Expanding OpenVMS Memory

When you expand OpenVMS memory, there are a number of other parameters you may wish to revisit. If you increase your memory dramatically, you will certainly have to change these SYSGEN parameters. You can also look each parameter up using HELP:


(The parameter SYS is short for SYS_PARAMETERS.)

Some parameters to consider changing are the following:

  • GBLPAGES. If you don’t increase this, you’ll be getting warning messages when you try to take advantage of all that memory. In short, this parameter sets the amount of memory that the kernel can keep track of; if you use too much this parameter is a limiting factor.
  • GBLPAGFIL. The page file needs to be able to take all of the pages that it might be called upon to reserve; increase this parameter.
  • VCC_CACHE_MAX. If you’ve not tuned your cache (XFC) then you’ll find half of your memory to be taken by the cache. This is almost certainly not what you want; modify this parameter to reduce the amount of memory your cache is allowed to take. Even so, do remember that your cache will decrease and increase dynamically in any case – but if you scale it back, then you’re not wasting memory so much.
  • MAXPROCESSCNT. This sets the maximum number of process slots – in essence, the maximum process count (which is what the parameter is called, after all). If you have a lot more memory, you’ll want to use it to run more, right? That’s not any good if you use too many processes and can’t run any more.
  • BALSETCNT. If you set MAXPROCESSCNT, you should set BALSETCNT to the same amount minus two – and never higher.

These changes can be made in the SYS$SYSTEM:MODPARAMS.DAT file and then use the AUTOGEN command to configure the sysetm. The MODPARAMS.DAT file uses a simple format; for our purposes, you can use something like this:


In place of ADD_* you can also use MAX_* or MIN_*. You can see more examples in HELP AUTOGEN MODPARAMS.DAT. AUTOGEN is described in the HELP; be careful using it! You don’t want to muck up the system so bad you have to reboot or to reinstall.

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One thought on “Expanding OpenVMS Memory”

  1. GBLPAGES, GBLPAGFIL, and VCC_MAX_CACHE are all dynamic parameters in recent versions of OpenVMS.

    For GBLPAGES, I’d suggest letting AUTOGEN calculate it unless you have a large number of user installed images. Ditto for GBLPAGFIL, unless you use lots of RMS global buffers (see the help in SYSGEN for more info).

    As for VCC_MAX_CACHE, I like it at the default, which says to use up to 50% of your physical memory for cache. Because it’s a dynamic parameter, if dynamic expansion and contraction of the cache doesn’t suit you, you can alway lower it on the fly. I also always recommend setting a minimum for the XFC cache using the reserved memory mechanism in SYSMAN.

    As for MAXPROCESSCNT and BALSETSLOTS, I can think of a number of reasons you’d add memory other than just running more processes. For example, if a new version of the application caused too many pagefaults, you might like to add memory and expand WSMAX, PQL_MWSQUOTA, and PQL_MWSEXTENT, using the new memory to increase processes working set sizes, reducing pagefaulting.

    Don’t forget to check the amount of memory after reboot with SHOW MEM. I’ve seen a few people caught out with SYSGEN parameter PHYSICAL_PAGES (or the old MEMSIZE parameter on VAXen) over the years.

    Don’t forget to check the size of your required dump file by commenting out DUMPFILE=0 in MODPARAMS.DAT and running AUTOGEN through the TESTFILES phase.

    As to the dangers of AUTOGEN, I find it to be rather conservative. At least when you use AUTOGEN, as long as you review (and correct) warnings in SYS$SYSTEM:AGEN$PARAMS.REPORT before a reboot, you can be assured of a bootable system.

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