CLAYTON@XRT.UPENN.EDU ("Clayton, Paul D.") (10/25/87)
Information From TSO Financial - The Saga Continues... Chapter 31 - October 24, 1987 Jeff Siegal has asked a number of questions about the use of page and swap files under VMS. The answers that Jerry supplied in a earlier message are accurate and well stated. There are a number of other items that need to be addressed also. 1. If the swap and page files are not contigious, then EXTRA work has to be done by the system to accomplish the same result. This EXTRA work is DELAYING the processor from doing work that your program is requesting. The overhead in dealing with fragmented files is to be considered OVERHEAD. While you can not get rid of OVERHEAD, you NEED to do everything possible to LIMIT it. In simple terms, make the files contigious, and speed things along. 2. I recommend that the SYSGEN parameters SWAPFILECNT and PAGEFILCNT be set to something in EXCESS of the number you are currently using. This will enable you to ADD additional files when they are needed to get a machine back on its feet. The alternative is a hung system and a reboot to get it back. 3. BEWARE the use of GLOBAL BUFFERING and its impact to the PRIMARY page file under VMS 4.X. We make extensive use of global bufferring on our systems and it needs to be understood that the GLOBAL PAGES used in creating the GLOBAL SECTION have a backing store in the PRIMARY page file ONLY. We have been NAILED to the wall on this several times. There are two SYSGEN parameters that influence the use of global buffering. They are GBLSECTIONS and GBLPAGFIL. The GBLSECTIONS parameter is the number of total global sections that can be existing at any point in time. This includes program sections from installed images, and any user sections for buffering data. The other parameter, GBLPAGFIL is the maximum number of pages that can be used in the primary page file for creating global buffers. The global buffer sections can be seen by doing a SHOW/GLOBAL command under INSTALL/COMMAND_MODE. The sections with the prefix of 'RMS$' are global buffer sections. Note that these may still exist on a CPU in a VAXCluster, which has no processes currently using those files on that specific CPU. The thing to remember here is that VAXClusters have files open, and sections created until ALL CPU nodes in the cluster have closed the file. Rumor has it that a NEW SYSGEN parameter will be in VMS 5.X to allow the global sections to have an alternate page file for the backing media when the sections are created. Have fun. Paul D. Clayton - Manager Of Systems TSO Financial - Horsham, Pa. USA Address - CLAYTON%XRT@CIS.UPENN.EDU