aio_proc_threads(5)                                     aio_proc_threads(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      aio_proc_threads - maximum number of process threads allowed in AIO
      pool

 VALUES    [Toc]    [Back]
    Failsafe
      256

    Default    [Toc]    [Back]
      1024

    Allowed values    [Toc]    [Back]
      4-2048

    Recommended values    [Toc]    [Back]
      32-2048

 DESCRIPTION    [Toc]    [Back]
      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 aio_proc_threads specifies, on a per-process basis, the
      maximum number of process threads that can be used by the POSIX AIO
      system as kernel threads for issuing I/Os.

      This tunable interacts with aio_proc_thread_pct in the following way:
      the maximum number of threads used for AIO will be the smaller of the
      two values defined by the two tunables, i.e.:

           MIN (aio_proc_threads, aio_proc_thread_pct * max_thread_proc)

      This allows the number of AIO threads to vary dynamically with
      max_thread_proc, but to always be bound by an absolute limit of
      aio_proc_threads.

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

    Restrictions on Changing    [Toc]    [Back]
      This tunable is dynamic.  Changes to to this tunable take effect
      immediately for new processes started after the change.  They also



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 aio_proc_threads(5)                                     aio_proc_threads(5)
                          Tunable Kernel Parameters



      impact existing processes, but the speed with which the changes
      propagate to running processes is determined by the tunable
      aio_monitor_run_sec.

    When Should the Value of This Tunable Be Raised?
      aio_proc_threads should be raised for applications that do not use
      very many threads for their own work, but desire high performance from
      the POSIX AIO subsystem.

    What Are the Side Effects of Raising the Value of This Tunable?
      Some applications that use POSIX AIO but also require a large number
      of threads may find that they are unable to create new threads.  (This
      could happen if the POSIX AIO thread pool ends up using too many of a
      process' allowable threads.)

      In addition, using a larger number of kernel threads might lead to
      increased CPU utilization.

    When Should the Value of This Tunable Be Lowered?
      aio_proc_threads should be lowered when POSIX AIO performance is
      acceptable but applications using POSIX AIO are seeing errors when
      trying to create new threads for other work.

    What Are the Side Effects of Lowering the Value of This Tunable?
      By ultimately reducing the number of threads available to handle POSIX
      AIO requests, overall I/O throughput of the POSIX AIO subsystem could
      be reduced.

    What Other Tunables Should Be Changed at the Same Time as This One?
      aio_proc_thread_pct interacts with this tunable by setting a limit on
      the number of AIO threads based on a percentage of the maximum number
      of allowable process threads.  This allows the AIO thread pools to
      respond dynamically to changes in max_thread_proc.

      aio_req_per_thread defines the desired relationship between the number
      of POSIX AIO kernel threads and the number of I/Os to be serviced.

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

 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



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 aio_proc_threads(5)                                     aio_proc_threads(5)
                          Tunable Kernel Parameters



      installed. For information about optional kernel software that was
      factory installed on your system, see HP-UX Release Notes at
      http://docs.hp.com.

 AUTHOR    [Toc]    [Back]
      aio_proc_threads was developed by HP.

 SEE ALSO    [Toc]    [Back]
      kctune(1M), sam(1M), gettune(2), settune(2), aio_proc_thread_pct(5),
      aio_req_per_thread(5), aio_monitor_run_sec(5).


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