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I am using sar utility for collecting and saving system activity information. After saving the information, I am seeing/reporting the information but the output is not matching. Can some one please help me in understanding the behavior or if I am doing a silly mistake in my understanding. Below are the details.

Step 1: To collect and save system activity

sar -o sa_30_dec_2012 1 > /dev/null 2>&1 &

Step 2: To report first 10 minutes readings with an interval of 1 second

sar 1 10 -f sa_30_dec_2012

Step 3: To report first 10 readings within an interval of 2 seconds

sar 2 5 -f sa_30_dec_2012

Output of Step 2

07:18:34 IST CPU %user %nice %system %iowait %steal %idle 07:18:35 IST all 1.51 0.00 1.51 3.02 0.00 93.97 07:18:36 IST all 1.50 0.00 1.00 0.00 0.00 97.50 07:18:37 IST all 1.02 0.00 0.51 0.00 0.00 98.48 07:18:38 IST all 2.55 0.00 0.51 0.00 0.00 96.94 07:18:39 IST all 3.03 0.00 0.51 0.00 0.00 96.46 07:18:40 IST all 1.49 0.00 1.49 3.48 0.00 93.53 07:18:41 IST all 1.52 0.00 0.51 0.00 0.00 97.97 07:18:42 IST all 1.01 0.00 1.01 0.00 0.00 97.99 07:18:43 IST all 1.53 0.00 0.00 0.00 0.00 98.47 07:18:44 IST all 2.53 0.00 1.52 0.00 0.00 95.96 Average: all 1.77 0.00 0.86 0.66 0.00 96.72

Output of Step 3

07:18:34 IST CPU %user %nice %system %iowait %steal %idle 07:18:36 IST all 1.50 0.00 1.25 1.50 0.00 95.74 07:18:38 IST all 1.78 0.00 0.51 0.00 0.00 97.71 07:18:40 IST all 2.26 0.00 1.00 1.75 0.00 94.99 07:18:42 IST all 1.26 0.00 0.76 0.00 0.00 97.98 07:18:44 IST all 2.03 0.00 0.76 0.00 0.00 97.21 Average: all 1.77 0.00 0.86 0.66 0.00 96.72

If you observe, the readings for timestamp 07:18:36 match but the readings after that do not match (the values are not equal). For example, the values of the following timestamps are not equal:-

Step 2 output

07:18:38 IST all 2.55 0.00 0.51 0.00 0.00 96.94 07:18:40 IST all 1.49 0.00 1.49 3.48 0.00 93.53

Step 3 output

07:18:38 IST all 1.78 0.00 0.51 0.00 0.00 97.71 07:18:40 IST all 2.26 0.00 1.00 1.75 0.00 94.99

Why are the Step 2 and Step 3 outputs not matching?

These commands were executed on Ubuntu 12.04 LTS.

Any help is very much appreciated.

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1 Answer 1

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Since the interval is changed to 2 seconds, it averages the data for those two seconds. If we take two lines from the first output (two 1-second intervals):

07:18:34 IST CPU %user %nice %system %iowait %steal %idle ... 07:18:37 IST all 1.02 0.00 0.51 0.00 0.00 98.48 07:18:38 IST all 2.55 0.00 0.51 0.00 0.00 96.94

%user (column 4) averages to 1.78 -> (1.02 + 2.55)/2=1.78. Same goes for all other columns.

07:18:38 IST all 1.78 0.00 0.51 0.00 0.00 97.71
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  • Oh, that is a nice observation. By sar <interval> <count> command, I was expecting that if I specify sar 2 5, it will display every alternate reading. Thank you very much. Commented Dec 30, 2012 at 18:00

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