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After removing APC (alternative php cache), I noticed that there are less pagination access and less dirty pages but in the same time:

  • High idle CPU
  • Very low CPU load (relatively to the time before removing APC)

If I am not mistaken CPU Idle can be a bad thing. So I checked the "interruptible sleeping process/threads".

while true; do date; ps auxf | awk '{if($8=="D") print $0;}'; sleep 1; done

I noticed that there is no waiting processes.

Do you think that this idle can be caused by something else? Removing APC may be the cause to more disk access, because there are no php opcodes(generated by APC) and saved to the memory.

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I believe that all is perfect...

  • No waits
  • Very low CPU load

CPU Idle is a good thing. This means that CPU is not overloaded with tasks.

I don't know nothing about APC, but as I see it if you don't use cache you will have more access to disk. The cache store in memory would avoid access to disk and would have a better performance.

  • So do you confirm that : - If "interruptible sleeping process/threads" are not the cause of the "Idle CPU", nothing else could cause this, and as a result there is no problem. ? – 4m1nh4j1 Apr 29 '14 at 12:21
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Idle CPU by itself isn't a problem. It would only potentially be a problem if you were noticing a performance issue where it wasn't utilizing enough CPU to handle the load in a timely matter.

Also, most well designed caching follows a pattern where it takes up more CPU right at first but after a while the CPU load dies down and it becomes more of a memory footprint. So it wouldn't cause a spike in CPU time to get rid of it, it would just cause a spike in system-wide iowait% since it would be going to disk.

The fact that you're not seeing any processes in state D after disabling APC probably indicates that the caching was unnecessary. I would check the output of iostat and the iowait% in the top command to see how much disk activity there actually is going on.

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