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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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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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