Loads of fun with DBA_HIST_OSSTAT

I saw a load of 44 on a node of our production Exadata and it worried me.  The AWR report looks like this:

Host CPU
            Load Average
 CPUs     Begin       End     %User   %System      %WIO     %Idle
----- --------- --------- --------- --------- --------- ---------
   16     10.66     44.73      68.3       4.3       0.0      26.8

So, why is the load average 44 and yet the CPU is 26% idle?

I started looking at ASH data and found samples with 128 processes active on the CPU:

     select
  2  sample_time,count(*)
  3  from DBA_HIST_ACTIVE_SESS_HISTORY a
  4  where
  5  session_state='ON CPU' and
  6  instance_number=3 and
  7  sample_time
  8  between
  9  to_date('05-MAR-2015 01:00:00','DD-MON-YYYY HH24:MI:SS')
 10  and
 11  to_date('05-MAR-2015 02:00:00','DD-MON-YYYY HH24:MI:SS')
 12  group by sample_time
 13  order by sample_time;

SAMPLE_TIME                    COUNT(*)
---------------------------- ----------
05-MAR-15 01.35.31.451 AM           128

... lines removed for brevity

Then I dumped out the ASH data for one sample and found all the sessions on the CPU were running the same parallel query:

select /*+  parallel(t,128) parallel_index(t,128) dbms_stats ...

So, for some reason we are gathering stats on a table with a degree of 128 and that spikes the load.  But, why does the CPU idle percentage sit at 26.8% when the load starts at 10.66 and ends at 44.73?  Best I can tell load in DBA_HIST_OSSTAT is a point measurement of load.  It isn’t an average over a long period.  The 11.2 manual describes load in v$osstat in this way:

Current number of processes that are either running or in the ready state, waiting to be selected by the operating-system scheduler to run. On many platforms, this statistic reflects the average load over the past minute.

So, load could spike at the end of an hour-long AWR report interval and still CPU could average 26% idle for the entire hour?  So it seems.

– Bobby

About Bobby

I live in Chandler, Arizona with my wife and three daughters. I work for US Foods, the second largest food distribution company in the United States. I have worked in the Information Technology field since 1989. I have a passion for Oracle database performance tuning because I enjoy challenging technical problems that require an understanding of computer science. I enjoy communicating with people about my work.

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