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I am running an python script on an ec2 instance that inserts rows in an database on another instance. In ec2's monitoring I saw a 100% cpu utilization, whereas top only shows 20% for the python process. What is missing from top? Network overhead?

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What about all the other processes? –  Ladadadada Jun 20 '12 at 13:40
    
At around 0% utilization –  RickyA Jun 20 '12 at 13:48
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up vote 7 down vote accepted

The data exposed by top is often insufficient or misleading in virtualized environments like Amazon EC2 and the reported percentage depends on your instance type and the under­ly­ing proces­sor core utilization (which usually doesn't match the virtualized hardware you are presented with from the hypervisor), amongst other things - what you are seeing is most likely caused by respective CPU steal time as exposed in most related Unix/Linux monitoring tools nowadays - see e.g. columns %steal or st in sar or top:

st -- Steal Time
The amount of CPU 'stolen' from this virtual machine by the hypervisor for other tasks (such as running another virtual machine).

The blog post EC2 monitoring: the case of stolen CPU provides a nice exploration and illustration of this topic:

When the top com­mand dis­plays 40% CPU busy but Cloud­Watch says the server is maxed out at 100% — which side do you take? The answer is sim­ple (Cloud­Watch is cor­rect, top is not) [...]

Please note that this hypervisor metric seems to be (easily) accessible on Unix/Linux systems only, but doesn't seem to be observable on Windows (yet), see my question Is there a Windows equivalent of Unix 'CPU steal time'? for more regarding this problem.

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Thanks for the blogpost. That really makes it clear. It is really good to know this since I am about to roll out Ganglia, and it would be a shame to measure the wrong metrics. Measure %idletime! –  RickyA Jun 20 '12 at 15:01
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Amazon probably check load and no percentage usage from TOP. If you have two processes on cpu, they can be utilized on 20% but you can have load 2.

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I am not sure what you mean with load 2... –  RickyA Jun 20 '12 at 13:54
    
You can have system load at 2. Two processes on CPU but with low utilization. –  MartinSVK Jun 20 '12 at 13:57
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