Why is there a huge difference between the outputs' of top command and the system monitor when both programs were run at the same time. Please find attached is the screen shot showing the huge difference:-

See the Ram usage, in GUI it just shows 1.1G whereas in CLI the usgage climbs upto 3.2G

Is this a bug?

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    Difference in terms of memory usage, processes running?? Choose view > all processes to show them all in System Monitor. See this discussion also askubuntu.com/q/49608 – user76204 Aug 16 '12 at 16:52
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    I think the link I quoted should help as that is about RAM usage. I find htop, which is in the repositories, is easier to use than top and gives clearer results. – user76204 Aug 16 '12 at 17:00

If you are referring to RAM usage, it is because system monitor does not include buffers and caching, just actual Ram used to run applications, top is slightly confusing because it does not obviously show disk usage after buffers / caching. Try using the following command :

free -m

See the image below, and note that the second line of free -m indicates available Ram, whilst the first line indicates actual Ram usage including disk caching.

enter image description here

I would recommend installing and using htop as it is better and simpler than top in my opinion.

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