SysMemory

SysMemory

The SysMemory option in XLCONFIG tells 4C how much shared memory to allocate. SysMemory is used for storing data that may be needed my multiple processes at the same time. This includes: 4C currently only allocates the shared memory segment once and it cannot grow, so it is important to make sure you specify it correctly. Nowadays, with so much memory available there is little sense in trying to keep shared memory to a minimum. I would recommend setting it high to start with and monitor how it is used. 16 or 32 meg are reasonable numbers to start with. Here are several ways to specify these values. The only reason in using multiple segments is that some Unix kernels default to low values for maximum size of a shared memory segment. Allocating multiple smaller segments helps you avoid having to reconfigure the kernel. When possible, you should try to use as few segments as possible.

In addition to the size of SysMemory there are two other parameters in XLCONFIG that affect how shared memory is used. They are: Both of theses parameters take one argument, the number of file specs or the number of program specs that you want to try to keep in shared memory. Determining this number can be tricky, is installation and application dependant, and may change over time as the number of users increase on a system. Here are some guidelines that may help: While 4C tries to deal with shared memory problems by allocating program specs and file specs in UsrMemory when necessary, it is important to get these settings as correct as possible. You will not get these values right by guessing or by using the default 4CConfig file. You can only get them right by monitoring new installations and by checking older ones as usage changes.

Here's one scenario to start with.

Set the SysMemory, Files, and Programs as follows:
SysMemory 16384 1
Files 1000
Programs 100
This allocates 16meg shared memory all as one segment, allows up to 1000 different file specs and 100 different program specs to be kept in memory at the same time.

When the system is up and running under heavy load, run the sys.mem.summary 4C program and look for the following: Until 4C adapts to changing usage and shared mem requirements automatically you should run this program: Another way of checking for 4C shared memory problems is to run:
xlview -v
or
xlview -V
from a command line.

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