Thursday, 21 November 2013

Basic Performance Analysis using AWR data

I've previously covered a number of queries that can be combined together to pull out key statistics from the Oracle AWR data for each snapshot it makes, which you could use to analyse the performance of a system. One example would be to use this to look at how the workload changes during the day, and to identify the period of peak activity. Another use would be to see if there were any periods experiencing significantly higher waits than at other times.

I'll now go through how you can use some of the AWR data to do an overall performance analysis of a system. But before we do that, we need to review some basics on performance tuning, just to clarify some things and avoid potential confusion.

Performance Tuning Basics

I'm of the same school of tuning as people like Cary Millsap and Jeff Holt and others - generally speaking you should be tuning specific performance problems as experienced by individual Oracle sessions, and not tuning the system as a whole. This is because any "improvements" you make to the system that do not benefit the slow sessions are completely wasted and pointless. For instance, the system may be experiencing slow disk I/O's, but it might be that the reported slow interactive response time by some users is due to very high CPU usage resulting from a poor execution plan for the particular SQL statement that session is executing. Making the disk I/O's happen quicker might "improve" things for the system as a whole, but it will have no impact at all on the CPU bound SQL statement that is the cause of slow response times on an end user's screen.

That said, there are times where system level tuning is appropriate or when you can only do system level tuning, and that is where AWR can help as it gathers system wide activity data. For session level tuning you need to look elsewhere - such as ASH (Active Session History) or using SQL Trace.

There are some big assumptions being made when we do system level tuning:
  • All sessions are executing the same or similar SQL statements
    • So a system wide "average" view across all sessions is valid
  • Any "poor performance" on the system impacts all sessions equally
    • So even one rogue SQL statement can slow down all other SQL statements being executed from other sessions
  • The system is under a high enough load that performance problems are due to interactions between the SQL statements
    • It is assumed that poorly performing individual SQL statements would have been identified earlier under initial testing
Providing these assumptions or similar are true, then you can do system level tuning.

System Level Tuning Measurements

A computer system exists to do work, and its performance is really about how long it takes to to that work and which resources it uses while doing it. Performance is an issue when a task takes too long to complete.

We need to measure both sides of this – the input work requests, and the output usage of the system resources that results. We can only do performance tuning properly when we know both of these, as this lets us see the impact of any changes we make - does resource usage go down or not for the same input workload?

Measuring the input workload is best done at business or application level transactions. And this is best achieved by instrumenting the application software itself to record both the type of transaction it is performing and the elapsed time it takes when submitted. However, very few applications if any are instrumented this way.

This leaves a database level measurement as the only viable and consistently available way of recording the input workload, and in my view the best available measurement is the number of SQL statements executed per second. While this is very imperfect as a measure of the input workload, it is the nearest we are going to get to it, and all input application transactions result in SQL statements being executed. So it does correlate with input workload.

For a measurement of workload on the system - the output resulting from the execution of the SQL statements - the best measurement is probably Average Active Sessions. AAS is simply the Database Time ("DB Time" from System Time Model) divided by elapsed time for the period over which Database Time was measured. The Database Time value is a measurement of the amount of elapsed time sessions were active executing SQL statements, and includes both active execution time (running on a CPU) and waiting time (for disk I/O or any other wait event). AAS indicates how many sessions were active at the same time executing SQL statements i.e. the level of concurrency on your system.

AAS correlates with the number of CPU's on your system - if all SQL statements executed with no waiting at all they would only use up CPU resource and your transaction rate would be limited by the number of CPU's in the system. If AAS is less than the number of CPU's in your system then you are not reaching capacity. If AAS is close to or more than the number of CPU's then you have an overloaded system and waiting of some form or another is occurring.

To make real use of the AAS value we also want to know the percentage of time spent waiting during each period i.e. total wait time as a percentage of database active time (DB time again). The Percentage Wait (which I label Wait%) indicates the efficiency of the SQL statement execution - if this waiting was eliminated then that wait time would be removed from the elapsed time of each SQL statement executed. This is only relevant when AAS is high enough. When the level of concurrency is very low you can get various anomalies, and there will always be some waiting somewhere. Again remember the assumptions I stated earlier - the system must be under a high enough load to experience performance problems due to the combined workload itself.

That's it - 3 simple measurements should be enough to tell you whether your system is overloaded or not, and whether it is performing efficiently. These can be easily extracted from AWR (see later in this post), and can graphed in your favourite spreadsheet tool to spot trends and anomalies.

Using these measurements

If you have a system wide bottleneck / capacity limit then checking on the total system resource usage during a period of time will hopefully identify any such bottlenecks. As stated, if AAS is high enough (I would say more than half of the number of CPU's), and the Wait% is also high (say 50% or more), then you have poor performance and it can be improved.

At this point I would use the expanded version of the AWR query that I have built up over the previous posts to get all of the data out of AWR for each snapshot, and again into a spreadsheet or something else. Within this AWR data is a breakdown of the Oracle Wait Time by Wait Class, which tells you which "type" of wait was consuming most of the wait time. This can either primarily be Disk (I/O), Network, Cluster, Commit (Redo), or Concurrency (locking).

Knowing the wait class you can then query AWR for SQL statement execution and sum them over the wait class identified earlier, and sort by the wait class time summed. I showed how to do this in my previous post, summing the number of disk reads per SQL statement executed in each snapshot. This way you easily get to see the top SQL by that wait type, and see how much time they spent waiting relative to the total wait time.

Equally you could just do all SQL statements by their total wait time, to see those that experienced the worst waits whatever type of wait they were. Wait time for a SQL statement would be calculated as its Elapsed time minus the CPU time.

AWR System Performance Query

Here is a query that gives you just the 3 key measurements mentioned before for a system from the AWR data. It reports all the measurements for yesterday - modify the date range constraint to report on different periods.
set feedback off
set verify off
set linesize 1000
set trimout on
set trimspool on
col snap_time           format a15      heading 'SNAP TIME'
col user_calls_sec      format 9,990    heading 'UCALL/S'
col aas                 format 90.0     heading 'AAS'
col wait_pct            format 990.0    heading 'WAIT%'
snaps as 
(select snap_id
      , dbid
      , end_snap_time
      , snap_interval
      , extract (second from snap_interval) 
       + (extract (minute from snap_interval) 
          + (extract (hour from snap_interval)
             + (extract (day from snap_interval) * 24)
             ) * 60
          ) * 60 snap_duration
  from (select csnaps.snap_id
             , csnaps.dbid
             , min (csnaps.end_interval_time) end_snap_time
             , min (csnaps.end_interval_time) - min (csnaps.begin_interval_time) snap_interval
          from dba_hist_snapshot csnaps
         group by csnaps.snap_id, csnaps.dbid
) -- snaps
, systimes as 
-- One row per Database Time Model with change in value between snapshots
(select systime.snap_id
      , systime.dbid
      , systime.stat_name
      , sum (systime.value - psystime.value) value
         from dba_hist_sys_time_model systime, dba_hist_sys_time_model psystime
        where systime.snap_id = psystime.snap_id + 1
          and systime.dbid = psystime.dbid
          and systime.instance_number = psystime.instance_number
          and systime.stat_id = psystime.stat_id
-- Assume if stat_id the same so is the stat_name
        group by systime.snap_id, systime.dbid, systime.stat_name
) -- systimes
, sysstats as 
-- One row per System Statistic with change in value between snapshots
(select sysstat.snap_id
      , sysstat.dbid
      , sysstat.stat_name
      , sum (sysstat.value - psysstat.value) value
         from dba_hist_sysstat sysstat, dba_hist_sysstat psysstat
        where sysstat.snap_id = psysstat.snap_id + 1
          and sysstat.dbid = psysstat.dbid
          and sysstat.instance_number = psysstat.instance_number
          and sysstat.stat_id = psysstat.stat_id
-- Assume if stat_id the same so is the stat_name
        group by sysstat.snap_id, sysstat.dbid, sysstat.stat_name
) -- sysstats
, syswaits as 
-- One row for total wait time, plus break down into major wait classes, and events
(select sysevent.snap_id
      , sysevent.dbid
      , sum (sysevent.time_waited_micro - psysevent.time_waited_micro) time_waited_micro
      , sum (sysevent.total_waits - psysevent.total_waits) wait_count
   from dba_hist_system_event sysevent, dba_hist_system_event psysevent
  where sysevent.snap_id = psysevent.snap_id + 1
    and sysevent.dbid = psysevent.dbid
    and sysevent.instance_number = psysevent.instance_number
    and sysevent.event_id = psysevent.event_id
    and sysevent.wait_class != 'Idle'  -- Ignore Idle wait events
  group by sysevent.snap_id
      , sysevent.dbid
) -- syswaits
select to_char (snaps.end_snap_time, 'DD/MM/YY HH24:MI') snap_time
     , (user_calls_st.value / snaps.snap_duration)              user_calls_sec
     , (dbtime.value / 1000000)       / snaps.snap_duration     aas
     , (100 * syswaits.time_waited_micro / dbtime.value)        wait_pct
  from snaps
     join (select * from systimes where stat_name = 'DB time') dbtime
       on snaps.snap_id = dbtime.snap_id and snaps.dbid = dbtime.dbid
     join syswaits
       on snaps.snap_id = syswaits.snap_id and snaps.dbid = syswaits.dbid
     join (select * from sysstats where stat_name = 'user calls') user_calls_st
       on snaps.snap_id = user_calls_st.snap_id and snaps.dbid = user_calls_st.dbid
 where snaps.end_snap_time between 
       (trunc (sysdate) - 1) and (trunc (sysdate))
 order by snaps.end_snap_time
set feedback on
set lines 80

1 comment:

OracleMan Consulting said...

thanks for all this hard work
it is a great help to us

i get some odd results on my very lightly loaded DB 12c 12101 DB running on Win7-64b

User %
Snap Time Calls AAS Wait
--------------- ------ ----- -------
03/12/13 00:00 0 0.0 1240.0
03/12/13 01:00 0 0.0 1019.6
03/12/13 08:41 0 0.0 #######
03/12/13 10:00 0 0.0 623.3
03/12/13 17:45 0 0.0 3256.0
03/12/13 19:00 0 0.0 2438.0
03/12/13 20:00 0 0.0 303.5
03/12/13 21:00 0 0.0 1103.3
03/12/13 22:00 0 0.0 120.3
03/12/13 23:00 0 0.2 28.7

just fyi