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Benefits of FAST_PATH?

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The Question is:

Without recoding our applications, where can
we expect to see an increase in I/O performance
by setting FAST_PATH to 1?

The Answer is :

  Fast Path allows I/O processing to target a particular processor in an
  OpenVMS multiprocessing (SMP) system.
  Fast Path allows the system manager to spread the I/O processing load
  across multiple processors in a multiprocessor system, and to offload
  the primary processor.  This relieves the interrupt processing load on
  the primary processor, and secondarily -- by taking advantage of the
  caches -- by finishing an I/O operation on the processor on which it
  started.  This also translates into a reduction of CPU cost per I/O
  Fast Path also reduces the IOLOCK8 spinlock activity, which can further
  improve system throughput in an SMP configuration.  (IOLOCK8 is one of
  the major bottleneck in parallel I/O operations.)
  The amount of benefit a particular system derives from selection and
  use of Fast Path depends on a number of factors, and is impossible to
  quantify without a detailed analysis of the system and application load.
  If you are able to recode your applications, consider using the $IO_SETUP
  and $IO_PERFORM system services.  These services provide a streamlined way
  to do I/O, and may provide a performance benefit, by reducing the CPU cost
  per I/O operation.  Again, the benefit is impossible to quantify without
  a detailed analysis of your system and application.
  For the most current details on Fast Path, please see the OpenVMS
  documentation for the particular OpenVMS release of interest -- the most
  recent of OpenVMS releases tend to have the widest Fast Path support; the
  most devices and drivers.  (Fast Path support is specifically implemented
  within the code of the various device drivers involved.  Only device
  drivers with this code will have support for Fast Path I/O processing.)
  Related management and programming information is available within the
  OpenVMS I/O User's Reference Manual, and elsewhere in the OpenVMS
  documentation set.
  Information on the $IO_SETUP and $IO_PERFORM system service calls is
  available in the OpenVMS Programming Concepts manual and in the OpenVMS
  System Service Reference Manual.

answer written or last revised on ( 29-NOV-2004 )

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