Wait Metrics vs Wait Events

Here is a quick table comparison of  different types of metrics views

The first line of the table is the classic wait event and statistic views. The following lines are the metric views.  The metric views were introduced in Oracle 10g.

Why Metrics are good

Metric views compute deltas and rates  which hugely simplifying the ability to answer simple questions like “what is the I/O rate on my databases right now?” This question, before 10g, was surprisingly tedious to answer. To answer the question one would have to query v$sysstat  for example:

Select value from v$sysstat where name=’physical reads’;

but querying v$sysstat just once fails to answer the question but instead answers the question “How much I/O has been done since the database was started?” To answer the original question one would have to query v$sysstat twice and take the delta between the two values:

• Take value at time A
• Take value at time B
• Delta = (B-A)
• and/or get Rate = (B-A)/elapsed time

Getting these deltas and rates could be a pesky task especially working with a customer over the phone. Then 10g Oracle introduced metric tables which answer the questions in one single query .

Using Metrics with Waits

The metric views  apply to wait events as well as  statistics. In a future posting we will go over statistics. In this posting we will go over wait events.  The number of views available to analyze wait events can be confusing. The point of this post is to clarify what the different views available are and how they can be used.

The wait event views are   (at system level)

• V$SYSTEM_EVENT – wait events cumulative since startup
• V$EVENTMETRIC – wait event deltas last 60 seconds
• DBA_HIST_SYSTEM_EVENT – wait events by snapshot (hour) for last week, cumulative since startup

The wait events are rolled up in to groups called wait classes. For wait class we have the following views:

• V$SYSTEM_WAIT_CLASS – cumulative since start up
• V$WAITCLASSMETRIC – last 60 seconds deltas
• V$WAITCLASSMETRIC_HISTORY – 60 seconds deltas for last hour

Note: DBA_HIST_WAITCLASSMETRIC_HISTORY is used for alerts and or baselines not everyday values.

Use Wait Event Metrics for Latency

I use wait event metrics for I/O latencies.

It may be surprising that I don’t mention using waits to identify bottlenecks and load on the system. For bottlenecks and load on the system the data in V$ACTIVE_SESSION_HISTORY (ASH) is probably better for a few reasons. One the data in ASH is mult-dimesional so it can be grouped by SQL and Session Also CPU information is derivable from ASH. CPU information is not in the event/waitclass views but is in ASH along with the waits.

The second part, the  latencies, specifically I/O latencies,  are only available in the wait event and waitclass views (and the filestat views on a per file basis)

User I/O latency with WAIT CLASS

One  use  of wait  metrics is determining the average read I/O  for all the various kinds of read I/O and read sizes:

select 10*time_waited/nullif(wait_count,0) avg_io_ms -- convert centi-seconds to milliseconds
from   v$waitclassmetric  m
       where wait_class_id= 1740759767 --  User I/O
/
 AVG_IO_MS
----------
     8.916

One issue with V$WAITCLASSMETRIC is that the field WAIT_CLASS name is not in the view, so we either have to use the WAIT_CLASS_ID (the hash of the name)  as above or join to V$SYSTEM_WAIT_CLASS as below

select
        10*m.time_waited/nullif(m.wait_count,0) avgms -- convert centisecs to ms
from   v$waitclassmetric  m,
       v$system_wait_class n
where m.wait_class_id=n.wait_class_id
  and n.wait_class='User I/O'
/
 AVG_IO_MS
----------
     8.916

Another issue is that the documentation for 11gR2 says that the TIME_WAITED is microseconds but in my tests it’s actually centisecs

 desc V$SYSTEM_WAIT_CLASS
Name                                        Type
-----------------------------------------  ----------------------------
WAIT_CLASS_ID                                NUMBER
WAIT_CLASS#                                  NUMBER
WAIT_CLASS                                   VARCHAR2(64)
TOTAL_WAITS                                  NUMBER
TIME_WAITED                                  NUMBER  - centi-seconds

You can get a list of all the WAIT_CLASS names in the view V$SYSTEM_WAIT_CLASS.  

 select wait_class_id , wait_class from V$SYSTEM_WAIT_CLASS ;

WAIT_CLASS_ID WAIT_CLASS
------------- ----------------------------------------------------------------
   1893977003 Other
   4217450380 Application
   3290255840 Configuration
   4166625743 Administrative
   3875070507 Concurrency
   3386400367 Commit
   2723168908 Idle
   2000153315 Network
   1740759767 User I/O
   4108307767 System I/O

Latencies for specific I/O  Wait Events

For specific I/O latencies there are two choices – v$eventmetric and v$system_event. With v$system_event it requires  running multiple queries and taking the deltas but the deltas are are already calculated in v$eventmetric. Here is an example of getting I/O latencies from v$eventmetric

Latencies in the past minute

col name for a25
select m.intsize_csec,
       n.name ,
       round(m.time_waited,3) time_waited,
       m.wait_count,
       round(10*m.time_waited/nullif(m.wait_count,0),3) avgms
from v$eventmetric m,
     v$event_name n
where m.event_id=n.event_id
  and n.name in (
                  'db file sequential read',
                  'db file scattered read',
                  'direct path read',
                  'direct path read temp',
                  'direct path write',
                  'direct path write temp',
                  'log file sync',
                  'log file parallel write'
)
/
INTSIZE_CSEC NAME                      TIME_WAITED WAIT_COUNT      AVGMS
------------ ------------------------- ----------- ---------- ----------
        6017 log file parallel write         2.538          4      6.345
        6017 log file sync                   2.329          1     23.287
        6017 db file sequential read             0          0
        6017 db file scattered read              0          0
        6017 direct path read                    0          0
        6017 direct path read temp               0          0
        6017 direct path write                   0          0
        6017 direct path write temp              0          0

Latencies averaged over each hour for specific I/O Wait Events

select
       btime,
       round((time_ms_end-time_ms_beg)/nullif(count_end-count_beg,0),3) avg_ms
from (
select
       to_char(s.BEGIN_INTERVAL_TIME,'DD-MON-YY HH24:MI')  btime,
       total_waits count_end,
       time_waited_micro/1000 time_ms_end,
       Lag (e.time_waited_micro/1000)
              OVER( PARTITION BY e.event_name ORDER BY s.snap_id) time_ms_beg,
       Lag (e.total_waits)
              OVER( PARTITION BY e.event_name ORDER BY s.snap_id) count_beg
from
       DBA_HIST_SYSTEM_EVENT e,
       DBA_HIST_SNAPSHOT s
where
         s.snap_id=e.snap_id
   and e.event_name like '%&1%'
order by begin_interval_time
)
order by btime
/
BTIME               AVG_MS
--------------- ----------
20-JUL-11 06:00      5.854
20-JUL-11 07:00      4.116
20-JUL-11 08:00     21.158
20-JUL-11 09:02      5.591
20-JUL-11 10:00      4.116
20-JUL-11 11:00      6.248
20-JUL-11 12:00     23.634
20-JUL-11 13:00     22.529
20-JUL-11 14:00      21.62
20-JUL-11 15:00     18.038
20-JUL-11 16:00     23.127

What are the sizes of the I/O requests?

One issue with looking at I/O latencies is determining the I/O sizes.  It would be awesome if there was a view with I/O counts, sizes and latencies in one place. ASH does have this information  but ASH data is weighted to the longer latencies and sizes and not the average. The average sizes have to be gotten from system statistics. The I/O sizes for ‘db file sequential read’ are single block reads so are single value that can be determined , but the other read events can vary in size. To get a general idea of I/O sizes one could just average across all I/O using the system statistics

Average I/O Size (across all I/O waits)

select
          sum(decode(metric_name,'Physical Reads Per Sec',value,0))*max(intsize_csec)/100  blocks_read,
          nullif(sum(decode(metric_name,'Physical Read IO Requests Per Sec',value,0)),0)*max(intsize_csec)/100  reads,
            sum(decode(metric_name,'Physical Reads Per Sec',value,0))/
          nullif(sum(decode(metric_name,'Physical Read IO Requests Per Sec',value,0)),0) avg_blocks_read
from v$sysmetric
where group_id = 2  -- 60 second deltas only (not the 15 second deltas);

BLOCKS_READ      READS AVG_BLOCKS_READ
----------- ---------- ---------------
       4798       4798               1

Load and Bottlenecks

The good thing about wait classes is that they simplify dealing with 1000s of wait events and group them into just a few wait classes. We can get a quick view of load on the system with

select n.wait_class, round(m.time_waited/m.INTSIZE_CSEC,3) AAS
from   v$waitclassmetric  m,
       v$system_wait_class n
where m.wait_class_id=n.wait_class_id
and n.wait_class != 'Idle'
;
WAIT_CLASS             AAS
--------------- ----------
Other                    0
Application              0
Configuration            0
Administrative           0
Concurrency              0
Commit                   0
Network                  0
User I/O              .149
System I/O            .002

but the big drawback with wait event and/or wait class views is that they lack information on CPU load. CPU load can be found in the system statistics but it’s just easier to do it all in one query using v$active_session_history. Here is a query using ASH to calculate AAS load on the database over the last 60 seconds:

select
            round(count(*)/secs.var,3)     AAS,
            decode(session_state,'ON CPU','CPU',wait_class)  wait_class
       from v$active_session_history ash,
            (select 60 var from dual)  secs
       where
            SAMPLE_TIME > sysdate - (secs.var/(24*60*60)) and
            SESSION_TYPE = 'FOREGROUND'
       group by decode(session_state,'ON CPU','CPU',wait_class) , secs.var
/
      AAS WAIT_CLASS
---------- ---------------
      .016 Concurrency
      .001 Network
         0 Other
      .083 Configuration
      .001 Administrative
      .034 CPU
         0 System I/O
      .001 Commit
      .054 Application
         0 User I/O

Though with v$sysmetric it’s pretty easy to join to v$waitclassmetric

  select n.wait_class, round(m.time_waited/m.INTSIZE_CSEC,3) AAS
   from   v$waitclassmetric  m,
          v$system_wait_class n
   where m.wait_class_id=n.wait_class_id
   and n.wait_class != 'Idle'
  union
   select  'CPU', round(value/100,3) AAS from v$sysmetric where metric_name='CPU Usage Per Sec' and group_id=2 ;
WAIT_CLASS                                                              AAS
---------------------------------------------------------------- ----------
Administrative                                                            0
Application                                                            .009
CPU                                                                   1.696
Commit                                                                    0
Concurrency                                                            .001
Configuration                                                             0
Network                                                                .002
Other                                                                     0
System I/O                                                                0
User I/O                                                                  0

and adding v$sysmetric into the query allows me to do something I’ve always wanted which is to include the OS CPU in AAS

 select n.wait_class, round(m.time_waited/m.INTSIZE_CSEC,3) AAS
   from   v$waitclassmetric  m,
          v$system_wait_class n
   where m.wait_class_id=n.wait_class_id
   and n.wait_class != 'Idle'
  union
   select  'CPU', round(value/100,3) AAS from v$sysmetric where metric_name='CPU Usage Per Sec' and group_id=2
  union
   select 'CPU_OS', round((prcnt.busy*parameter.cpu_count)/100,3) - aas.cpu
  from
  ( select value busy from v$sysmetric where metric_name='Host CPU Utilization (%)' and group_id=2 ) prcnt,
  ( select value cpu_count from v$parameter where name='cpu_count' )  parameter,
  ( select  'CPU', round(value/100,3) cpu from v$sysmetric where metric_name='CPU Usage Per Sec' and group_id=2) aas
;
WAIT_CLASS                                                              AAS
---------------------------------------------------------------- ----------
Administrative                                                            0
Application                                                               0
CPU                                                                    .009
CPU_OS                                                                 .024
Commit                                                                    0
Concurrency                                                               0
Configuration                                                             0
Network                                                                .002
Other                                                                     0
System I/O                                                                0
User I/O                                                                  0

One huge loss over using ASH is the loss of the information users waiting for CPU but not running on CPU.

For further reading see

• oramon.sh – shell script to display I/O latency from v$system_event
• Oracle CPU time – how to see Oracle’s usage and demand of CPU