MPE/iX 5.5 Operating System Limits

how many global DSTs will be required on a particular system. Below are some
examples of system DSTs:
- 63 Reserved system DSTs (eg. FMAVT, LOG TABLE, etc)
- Up to 118 File System Control Block Tables
- Dynamically allocated system DSTs (eg. BIPC DST, LOG BUF DST, LSTX)
- CM Stacks for system processes (eg. SESSION, JOB, PROGEN)
Each Job uses 4 DSTs and each Session uses 5 DSTs to get to the Command Interpreter
prompt. Running additional programs will require additional DSTs. The DSTs required
for a Job or a Session are listed below:
- Job Information Table (JIT)
- Job Directory Table (JDT)
- CM Stack for JSMAIN process
- CM Stack for CI process
- [FOR SESSIONS ONLY] 1 PACB for the $STDIN/$STDLIST device
Each additional user process uses 1 or more DSTs. At a minimum each process will
require 1 DST for a CM Stack. ALL processes need a CM stack (including those created
from NM program files). Process DST usage is summarized below:
- CM Stack
- DSTs explicitly allocated using GETDSEG
- PACBs for CM files (RIO, Circular, Message, & Device Files) opened
for
Buffered Access
In order to estimate the number of DSTs used by the system we will use the following
assumptions:
- The system has the number of sessions logged on is 1700
- 1700 of these sessions are "active"
See the Total Concurrent Logons - Concurrent Process Limit discussion
in the JOB/SESSION section.
- 500 global system DSTs are used by the operating system.
(500 is NOT an exact number, but is an educated guess.)
For the model above the number of DSTs used by the system would be equal to the
following formula:
# Of System DSTs = 500 + (1700 * 6) = 10700
NOTE: Each of the 1700 application programs executing in the above example may
require additional DSTs. Since the maximum number of DSTs is 16383, this leaves
approximately 5683 (16383-10700) DSTs for use by user applications. This is
approximately 3 per application (5684/1700). This limitation of 3 DSTs available for the
user application can be a problem on systems that run CM applications.
Here is another sernario with different assumptions.