This is going to be a long post but don’t be discouraged: most of it will involve snapshots and screen examples, so it shouldn’t be too bad …
- ORACLE Instance Memory consists of 2 parts: process memory and shared (SGA) memory
- Process memory is a bunch of memory segments allocated in individual ORACLE processes and their collective size is (attempted to be) managed by pga_aggregate_target parameter. AIX improves process memory usage by identifying sharable segments (such as program or shared library text) and not duplicating them for each individual process.
- SGA memory is allocated as a single AIX shared memory segment (which, in reality, turns out to be a bunch of smaller VMM segments) and (in ORACLE 10g) is managed by sga_target and sga_max_size parameters. AIX, by default, helps with shared memory usage by allocating it only as needed. However, you can overwrite this behavior and force AIX to allocate all the shared memory at once and, additionally, put a “pin” on it in order to prevent paging.
In this post I am going to talk about how ORACLE allocates and uses memory when running on AIX, but I will also talk about the power of approximation and how it can sometimes be misused for ill purposes
On the outset, ORACLE/AIX memory “deal” seems simple enough – obviously, ORACLE will use memory when it runs and many AIX commands (such as vmstat or ps) will show memory usage both system wide and specific to particular process. But, as always, the devil is in the details and the effect of those “details” may be far from subtle.
So, why don’t we go ahead and find that devil, shall we ?
The straightforward (but naive) approach
Let’s say that we have an ORACLE instance running on one of our AIX servers. How do we find out how much memory it is using ?