Why does operf seems stop working after several samplings?

Hi All,

I use oprofile to try profile the database server's (MySQL) performance.

I start with this command before I start my test:
sudo nohup operf --pid `pgrep mysqld` &

All the tests will be run for 150s.

When the CPU utilization is low, the sample number seems reasonable.
However, when the CPU utilization is very high, more than 100%, the sample
number decreases dramatically.

The total sample numbers are 42417612 samples and 1226 samples, for 70% CPU
utilization and 100% CPU utilization.

*I find when I use opreport to get the counts in the course of the testing,
the number of counts never changed. I think the oprofile stops working
after several samplings.*

*I first try different reset counts. I find when this value is small, I can
get more samplings.*

*I also try perf. It works well. The sampling number becomes larger as time
goes on.*

Any advice?

Thanks,
TIm
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My machine information:
CPU: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz
Memory: 64GB

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