Need to understand how system time is computed by mpstat

Hi!
i've got difficulties analyzing system utilization. Linux utilities (vmstat, iosat, mpstat) report system utilization on level of 3-6%, but when it's 6% system slows down in a visible way. Generally speaking spikes up to 6% are visible in application response times. It's linux 2.16.18 with Xeon X5677 (8 cores+HT). Linux reports 16 logical processors. It's interesting that sum of %sys for each CPU is always <100%.
How %sys is computed by mpstat? Does it mean that 6% is a this system boundary?
Thanks,
Ryszard

Hi Dude!
I agree. You are right in each point.
But my exact question is about %sys calculation by mpstat. I was probably not very precise with term of "system boundary". My second question was about maximum CPU time that kernel may use. I've discovered very interesting thing related to the subject: sum of %sys column for each CPU (as reported by mpstat) is always below 100%. It was measured during quite heavy load test of the system.
Take a look at below mpstat output:
CPU     %user     %nice     %sys     %iowait     %irq     %soft     %steal     %idle     intr/s
all     48.88     0     4.66     0.04     0.05     0.44     0     45.93     8904
0     24     0     1     0     0     0     0     75     1000.2
1     28.8     0     7.8     0.2     0     0     0     63.2     0.4
2     25.95     0     13.77     0.2     0     0.2     0     59.88     0.2
3     34.4     0     1     0     0     0     0     64.6     0
4     56.8     0     3.2     0     0     1     0     39     800.2
5     53.2     0     2.2     0     0.2     0.6     0     43.8     571.4
6     51.6     0     1.8     0     0.2     0.4     0     46     211.2
7     35.2     0     3.6     0     0     0.2     0     61     0
8     82.24     0     16.77     0     0     0.2     0     0.8     0
9     30.2     0     11.8     0     0     0     0     58     0.4
10     65.2     0     0.4     0     0     0     0     34.4     0.2
11     35.53     0     0.8     0     0     0.2     0     63.47     0.4
12     65.73     0     3.41     0     0     0.6     0     30.26     539.6
13     60.8     0     2.6     0     0     1.2     0     35.4     1228.4
14     58.2     0     2.2     0     0     1.4     0     38.2     1581.2
15     74.4     0     2.2     0     0.2     1     0     22.2     2970
Let's take transposed %sys column:
%sys     4.66     1     7.8     13.77     1     3.2     2.2     1.8     3.6     16.77     11.8     0.4     0.8     3.41     2.6     2.2     2.2
all=4.66%
sum 0.15 = 1+7.8+13.77+...+2.2+2.2 = 74.55%
My bet is that 74.55% is more realistic than 4.66%. How this 4.66% is calculated? What does it mean? And why sum(%sys, 0..15) is always below 100%?
Thanks,
Ryszard

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