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CHAPTER 4 MEMORY STRUCTURES
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So, assuming that WORKAREA_SIZE_POLICY is set to AUTO and PGA_AGGREGATE_TARGET has a nonzero value, you will be using the new (as of Oracle 9i) automatic PGA memory management. You can turn it on in your session via the ALTER SESSION command or at the system level via the ALTER SYSTEM command.
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Note Bear in mind the previous caveat that in Oracle9i, shared server connections will not use automatic
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memory management; rather, they will use the SORT_AREA_SIZE and HASH_AREA_SIZE parameters to decide how much RAM to allocate for various operations. In Oracle 10g and up, automatic PGA memory management is available to both connection types. It is important to properly set the SORT_AREA_SIZE and HASH_AREA_SIZE parameters when using shared server connections with Oracle9i.
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So, the entire goal of automatic PGA memory management is to maximize the use of RAM while at the same time not using more RAM than you want. Under manual memory management, this was a virtually impossible goal to achieve. If you set SORT_AREA_SIZE to 10MB, when one user was performing a sort operation that user would use up to 10MB for the sort work area. If 100 users were doing the same, they would use up to 1,000MB of memory. If you had 500MB of free memory, the single user performing a sort by himself could have used much more memory, and the 100 users should have used much less. That is what automatic PGA memory management was designed to do. Under a light workload, memory usage could be maximized as the load increases on the system, and as more users perform sort or hash operations, the amount of memory allocated to them would decrease to reach the goal of using all available RAM, but not attempting to use more than physically exists.
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Determining How the Memory Is Allocated
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Questions that come up frequently are "How is this memory allocated " and "What will be the amount of RAM used by my session " These are hard questions to answer for the simple reason that the algorithms for serving out memory under the automatic scheme are not documented and can and will change from release to release. When using things that begin with "A" for automatic you lose a degree of control, as the underlying algorithms decide what to do and how to control things. We can make some observations based on information from MetaLink note 147806.1: The PGA_AGGREGATE_TARGET is a goal of an upper limit. It is not a value that is preallocated when the database is started up. You can observe this by setting the PGA_AGGREGATE_TARGET to a value much higher than the amount of physical memory you have available on your server. You will not see any large allocation of memory as a result. A serial (nonparallel query) session will use a small percentage of the PGA_AGGREGATE_TARGET, typically about 5 percent or less. So, if you ve set the PGA_AGGREGATE_TARGET to 100MB, you d expect to use no more than about 5MB per work area (e.g., the sort or hash work area). You may well have multiple work areas in your session for multiple queries, or more than one sort or hash operation in a single query, but each work area will be about 5 percent or less of the PGA_AGGREGATE_TARGET. Note that this 5 percent is not a hard and fast rule; things change over time, the automatic algorithms can and will change in the database.
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CHAPTER 4 MEMORY STRUCTURES
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As the workload on your server goes up (more concurrent queries, concurrent users), the amount of PGA memory allocated to your work areas will go down. The database will try to keep the sum of all PGA allocations under the threshold set by PGA_AGGREGATE_TARGET. This is analogous to having a DBA sit at a console all day, setting the SORT_AREA_SIZE and HASH_AREA_SIZE parameters based on the amount of work being performed in the database. We will directly observe this behavior shortly in a test. A parallel query may use up to about 30 percent of the PGA_AGGREGATE_TARGET, with each parallel process getting its slice of that 30 percent. That is, each parallel process would be able to use about 0.3 * PGA_AGGREGATE_TARGET / (number of parallel processes).
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OK, so how can we observe the different work area sizes being allocated to our session By applying the same technique we used earlier in the manual memory management section to observe the memory used by our session and the amount of I/O to temp we performed. I performed the following test on a Red Hat Advanced Server 4.0 Linux machine using Oracle 11.2.0.1 and dedicated server connections. This was a two-CPU Dell PowerEdge with hyperthreading enabled, so it was as if there were four CPUs available. We begin by creating a table to hold the metrics we d like to monitor: create as select from ( select from where and and union select from where and and and group ); table sess_stats name, value, 0 active a.name, b.value v$statname a, v$sesstat b a.statistic# = b.statistic# b.sid = (select sid from v$mystat where rownum=1) (a.name like '%ga %' or a.name like '%direct temp%') all 'total: ' || a.name, sum(b.value) v$statname a, v$sesstat b, v$session c a.statistic# = b.statistic# (a.name like '%ga %' or a.name like '%direct temp%') b.sid = c.sid c.username is not null by 'total: ' || a.name
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The columns in this table we ll be using for the metrics represent: NAME: the name of the statistic we are gathering (PGA and UGA information from V$SESSTAT for the current session, plus all of the memory information for the database instance as well as temporary tablespace writes). VALUE: the value of the given metric. ACTIVE: the number of other sessions doing work in the database. Before we start, we assume an idle database; we are the only user session right now, hence the value of zero.
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