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In /proc/<pid>/stat I add user cpu, system cpu, child user and child system cpu together for all processes. I take a delta from a previous sample.

Immediately after I sum the user, nice and system CPU from /proc/stat which should be for the entire box. Again I take a delta.

The sum from the processes is almost always slightly greater than that for the overall processor and I can't figure out why.

  • I ran another test where I drove a single process to have 100% CPU on one of the four cores and measured just that process and /proc/stat. The box was otherwise idle. The process registered 100 clock ticks per second normally (expected) and occasionally 101 ticks. When it registered 101, that extra one never accumulated in /proc/stat which kept going up 100 ticks every second. It appears it sometimes misses them. – user331809 Jan 17 at 15:23
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There are a few reasons why that would be the case, the most obvious one is that the per-process accounting uses different logic for counting different things.

Two specific examples:

  1. The system stat has a nice column that counts user time spent in nice tasks (lower priority) - which you didn't specify that you use, so I'm guessing you didn't sum it. In the process this will be counted as standard user time.
  2. iowait time - which also you didn't account for - is likely counted in the process stat as system time. But its a bad idea to also count in iowait time as it is very unreliable account of actual time, as man proc explains:

iowait (since Linux 2.5.41)

(5) Time waiting for I/O to complete. This value is not reliable, for the following reasons:

  1. The CPU will not wait for I/O to complete; iowait is the time that a task is waiting for I/O to complete. When a CPU goes into idle state for outstanding task I/O, another task will be scheduled on this CPU.

  2. On a multi-core CPU, the task waiting for I/O to complete is not running on any CPU, so the iowait of each CPU is difficult to calculate.

  3. The value in this field may decrease in certain conditions.

Lastly, I have to note that I believe this type of accounting is useless as it can never be accurate enough for any serious purpose - system and process counters are processed in different parts of the system for different purposes and will never match to any useful degree.

  • Thank you, I suspect the last paragraph is the answer sadly though it boggles me that we just skip clock ticks sometimes. As for nice, yes I did say that I added it. And iowait, not added but I actually checked all the counters to make sure nothing unexpected was getting incremented and it was not. It would be a bug if this was counted as real CPU in the process. – user331809 Jan 18 at 18:08
  • As far as I understand, iowait (D state in ps) is counted as process system time. – Guss Jan 18 at 19:20

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