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Don Burleson Blog 







Oracle RAM constrained database

Oracle Database Tips by Donald BurlesonAugust 30, 2016

Question: What do I do to see if my Oracle server is RAM constrained?  I fear that I am RAM bound on my Oracle server but I am not sure how to detect it.

Answer:  It is impossible for an Oracle database to be RAM bound unless the instance is misconfigured by the DBA.  The DBA controls the amount of RAM on an instance by using the following SGA pool parameters.

You can tell when an Oracle server is RAM bound only with these conditions:

  • Server RAM bound:  The RAM page-in (pi) will increase along with the scan-rate (sr)  See Server_RAM for details.
  • PGA advisory RAM bound:  When you see multi-pass executions in the v$_pga_advice.htm utility.  This appears in the AWR report.

In a properly configured database server all of the T2 RAM will be allocated to the database instances and the free RAM will show optimal as 5% of less of free RAM.  All RAM is fixed in size and allocated at instance startup time.  The only variable RAM in Oracle is for the PGA, and it is constrained by the _pga_max_size undocumented parameter.

Oracle 11g and beyond:

  • memory_target
  • memory_max_target

Oracle 10g (not using AMM)

  • sga_target
  • sga_memory_max

Oracle 9i and prior: (unless not using AMM)

  • pga_aggregate_target
  • sort_area_size
  • pga_aggregate_target

See details here on using the PGA and SGA parameters.

you can adjust your PGA regions to allow for system-specific sorting and hash joins. Please read these important notes on PGA behavior.

The moral of the story is that the DBA controls the amount of RAM used by Oracle, and this amount of RAM (SGA and PGA) can be reduced as much as desired, knowing that as the available RAM decreases, the amount of physical I/O increases.

You can also increase your pga_aggregate_target above the default 200 megabyte setting by setting the hidden _pga_max_size parameter.

  • _pga_max_size = 1000m
  • _smm_px_max_size = 333m

With pga_aggregate_target and _pga_max_size hidden parameter set to 1 meg we see a 5x improvement over the default for parallel queries and sorts:

  • A RAM sort or hash join may now have up to 50 megabytes (5% of pga_aggegate_target).
  • Parallel queries may now have up to 330 megabytes of RAM (30% of pga_aggegate_target), such that a DEGREE=4 parallel query would have 83 megabytes (333 meg/4).


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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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