Tag Archives: Performance

An introduction to collectl

Some of you may have seen on twitter that I was working on understanding collectl. So why did I start with this? First of all, I was after a tool that records a lot of information on a Linux box. It can also play information back, but this is out of scope of this introduction.

In the past I have used nmon to do similar things, and still love it for what it does. Especially in conjunction with the nmon-analyzer, an Excel plug in it can create very impressive reports. How does collectl compare?

Getting collectl

Getting collectl is quite easy-get it from sourceforge: http://sourceforge.net/projects/collectl/

The project website including very good documentation is available from sourceforge as well, but uses a slightly different URL: http://collectl.sourceforge.net/

I suggest you get the archive-independent RPM and install it on your system. This is all you need to get started! The impatient could type “collectl” at the command prompt now to get some information. Let’s have a look at the output:

$ collectl
waiting for 1 second sample...
#cpu sys inter  ctxsw KBRead  Reads KBWrit Writes   KBIn  PktIn  KBOut  PktOut
1   0  1163  10496    113     14     18      4      8     55      5      19
0   0  1046  10544      0      0      2      3    164    195     30      60
0   0  1279  10603    144      9    746    148     20     67     11      19
3   0  1168  10615    144      9    414     69     14     69      5      20
1   0  1121  10416    362     28    225     19     11     71      8      35

The “ouch” has been caused by my CTRL-c to stop the execution.

Collectl is organised to work by subsystems, the standard option is to print CPU, disk and network subsystem, aggregated.

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Selectively purging the shared pool

Another very useful thread has developed on the oracle-l mailing list. On a side note, this is really _the_ list to be subscribed to-there are so many experienced Oracle DBAs out that it’s pure joy.

But I digress, back to the problem I often face when performance troubleshooting systems is to purge a SQL statement from the shared pool. Most often, bind variable peeking in conjunction with an empty partition (or otherwise inappropriate stats) caused the optimiser to choose a really inadequate plan for subsequent executions of the same SQL ID.

Oracle 11.1 extended DBMS_SHARED_POOL to include a “purge” function which can be used to manage the shared pool. It’s actually a lot more powerful than just purging SQL plans but that’s for another blog entry :) Anyway, if you are interested, additional documentation can be found in metalink notes 457309.1 and 751876.1. If anyone can shed more light on the various heaps you can pass as an argument, please comment here!

The new version of the package has been back ported and included in the patch set (where it is event protected) and can also be downloaded for and for patch 5614566 on metalink.

Prior to this, I usually used the “comment on table <owner>.<table_name> is ‘<some comment>’ to force an invalidation of the statement in v$sql. Next time the statement is executed you’d hope that you have escaped the bind variable peeking disaster.

Kerry Osborne, a great source for inspiration has posted an easy way to using it. The following is shamelessly taken from the mailing list, I won’t take any credit for it (but I’ll use it whenever I can!)

Beginning the quote, slightly amended to better fit into context:

Heap 0 flushes the whole statement, heap 6 flushes the plans (although I’ve never had a reason to flush just the plans. By the way, when heap=6, plans for all children are flushed.

The backport to seems to work fine as well, but it doesn’t appear to work in versions prior to ( for example).

Here’s the script I use (because I don’t like the funky “address,hash_value” format that the purge procedure requires:

-- flush_sql.sql
 name varchar2(50);
 version varchar2(3);
 select regexp_replace(version,'\..*') into version from v$instance;

 if version = '10' then
 execute immediate
 q'[alter session set events '5614566 trace name context forever']'; -- bug fix for backport
 end if;

 select address||','||hash_value into name
 from v$sqlarea
 where sql_id like '&sql_id';



If you want to have the flexibility to select the heap – change the call to purge like this:


Then just put in 0 or 6 when prompted for the heap.

End qoute. Credit where credit is due:

Kerry Osborne
blog: kerryosborne.oracle-guy.com <http://kerryosborne.oracle-guy.com&gt;