Index Contention / Sequence Caching

I'm having a slight disagreement with my developers, and hoping you guys can help me out (either confirm what I think or tell me I'm flat-out wrong, I can take either, I just need to get this fixed).

One of our applications is suffering pretty severe waits on a particular insert. From querying the waits from gv$active_session_history for *one* second:

14-MAR-14 11.11.00.041 AM      826usupm5pkwu gc buffer busy                   13557923         13
14-MAR-14 11.11.00.041 AM      826usupm5pkwu enq: TX - index contention       13557923         19
14-MAR-14 11.11.00.142 AM      826usupm5pkwu gc buffer busy                   13557923         49
14-MAR-14 11.11.00.142 AM      826usupm5pkwu enq: TX - index contention       13557923         53
14-MAR-14 11.11.00.041 AM      826usupm5pkwu enq: TX - index contention             -1         67
14-MAR-14 11.11.00.483 AM      826usupm5pkwu gc buffer busy                   13557923        109

To me, this spells classic hot block on object 13557923 - which happens to be the PK for a table where we had a lot of waits happening. My idea to resolve is to partition that index to split out the contention - instead of all sessions trying to grab that last block, the sessions will have X blocks to work with, depending on the degree of partitioning.

Frankly, I didn't quite understand development's response (which may indicate a lack in my knowledge, or may indicate that they're not talking Oracle terms). They want to increase the caching for the sequence that they use to populate the PK from 20 to 1000, because:

"The fact that the biggest waits were index contention and gc buffer busy waits indicates that the contention is mostly between RAC nodes trying to lock the same index page. Increasing cache from 20 to 1000 (assuming that the default page is 8K and a PK column size is 8 bytes) will cause multiple nodes to work with their own index pages. There still will be contention between sessions running on the same node due to high number of CPUs (48) on each node. But in a short term increasing sequence cache might decrease contention up to three times (it's a 3 node RAC)" I'm fairly certain that the gc buffer busy waits were because sessions trying to grab that index block were spread among multiple nodes, but I really don't understand how increasing the sequence cache value should help that - no matter what's cached in memory, the same block is going to be the hot spot, no? Short version: 1) am I crazy? 2) is the developer who said that crazy? 3) what the heck is object "-1" 4) if you were confronted with waits like that - any immediate thoughts for resolving other than partitioning that PK index? Oracle 10.2.0.4 on Windows 2003 x64, in case that matters. Thanks!
stephan

Stephan Uzzell

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