Re: RAC PARALLEL

From: Sanjay Mishra <smishra_97_at_yahoo.com>
Date: Mon, 17 Oct 2011 11:35:57 -0700 (PDT)
Message-ID: <1318876557.93944.YahooMailNeo_at_web161204.mail.bf1.yahoo.com>

Riyaj
One important thing found is that several simple queries are using PX.... in Explain when moved to 11g RAC and taking much more time than non-rac 10g.In 10g it is simple NL and Index scan but in 11g has much more in explain plan using the same Indexes. Also Cost comes out in two digit in 10g which is 5 digit in 11g.

Sanjay

---

From: Riyaj Shamsudeen <riyaj.shamsudeen_at_gmail.com> To: smishra_97_at_yahoo.com
Cc: Andrew Kerber <andrew.kerber_at_gmail.com>; "mwf_at_rsiz.com" <mwf_at_rsiz.com>; "tim_at_evdbt.com" <tim_at_evdbt.com>; "oracle-l_at_freelists.org" <oracle-l_at_freelists.org> Sent: Monday, October 17, 2011 10:37 AM
Subject: Re: RAC PARALLEL

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, Pro Oracle SQL, Expert PL/SQL Practices

On Fri, Oct 14, 2011 at 10:46 AM, Sanjay Mishra <smishra_97_at_yahoo.com> 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
>

--
http://www.freelists.org/webpage/oracle-l

Received on Mon Oct 17 2011 - 13:35:57 CDT