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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

up vote 1 down vote accepted

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
share|improve this answer
    
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. –  Anuroop Dec 30 '12 at 18:00

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