Re: RAC PARALLEL
- From: Riyaj Shamsudeen <riyaj.shamsudeen@xxxxxxxxx>
- To: smishra_97@xxxxxxxxx
- Date: Mon, 17 Oct 2011 09:37:52 -0500
Sanjay
There are few observations:
1. Reviewing top 5 wait events, notice that there is no direct path read
waits. If you have parallel queries performing full table scans, then direct
path read would show up in the top 5 wait events.
So, your application might be using a special case of PQ where the
slaves perform nested loops join (yes, this is a possible execution plan).
If that is the case, then slaves will read the blocks in to buffer cache and
access them. This type of PQ execution stats are markedly different from the
traditional full table scan reading the blocks in to their PGA (aka direct
reads). Also, It goes without saying that, this type of execution plans will
induce more cache fusion traffic for the cache fusion clients (as against PQ
client), leading to the fact that keeping them in the same instance will
improve performance.
On the other hand, I don't know, why parallel hints are added to
these type of queries. Can you review the execution plans for these queries
and determine if they have much NL join? use dbms_xplan.display_cursor
please.
Also, what are the stats indicate in dba_hist_ic_client_stats during
the problem time frame?
2. Further, setting parallel_force_local has side effects and bugs.
Notably bug "Bug 9671271 - All active instances used in calculation of dop
when parallel_force_local=true / High version count on PX_MISMATCH [ID
9671271.8]". I had an issue with client database throwing spurious ORA-4031
errors, above bug as a final root cause.You should services to control slave
allocation.
HTH
Cheers
Riyaj Shamsudeen
Principal DBA,
Ora!nternals - http://www.orainternals.com - Specialists in Performance,
RAC and EBS11i
Blog: http://orainternals.wordpress.com
OakTable member http://www.oaktable.com
Co-author of the books: Expert Oracle
Practices<http://tinyurl.com/book-expert-oracle-practices/>,
Pro Oracle SQL, Expert PL/SQL
Practices<http://tinyurl.com/book-expert-plsql-practices>
On Fri, Oct 14, 2011 at 10:46 AM, Sanjay Mishra <smishra_97@xxxxxxxxx>wrote:
> Changing parallel_force_local to True has brought back the SQL to same and
> even better performance level. Thanks to all for the help and suggestions.
> Will be doing more regression testing to make sure if this is the only
> issue.
>
> Sanjay
>
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