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inmemory internals

library cache lock on BUILD$ object

I was testing an application performance in 12c, and one job was constantly running slower than 11g. This post is to detail the steps. I hope the steps would be useful if you encounter similar issue.


In an one hour period, over 90% of the DB time spent on waiting for library cache lock waits. Upon investigation, one statement was suffering from excessive waits for ‘library cache lock’ event. We recreated the problem and investigated it further to understand the issue.

Following is the output of wait_details_rac.sql script (that I will upload here) and there are many PX query servers are waiting for ‘library cache lock’ wait event.

inmemory: Why did that table was not populated in the column store?

I enabled an huge 70G table for inmemory population, I expected the inmemory population to take a while, but the population didn’t complete even after letting it run for a day. Why?

ASH data

Initial review of the server shows no issues, no resource starvation. This must be a problem with Oracle processes itself. I started digging further, and ASH data shows that in numerous samples the process was seen reading block using single block I/I calls. Also object_id matches with the table I was trying to populate.

inmemory: sa00 process

After the restart of a 12c inmemory database with 300GB+ SGA, I noticed that an Oracle background process sa00 was consuming a bit of CPU. Documentation suggests that it is SGA Allocator process, however, ipcs -ma command shows that the shared memory segment is already allocated. I was curious, of course, what would that background process will be allocating?.


Process stack of the process shows that it is touching SGA pages to pre-page SGA memory pages.

Inmemory: Not all inmemory_size is usable to store tables.

I have been testing the inmemory column store product extensively and the product is performing well for our workload. However, I learnt a bit more about inmemory column store and I will be blogging a few them here. BTW, I will be talking about internals of inmemory in Oaktable world presentation, if you are in the open world 2014, you can come and see my talk: