Saturday Nov 08, 2008

Virtual CPUs effect on Oracle SGA allocations.

Several years ago, I wrote about how Oracle views multi-threaded processors. At the time we were just introducing a dual-core processor. This doubling of the number of cores was presented by Solaris as virtual CPUs and Oracle would automatically size the CPU_COUNT accordingly. But what happens when you introduce a 1RU server that has 128 virtual CPUs?

The UltraSPARC T1/T2/T2+ servers have many threads or virtual CPUs. The CPU_COUNT on these systems is sized no different than before. So, the newly introduced T5540 with 4xUltraSPARC T2+ processors would have 256 threads and CPU_COUNT would be set to 256.

So, what does CPU_COUNT have to do with memory?

Thanks to my friends in the Oracle Real World Performance group, I was made aware that Oracle uses CPU_COUNT to size the minimum amount of SGA allowed. In one particular case, the DBA was trying to allocate 70 database instances on a T5140 with 64GB of memory and 128 virtual CPUs. Needless to say, the SGA_TARGET would have to be set fairly low in-order to accomplish this task. A SGA_TARGET was set to 256MB, but the following error was encountered.

    ORA-00821: Specified value of sga_target 256M is too small
After experimentation, they were able to start Oracle with a target of 900MB, but with 70 instances this would not fly. Manually lowering the CPU_COUNT allowed the DBA to use an SGA_TARGET of 256MB. Obviously, this is an extreme case and changing CPU_COUNT was reasonable.

Core and virtual CPU counts have been on the rise for some years now. Combine rising virtual CPU count with the current economic climate and I would suspect that consolidation will be more popular than ever. In general, I would not advocate changing CPU_COUNT manually. If you had one instance on this box, the default be just fine. CPU_COUNT automatically sizes so many other parameters that you should be very careful before making a change.

About

This blog discusses performance topics as running on Sun servers. The main focus is in database performance and architecture but other topics can and will creep in.

Search

Archives
« April 2014
SunMonTueWedThuFriSat
  
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
   
       
Today
News

No bookmarks in folder

Blogroll

No bookmarks in folder