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HP OpenVMS Systemsask the wizard |
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The Question is: Dear Sir, Madam, I am busy tracking a performance problem in one of the application (written in "C"). This application is logging a lot of messages that might be used in future should the system crashes. We noticed that each write is followed by a fflush(). If we removed the fflush the system performance increases by more than 500 %. The questions are: When is the flushing necessary ? Does VMS have its own automatic flushing facility ( a periodic flushig that occures after a configured period of time, or when a process crashes, or when the UPS detects a loss of power ) ? Does VMS flu sh automatically the cache once we've reached the buffer capacity. Thanks and regards Philippe Delcroix The Answer is : What you are seeing results from explicitly flushing the contents of the I/O cache to disk, and -- implicitly -- an indication of the performance benefits of using I/O caches. OpenVMS does flush the contents of I/O buffers as the buffers fill or as the application closes the file or as the application image or process goes through an operation known as "rundown". OpenVMS includes an operation known as a process rundown, and this operation is performed during a typical image exit. The process rundown flushes the contents of the process I/O caches to disk. Contrast this with the behaviour on process deletion (sys$delprc call, STOP/ID command, etc) or system crash, when no rundown is performed. Better control over file writes than is possible in native C can be achieved by replacing C calls with direct calls to the RMS services. Alternatives include writing the log to another (logging) process on the local node, or to an application running on another node. OpenVMS does not have a mechanism which ties it to a UPS. That said, it is generally quite trivial to connect OpenVMS to the UPS to handle a UPS-requested shutdown via communications over a serial line or via modem control signals. The application shutdown can be handled via commands added into the SYSHUTDWN.COM command procedure. The OpenVMS Wizard would recommend a visit to the Guide to OpenVMS File Applications and the OpenVMS Record Management Services Reference Manual. Both are included in the OpenVMS documentation set.
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