topic Re: SHMMAX = Available Memory in Operating System - HP-UX

SHMMAX = Available Memory

Alan Meyer_4 — Fri, 12 May 2006 12:57:19 GMT

In the setup for Oracle 10G on HP-UX 11.23, it specifies that SHMMAX should be set to Available Memory.

How does one determine available memory?

Re: SHMMAX = Available Memory

Don Morris_1 — Fri, 12 May 2006 13:00:44 GMT

pstat_getstatic() or Glance would be the most reliable way -- but since this is a fence line setting anyway, just set it to the amount of RAM on the box (it doesn't matter much if you don't account for the memory used by the kernel, etc.)

Re: SHMMAX = Available Memory

Alzhy — Fri, 12 May 2006 13:01:10 GMT

Use vmstat.
The "free" column is your free memory.

Re: SHMMAX = Available Memory

Alan Meyer_4 — Fri, 12 May 2006 13:33:42 GMT

Thanks for all your help.

Re: SHMMAX = Available Memory

A. Clay Stephenson — Fri, 12 May 2006 15:27:59 GMT

Let me say that this is a very dumb way to set SHMMAX. What should be done is to ask the DBA's what their largest anticipated SGA will need and set SHMMAX a bit above that. SHMMAX has almost nothing to do with the amount of physical memory in a system because HP-UX is a virtual memory based system; however, allowing a single process to grab all the available memory is normally considered a bad thing. Moreover, SHMMAX is a per-process limit rather than a system-wide limit so even with values much smaller than total memory size, it is possible to drive your box to its knees because of excessive swapping.

Setting SHMMAX to a very large value does no harm in itself but it does allow the potential (intended or otherwise) for abuse there this should be a planned limit based upon maximum anticipated needs rather than
tying it to some value directly related to the total amount of physical memory.