What can be done if adding virtual Terminals to TPX results in Abends S80A-10 and/or S878-10?

Products

• TPX - Session Management

Issue/Introduction

 An Administrator of TPX is adding virtual Terminals dynamically using command VTADD. This might result in Abends S80A-10 and/or S878-10 because of Storage Shortages below-the-line.

 

 

Resolution

If you are receiving Abends S80A-10 and S878-10 when adding virtual terminals to the TPX-region this is caused by the fact that every virtual terminal needs 144 bytes of storage below the 16MB-line. For 10,000 terminals this sums up to 1.37 MB, for 20,000 terminals this sums up to 2.75 MB of storage needed below the 16 MB line. Increasing the region size of TPX does not help in this case as it does not extend the storage below 16 MB.

Here is a high-level overview of how the storage below 16MB is shared between TPX and z/OS:

16MB	CSA etc.
	System Storage (minimum 2MB)
	DSA
	Slots
	TPX Modules (~0,5 MB)
0MB	Nucleus

 

TPX resides in the bold box. Beside the modules the available Storage is divided into a DSA and a Slot Area.

The System Storage is used by z/OS, VTAM and other System Components to communicate with TPX.

The virtual terminals control blocks which are created during startup of TPX are stored in the DSA. If you are adding virtual terminals dynamically using TPXOPER command VTADD these are stored in the System Storage.

So you need to decide how large the System Storage on one side and the DSA on the other side is. Remember that you cannot use the whole System Storage for virtual terminals using VTADD because other System Components, like your security package or VSAM, need to use it also.

Both, the size of the System Storage and of the DSA can be set in the second panel of the SMRT.

1. The size of System Storage should not be lower than 2048 KB. Any change requires a restart of TPX to take effect.
   
   
2. The size of 'DSA percentage' is determined by the percentage of DSA and Slot size. If the percentage of DSA is increased, this lowers the available amount of storage for the slots at the same time. So you need to check the Slot usage with TPXOPER command 'D STOR'. The column 'Max.Use' shows how the several slots are used during lifetime of the TPX-Region. You can change the percentage of the several slots to give slots which are used more intensively more storage than others. Again, changes here require TPX to be restarted in order to get effective.

If you plan to increase the number of virtual terminals even higher than about 25,000 then consider dividing your TPX into a Generic Resource of two or more TPX-regions. Then every TPX does not need to have that large number of virtual terminals, but only a fraction.