A doc from Metalink:
Preferred and easiest way of monitoring and setting pga_aggregate_target parameter (PGA) is section ‘PGA Memory Advisory’ in the AWR and Statspack reports.
PGA Memory Advisory
- When using Auto Memory Mgmt, minimally choose a pga_aggregate_target value where Estd PGA Overalloc Count is 0
| PGA Target Est (MB) | Size Factr | W/A MB Processed | Estd Extra W/A MB Read/ Written to Disk | Estd PGA Cache Hit % | Estd PGA Overalloc Count |
|---|---|---|---|---|---|
| 128 | 0.13 | 258,572.74 | 127,776.77 | 67.00 | 2,010 |
| 256 | 0.25 | 258,572.74 | 31,433.28 | 89.00 | 0 |
| 512 | 0.50 | 258,572.74 | 29,560.27 | 90.00 | 0 |
| 768 | 0.75 | 258,572.74 | 28,863.55 | 90.00 | 0 |
| 1,024 | 1.00 | 258,572.74 | 28,863.55 | 90.00 | 0 |
| 1,229 | 1.20 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 1,434 | 1.40 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 1,638 | 1.60 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 1,843 | 1.80 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 2,048 | 2.00 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 3,072 | 3.00 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 4,096 | 4.00 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 6,144 | 6.00 | 258,572.74 | 4,643.11 | 98.00 | 0 |
| 8,192 | 8.00 | 258,572.74 | 4,643.11 | 98.00 | 0 |
In this section, you first find the row where field ‘Size Factr’ is 1.00. The field ‘PGA Target Est(MB)’ of this row will show your current PGA setting – figure 1024 in the above example. Other fields (columns) you will be interested in are: ‘Estd Extra W/A MB Read/ Written to Disk ‘ and ‘Estd PGA Overalloc Count’.
When you go down or up the advisory section from the row with ‘Size Factr’ = 1.00, you get estimations for Disk usage – column ‘Estd Extra W/A MB Read/ Written to Disk ‘ - for bigger or smaller settings of pga_aggregate_target. The less Disk usage figure in this column, usually the better.
Your first goal is to have such a setting of pga_aggregate_target, that number in the column ‘Estd Extra W/A MB Read/ Written to Disk ‘ does not substantially reduce any more, see figure 28863.55 in the example AWR report.
In other words, further increases of pga_aggregate_target won’t give any more benefit.
Column ‘Estd PGA Overalloc Count’ shows estimations of how many times database would need to request from OS more PGA memory than the amount shown in the ‘PGA Target Est(MB)’ field of the respective row. Ideally this field should be 0, and that is your equally important second goal.
In many cases ‘Estd PGA Overalloc Count’ figures reach 0 before the number in ‘Estd Extra W/A MB Read/ Written to Disk ‘ stabilizes
Question whether increase from the current actual size is possible for a given database, should be always investigated. The answer depends on how much of total memory (SGA+PGA) can be allocated for this database on this box, i.e. take into account memory needs of other databases, software and OS residing on the box.
Ref:DOC ID:786554.1
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