The Process Global Area and User Global Area- Memory Structures

Aug 22, 2023

The Process Global Area and User Global Area- Memory Structures

The PGA is a process-specific piece of memory. In other words, it is memory specific to a single operating system process or thread.

This memory is not accessible by any other process or thread in the system. It is typically allocated via either of the C runtime calls malloc() or memmap(), and it may grow (or even shrink) at runtime.

The PGA is never allocated in Oracle’s SGA; it is always allocated locally by the process or thread—the P in PGA stands for process or program; it is not shared.

The UGA is, in effect, your session’s state. It is memory that your session must always be able to get to. The location of the UGA is dependent on how you connect to Oracle.

If you connect via a shared server, the UGA must be stored in a memory structure that every shared server process has access to—and that’s the SGA. In this way, your session can use any one of the shared servers, since any of them can read and write your session’s data.

On the other hand, if you are using a dedicated server connection, there’s no need for universal access to your session state, and the UGA becomes virtually synonymous with the PGA; it will, in fact, be contained in the PGA of your dedicated server.

When you look at the system statistics, you’ll find the UGA reported in the PGA in dedicated server mode (the PGA will be greater than or equal to the UGA memory used; the PGA memory size will include the UGA size as well).

So, the PGA contains process memory and may include the UGA. The other areas of PGA memory are generally used for in-memory sorting, bitmap merging, and hashing. It would be safe to say that, besides the UGA memory, these are the largest contributors by far to the PGA.

There are two ways to manage memory in the PGA: manual and automatic. The manual method should not be used (unless you’re on an old version of Oracle and don’t have a choice).

The automatic PGA memory management is the recommended technique that you should use. The automatic method is much simpler and more efficient in managing memory. The manner in which memory is allocated and used differs greatly in each case, so we’ll discuss each in turn.