Definition of "%CPU field" of processes from top's man page:
The task's share of the elapsed CPU time since the last screen update, expressed as a percentage of total CPU time.
Definition "CPU time" from Wikipedia
CPU time (or process time) is the amount of time for which a central
processing unit (CPU) was used for processing instructions of a
computer program or operating system.
Questions
- Does top CPU include the kernel function execution times related to the socket send/receive API?
- I'm using epoll which is blocking mode and does top CPU consider the blocked time as busy time? (Likely no but want to confirm)
- (Not directly related) If I use blocking socket, does top CPU consider the blocked time as busy time?
Answer to questions 2 and 3
Blocking time is not counted into %CPU as "CPU time" defined as time spent executing instructions on a CPU.
Answer to question 1
According to Wikipedia CPU time is the sum of both "user CPU time" and "system CPU time".
We would expect both kernel mode execution time on behalf of process and user mode execution of process to count up into the calculation of %CPU. In order to test if that is the case you can run a system time usage heavy process and check the %CPU of that process via top, which I have seen in a blog post.
$ dd if=/dev/zero of=/dev/null bs=65536
Cpu(s): 0.2%us, 12.8%sy, 0.0%ni, 87.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 16330524k total, 7869052k used, 8461472k free, 464240k buffers
Swap: 18563064k total, 0k used, 18563064k free, 6716724k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
29937 user 20 0 102m 724 564 R 99.7 0.0 0:13.40 dd
In the same blog post, the writer inspects source codes of implementations of top utility program for Linux and OpenBSD, I would suggest you to take a look at there too.
