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martedì, giugno 20, 2017

On DB Time, DB CPU and Utilization


The "Load Section" of an AWR Report has two interesting lines
  • DB Time(s)
  • DB CPU(s)

Load Profile

This "Load Profile" is an AWR snapshot of 1h (60m/3600s). Look at the definitions of those metrics.

  • DB Time is a time model statistic that is the sum of Oracle process CPU consumption and non-idle wait time
  • DB CPU is Oracle server/foreground/shadow process CPU consumption  

(the above definitions are taken from an Orapub's Blog). Both of them are divided by "Elapsed Time" so they are an average on the snapshot interval.

Because of the definition of "DB Time", the "DB Time(s)" represent the Average Active Session on just one second.

"DB CPU(s)" tell us how many CPU are used in one second. You can compare this value with the number of CPU on your system, in order to understand if you are CPU bound.

In my example (from production system), the database is doing.....nothing: 1 active session each 2 seconds and a half (0.4 in 1s + 0,4 in 1s + 0.2 in 1/2s).

Also each 1s only 0.3 CPUs are used on 120 CPUs of the system (see next picture).


How you can see from this Orapub post, the utilization is calculated as
  • Utilization = Requirement / Capacity (a)
(a) capacity: the maximum amount that can be contained or accommodated

From our point of view

  • The requirement is the time used 
  • The capacity is the time available

so the previous formula became

  • Utilization = time used / time available

You can calculate the utilization using two different methods. These methods are taken from Craig Shallahamer's Video Lessons.
  1. Core method
  2. Busy:Idle method

In order to explain these methods, I post another picture from AWR report

Operating System Statistics

Core method

With this method, requirement and capacity are:

Requirement =>  (v$osstat.)BUSY_TIME
Capacity => #CPU * Elapsed (Time)

In AWR report, the "BUSY_TIME" came from V$OSSTAT, while "Elapsed" is the "Elapsed Time" of snapshot and #CPU is the number of CPU on the system (the NUM_CPU column of V$OSSTAT)

Remember to normalize the numbers

Utilization = BUSY_TIME / Elapsed = (3336707/100) / (120*60*60)
Utilization = 33367.07 / 432000
Utilization = 0,077
Utilization = 7,7%

Because BUSY_TIME is in hundredths of seconds, I had divided the BUSY_TIME by 100. Also, the "Elapsed Time" in AWR report is in minutes, I multiplicated it for 60.

Busy:Idle method

With this method, requirement and capacity are:

Requirement =>  (v$osstat.)BUSY_TIME
Capacity => (v$osstat.)BUSY_TIME + (v$osstat.)IDLE_TIME

In this case, all information comes from the "Operating System Statistics" section of AWR Report.

Utilization = 3336707 / (3336707 + 39244501)
Utilization = 3336707 / 42581208
Utilization = 0,078
Utilization = 7,8%

In this case, all measurement was in hundredths of seconds so no further operations are needed.

The low utilization is a confirm of the very low Average Active Section shown in firs part of this post.



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