Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 4 ■ MEMORY STRUCTURES
a 50MB sort area size might be reasonable for the single user logged in. However, at
12:00 pm with 500 users, 50MB might not be appropriate. This is where the WORKAREA_SIZE_
POLICY = AUTO setting and the corresponding PGA_AGGREGATE_TARGET come in handy. Setting the
PGA_AGGREGATE_TARGET, the amount of memory you would like Oracle to feel free to use to sort
and hash, is conceptually easier than trying to figure out the perfect SORT|HASH_AREA_SIZE,
especially since there isn’t a perfect value for these parameters; the perfect value varies by
workload.
Historically, the DBA configured the amount of memory used by Oracle by setting the
size of the SGA (the buffer cache; the log buffer; and the Shared, Large, and Java pools). The
remaining memory on the machine would then be used by the dedicated or shared servers in
the PGA region. The DBA had little control over how much of this memory would or would not
be used. She could set the SORT_AREA_SIZE, but if there were 10 concurrent sorts, then Oracle
could use as much as 10 * SORT_AREA_SIZE bytes of RAM. If there were 100 concurrent sorts,
then Oracle would use 100 * SORT_AREA_SIZE bytes; for 1,000 concurrent sorts, 1,000 *
SORT_AREA_SIZE; and so on. Couple that with the fact that other things go into the PGA, and
you really don’t have good control over the maximal use of PGA memory on the system.
What you would like to have happen is for this memory to be used differently as the
memory demands on the system grow and shrink. The more users, the less RAM each should
use. The fewer users, the more RAM each should use. Setting WORKAREA_SIZE_POLICY = AUTO is
just such a way to achieve this. The DBA specifies a single size now, the PGA_AGGREGATE_TARGET
or the maximum amount of PGA memory that the database should strive to use. Oracle will
distribute this memory over the active sessions as it sees fit. Further, with Oracle9i Release 2
and up, there is even PGA advisory (part of Statspack, available via a V$ dynamic performance
view and visible in Enterprise Manager), much like the buffer cache advisor. It will tell you
over time what the optimal PGA_AGGREGATE_TARGET for your system is to minimize physical I/O
to your temporary tablespaces. You can use this information to either dynamically change the
PGA size online (if you have sufficient RAM) or decide whether you might need more RAM on
your server to achieve optimal performance.
Are there times, however, when you won’t want to use it? Absolutely, and fortunately they
seem to be the exception and not the rule. The automatic memory management was designed
to be multiuser “fair.” In anticipation of additional users joining the system, the automatic
memory management will limit the amount of memory allocated as a percentage of the
PGA_AGGREGATE_TARGET. But what happens when you don’t want to be fair, when you know
that you should get all of the memory available? Well, that would be the time to use the ALTER
SESSION command to disable automatic memory management in your session (leaving it in
place for all others) and to manually set your SORT|HASH_AREA_SIZE as needed. For example,
that large batch process that takes place at 2:00 am and does tremendously large hash joins,
some index builds, and the like? It should be permitted to use all of the resources on the
machine. It does not want to be “fair” about memory use—it wants it all, as it knows it is the
only thing happening in the database right now. That batch job can certainly issue the ALTER
SESSION commands and make use of all resources available.
So, in short, I prefer to use automatic PGA memory management for end user sessions—
for the applications that run day to day against my database. Manual memory management
makes sense for large batch jobs that run during time periods when they are the only activities
in the database.