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HP OpenVMS Systemsask the wizard |
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The Question is: I have another question for you from Tallahassee, I would also appreciate your opinion about some SYSGEN Parameters. We have a three node alpha cluster running VMS 7.1-2 with 6 GIG of memory on each node. 1) Per suggestion from are contractor we have set the following parameters such that the system does not utilize the system pagefiles. The process tend to use memory until the Free Page List is almost empty. We then have to stop and start the process to recover the memory. They say this improves performance by reduceing the number of hard page faults, but I believe we have reached a point of diminshing return. Is it a good idea to set these values this high. PQL_DWSEXTENT = 1282048 PQL_DWSQUOTA = 37368 2) I have been trying to determine how much physical memory a process is using. There are a couple of argument for the lexical F$GETJPI that I have been playing with to determine the overall largest consumers of memory. Can you define these arguements mor e clearly. I would also appreciate your thought on the best way to determine how much memory a process is consuming. GPGCNT - Global Page Count in Working Set. PPGCNT - Process Page Count. VIRTPEAK - Peak Virtual Address Size. WSSIZE - Process's current working set size. Mike Casserly Please feel free to publish my question. The Answer is : Without particular details on the environment -- both in terms of the current process activity and the specific requirements -- an answer is rather difficult. Remote system tuning is exceedingly difficult. As a first rule of thumb, the OpenVMS Wizard strongly encourages you to ignore most tuning "rules of thumb" and to also ignore most "quick-and-easy" system tuning tips -- some are certainly correct, some were once correct, some are correct for very specific workloads, and some were never correct. Determining which of these possibilities is the basis of the "tip" is an interesting and difficult task for the OpenVMS system manager. That said, the use of the MODPARAMS.DAT file and AUTOGEN, saving and using the FEEDBACK mechanism, and following the recommendations for the system parameters and process quota settings discussed in the OpenVMS performance management manual are recommended -- AUTOGEN and a basic system performance examination will generally get you rather close to the theoretical maximum system performance, often with extensive manual effort (gathering data, analyzing, tweaking, and then analyzing results) and/or one or more hardware upgrades required to achieve additional system performance gains. (Failure to follow the analysis-feedback loop as set forth by the OpenVMS performance manual can actually lead to serious performance problems...) There are system performance analysis and tuning tools available from various sources -- in addition to AUTOGEN, of course -- that can sometimes be of assistance. These tools can range from reporting and analysis tools to tools that gather and dynamically tune the system. Also available from various sources are consulting services. PPGCNT is the number of private pages in the process's working set. GPGCNT is the number of global pages in the working set. WSSIZE is the current capacity of the process working set list -- the working set limit -- and the working set is not necessarily densely packed. The actual working set size can be determined by adding PPGCNT and GPGCNT. VIRTPEAK is the peak number of pages in the process's virtual address space. The actual amount of physical memory in use by a process is GPGCNT+PPGCNT. Since there is an unknown about of memory sharing, the total memory consumed by all processes is anywhere between SUM(all PPGCNT)+MAX(GPGCNT) and SUM(all PPGCNT)+SUM(all GPGCNT). The PQL_D* SYSGEN parameters are the Process Quota List (PQL) defaults. They are only considered for processes which are activated without the corresponding PQL entry being specified. Since a complete PQL is specified for *most* processes (ie: all interactive, batch, network and subprocesses), there are probably only a handful of processes on your system that will be affected by these settings. Of much more interest are the PQL_M* parameters. These are PQL Minimum values and will override lower PQL settings for ALL processes on the system. by default AUTOGEN will set PQL_MWSEXTENT to WSMAX, thereby allowing any process on the system to expand its working set size to WSMAX (assuming memory is available). In general, this can be a "good thing" and is unlikely to cause trouble as the system is free to take memory back from processes with working sets extended beyond their WSQUOTA. By default, WSMAX on a 6 GB system would be larger than 1282048. A WSQUOTA of 37368 pagelets is not very large -- that is only ~16MB. However, depending on the number of processes on your system, it may make sense to reduce it, to allow the system more latitude for recovering memory. On very large memory systems, reducing the WSEXTENT value will not necessarily increase hard pagefaults as processes will page against the free list and modified page list. Soft page faults are cheap, but they are not free. If you have free memory, but are still experiencing pagefaults, you need to determine the cause. You may find they are due to image activations, in which case tuning working set sizes won't help much, you would need to look at the application to see if it's possible to reduce the number of activations. And -- of course -- you will want to determine first if the application performance is currently insufficient, and then if the numbers of soft and hard faults are even of central concern when examining system and application performance...
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