physical_io_buffers(5)                               physical_io_buffers(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      physical_io_buffers - total buffers for physical I/O operations

 VALUES    [Toc]    [Back]
    Default
      0 buffers

    Allowed values    [Toc]    [Back]
      Minimum: 384 buffers

      Maximum: 65535 buffers

 DESCRIPTION    [Toc]    [Back]
      Many physical I/O operations in the kernel use a shared pool of small
      pieces of memory (buffers) to hold data being read or written to the
      I/O device.  If no buffer is provided to the I/O operation routine,
      and no kernel buffers are available because they are already in use
      for other physical I/O transactions, the caller must wait for an
      available buffer.  Under heavy I/O load with many threads attempting
      I/O operations buffer scarcity can have a significant performance
      impact.

      The physical_io_buffers tunable allows the system administrator to
      raise or lower the amount of kernel buffers to suit the I/O traffic on
      the machine to avoid this performance bottleneck.

    Who is Expected to Change This Tunable?
      This tunable should only be modified by those with a complete
      knowledge of both kernel behavior and underlying device hardware.

    Restrictions on Changing    [Toc]    [Back]
      When set to zero (default value), the kernel will attempt to compute
      the most appropriate value for this tunable.  Changes to this tunable
      take effect immediately.

      Attempts to raise this tunable may fail with the [ENOMEM] error if
      insufficient memory is available to the kernel to create the needed
      buffers.

    When Should the Value of This Tunable Be Raised?
      This tunable should be raised on machines with many I/O devices
      operating under simultaneous load.  Perceptible behavior would be long
      I/O operation times, where a ps -fl call shows these I/O processes
      sleeping with the same wait channel (the exact number of the wait
      channel may vary).

      The value of the tunable should be raised to the number of concurrent
      I/O thread operations times the number of devices.  Since this may be
      difficult to gauge, raising the tunable by the number of sleeping
      processes as reported by ps -fl above and then monitoring for further
      bottlenecks would be recommended.



 Hewlett-Packard Company            - 1 -      HP-UX 11i Version 2: Sep 2004






 physical_io_buffers(5)                               physical_io_buffers(5)
                          Tunable Kernel Parameters



    What Are the Side Effects of Raising the Value?
      Kernel dynamic memory usage is increased (buffers are not free).
      Specifically, each buffer requires 608 bytes of memory within the
      kernel.

    When Should the Value of This Tunable Be Lowered?
      If the tunable value is above the minimum, and physical memory is a
      scarce resource then the value can be lowered if one of two cases is
      true:

      +  There are many more buffers than concurrent threads performing
         reads or writes to the devices on the machine; or,

      +  The primary workload of the machine is memory bound and requires
         additional space so that a corresponding I/O bottleneck is
         irrelevant to the overall machine usage.

    What Are the Side Effects of Lowering the Value?
      Physical memory is returned for kernel or user memory needs.  Fewer
      buffers are available for reads or writes to devices and some threads
      may sleep waiting for buffers.  This may reduce system performance for
      those threads.

    What Other Tunable Values Should Be Changed at the Same Time?
      A change to physical_io_buffers is independent of any other tunables.

 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
      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]
      physical_io_buffers was developed by HP.


 Hewlett-Packard Company            - 2 -      HP-UX 11i Version 2: Sep 2004