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II.6.9 Diagnosing Performance Problems

  • Did performance problems happen overnight, or did they sneak up on you? Any clue what caused the performance problems (e.g. loading 20K users into .LRN)

  • Is the file system out of space? Is the machine swapping to disk constantly?

  • Isolating and solving database problems.

    • Without daily internal maintenance, most databases slowly degrade in performance. For PostGreSQL, see the section called “Vacuum Postgres nightly”. For Oracle, use exec dbms_stats.gather_schema_stats('SCHEMA_NAME') (Andrew Piskorski's Oracle notes).

    • You can track the exact amount of time each database query on a page takes:

      1. Go to Main Site : Site-Wide Administration : Install Software

      2. Click on "Install New Application" in "Install from OpenACS Repository"

      3. Choose "ACS Developer Support">

      4. After install is complete, restart the server.

      5. Browse to Developer Support, which is automatically mounted at /ds.

      6. Turn on Database statistics

      7. Browse directly to a slow page and click "Request Information" at the bottom of the page.

      8. This should return a list of database queries on the page, including the exact query (so it can be cut-paste into psql or oracle) and the time each query took.

        Figure6.8.Query Analysis example

        Query Analysis example

    • Identify a runaway Oracle query: first, use ps aux or top to get the UNIX process ID of a runaway Oracle process.

      Log in to SQL*Plus as the admin:

      [$OPENACS_SERVICE_NAME ~]$ svrmgrl
      Oracle Server Manager Release - Production
      Copyright (c) 1997, 1999, Oracle Corporation.  All Rights Reserved.
      Oracle8i Enterprise Edition Release - Production
      With the Partitioning option
      JServer Release - Production
      SVRMGR> connect internal	

      See all of the running queries, and match the UNIX PID:

      select p.spid  -- The UNIX PID
             ,s.sid  ,s.serial#
             ,p.username  as os_user
             ,s.username  ,s.status
             ,p.terminal  ,p.program
        from v$session s  ,v$process p
       where p.addr = s.paddr
       order by s.username ,p.spid ,s.sid ,s.serial# ;

      See the SQL behind the oracle processes:

      select s.username
             ,s.sid  ,s.serial#
        from v$session s, v$sqltext sql
       where sql.address    = s.sql_address
         and sql.hash_value = s.sql_hash_value
       --and upper(s.username) like 'USERNAME%'
       order by s.username ,s.sid ,s.serial# ,sql.piece ;

      To kill a troubled process:

      alter system kill session 'SID,SERIAL#';  --substitute values for SID and SERIAL#

      (See Andrew Piskorski's Oracle notes)

    • Identify a runaway Postgres query. First, logging must be enabled in the database. This imposes a performance penalty and should not be done in normal operation.

      Edit the file postgresql.conf - its location depends on the PostGreSQL installation - and change

      #stats_command_string = false


      stats_command_string = true

      Next, connect to postgres (psql service0 ) and select * from pg_stat_activity;. Typical output should look like:

        datid   |   datname   | procpid | usesysid | usename |  current_query
       64344418 | openacs.org |   14122 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14123 |      101 | nsadmin |
                                                               from acs_mail_lite_queue
                                                               where message_id = '2478608';
       64344418 | openacs.org |   14124 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14137 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14139 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14309 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14311 |      101 | nsadmin | <IDLE>
       64344418 | openacs.org |   14549 |      101 | nsadmin | <IDLE>
      (8 rows)

The first task is to create an appropriate environment for finding out what is going on inside Oracle. Oracle provides Statspack, a package to monitor and save the state of the v$ performance views. These reports help finding severe problems by exposing summary data about the Oracle wait interface, executed queries. You'll find the installation instructions in $ORACLE_HOME/rdbms/admin/spdoc.txt. Follow the instructions carefully and take periodic snapshots, this way you'll be able to look at historical performance data.

Also turn on the timed_statistics in your init.ora file, so that Statspack reports (and all other Oracle reports) are timed, which makes them a lot more meaningful. The overhead of timing data is about 1% per Oracle Support information.

To be able to get a overview of how Oracle executes a particular query, install "autotrace". I usually follow the instructions here http://asktom.oracle.com/~tkyte/article1/autotrace.html.

The Oracle Cost Based optimizer is a piece of software that tries to find the "optimal" execution plan for a given SQL statement. For that it estimates the costs of running a SQL query in a particular way (by default up to 80.000 permutations are being tested in a Oracle 8i). To get an adequate cost estimate, the CBO needs to have adequate statistics. For that Oracle supplies the dbms_stats package.