for other work.  On the other hand, POSIX AIO performance should increase.

   What Other Tunables Should Be Changed at the Same Time as This One?
       interacts with this tunable by setting a strict limit on the number of threads that can be used for POSIX AIO.

       interacts with this tunable by setting a limit on the number of threads that can be used for POSIX_AIO, but does so based on  a	percentage
       of the maximum number of allowable process threads.  This allows the AIO thread pools to respond dynamically to changes in

       defines how often (in seconds) the AIO thread mechanism will monitor itself for adherence to the constraints defined by the tunables above.

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.

       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 tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

SEE ALSO

       kctune(1M), sam(1M), gettune(2), settune(2), aio_proc_threads(5), aio_proc_thread_pct(5), aio_monitor_run_sec(5).

							     Tunable Kernel Parameters					     aio_req_per_thread(5)