One of my readers recently asked me if ORACLE 11g can use memory_target along with AIX large pages … (Thanks Randolf!)
At the first blush this seems to be impossible. The main reason, of course, is that memory_target and large pages are used for the purposes that are, in fact, completely opposite!
- The goal of 11g memory_target is to flow memory efficiently between ORACLE SGA and PGA to the place where it is most needed. This means that SGA may reduce its size from time to time, releasing memory to the operating system
- AIX large pages, on the other hand, are designed to never leave physical memory
I hope, you can see a contradiction here …
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 the previous post we discussed how ORACLE allocates shared memory for SGA in AIX and one of the conclusions was that AIX does not give all the requested memory to ORACLE instance right away but merely promises it.
While this technique allows to use memory more efficiently and you (at least temporarily), can request more memory for processes and shared segments than what AIX physically has, it also has a rather unpleasant consequence – when we get to the limit of physical memory, AIX will have no choice but to start paging memory.
Paging is not necessarily a bad thing – moving older and not-so-often used data out of memory is something that will be done rather routinely – this is how AIX keeps a healthy system. However, when SGA memory starts to page out (and, more importantly, page back in) things can go bad quickly as, well, ORACLE does not really expect SGA to be a disk based area … (ORACLE would have called it “SDA” if that was the case )
You probably know that in the vast majority of configurations, it is strongly advised to size SGA so that it fits entirely into physical memory and never pages out. The question becomes: how can we accomplish that on AIX?
In the previous post we discussed memory usage by ORACLE processes and, if you remember, it took us some effort to get to their actual memory usage.
In this post we are going to talk about the other large memory area that ORACLE instance uses – System Global Area or SGA.
In many respects, finding out SGA memory usage is going to be simpler as we have to deal with only one large entity (AIX shared memory segment) instead of many small memory chunks in separate ORACLE processes. But as we will see, this process is still rather “involved” for AIX has a few tricks up its sleeve for shared memory as well.
How much memory is used by ORACLE SGA. Really …
Let’s start with a simple question.
What happens if we try to allocate 12 Gb SGA on a machine with only 8 Gb of physical memory?