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HP OpenVMS Systemsask the wizard |
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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? Thanks. 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 operation. 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.
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