SERVGA parameter: Difference between revisions

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m (Corrected SERVGSA to SERVGA. Other minor wording changes.)
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==Description==
==Description==
<p>The <var>SERVGA</var> parameter, along with the <var>SERVGSZ</var> parameter, control swappable server areas that are placed above the 2G bar.</p>
<p>The <var>SERVGA</var> parameter controls which server tables are allocated in the ATB-swappable-server area.</p>
Each server table to be allocated in the above-the-bar swappable area is controlled by a bit in <var>SERVGSA</var>; if the bit is on, the corresponding server table is allocated in the above-the-bar swappable area.
Each server table to be allocated in that area is controlled by a bit in <var>SERVGA</var>; if the bit is on, the corresponding server table is allocated in the ATB-swappable-server area.


The bits are:
The bits are:

Revision as of 23:38, 4 November 2014

Above the bar server areas

Summary

Default value
0
Parameter type
System
Where set
On User 0's parameter line
Related products
All
Introduced
Model 204 V7.5

Description

The SERVGA parameter controls which server tables are allocated in the ATB-swappable-server area.

Each server table to be allocated in that area is controlled by a bit in SERVGA; if the bit is on, the corresponding server table is allocated in the ATB-swappable-server area.

The bits are:

BitServer table placed above the bar
X'00004000'NTBL
X'00002000'QTBL

SERVGSZ is the amount of space in bytes required for the swappable above-the-bar server tables per server. The total amount of storage allocated for swappable above-the-bar server areas equals SERVGSZ rounded to 4K and multiplied by NSERVS. When sizing SERVGSZ, it should accomodate the largest swappable above-the-bar table sizes that might be needed.

QTBL and NTBL can alternatively be placed above-the-bar in a non-swappable area. This can be indicated with the SERVNSA and SERVNSSZ parameters. The bits corresponding to NTBL and QTBL should not be set in both SERVNSA and SERVGA — if they are, an error message will be issued and the online will fail to come up.

While it might seem odd to have above-the-bar swappable server areas, especially if swapping to above-the-bar memory, placing NTBL and QTBL in a swappable area can save quite a bit of real memory. This is especially true if the RESPAGE parameter is set to a non-zero value and there are significantly more users (NUSERS) than servers (NSERVS). In such a case, most swapped out users would either be using a resident (shared) NTBL and QTBL, or they would be logged out and only using a single 4K page. As such, reserving the amount of space required for NTBL and QTBL (which can typically get quite large) in the non-swappable above-the-bar area for all the swapped out users can waste a significant amount of real storage.