Ubuntu – Why does memory usage shown in System Monitor differ from that in ps_mem

memory usagesystem-monitor

I came to Ubuntu 16.04 from Mint 17.3 (and Ubuntu Mate 15.10), and I'm shocked with the memory usage of Ubuntu 16.04.

With a clean install, 16.04 used about 650+ Mb RAM, and after 1 or 2 days, after when I've installed most need apps for regular usage, 16.04 uses ~ 1GB at idle, and climbs to 1.5GB only with 4 or 5 chrome tabs.

Then I use a terminal tool to check for memory ( I use ps_mem here) – and the memory here is significantly lower than which showed in System Monitor:

So, my questions are:

Which tool shows the right memory usage: System Monitor or ps_mem?
Why is Ubuntu 16.04 using so much memory?

Best Answer

You can run free -m to check memory usage:

Pc_mem show the actual memory usage(used col) but gnome system monitor shows the usage of (buffer/cache - used) according to this answer.
But i find that equation is changed with system monitor 3.18.2 to

(1-(used/total))*100 or (1-(used/available))*100.

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Ubuntu – Memory is taken up by “invisible” threads

By default, freeshows also memory used by cache and buffers

In the free -m output, as already said in the comments, you have to look to the line

-/+ buffers/cache:       1106       2842

to see exactly the quantity of memory used by applications, kernel and modules. In this case, you are using 1106 MB, which is almost 28% of the total memory. The rest is taken by the usual buffers and cache that helps the kernel to access files quicker.

The ps output shows the memory used by process

In the ps output I see that you still have the whole graphical desktop running, with a lot of components running. They take about 13% of the memory.

And the modules...

The delta between the ps calculation and the free is constituted of the memory taken by the kernel and the modules loaded. As the size column of the lsmod output only displays the size of the module on the filesystem (so the syse of the .ko file from which this module is loaded) it is not reliable to measure the memory used by them. A more reliable approach to measure the memory used by modules is to use the slabtop utilities (installed from the procps package.

Also see the content of /proc/meminfo, it gives the total usage of the Slab memory, the memory used to efficiently allocate memory to kernel objects. (See Wikipedia for more details on Slab).

In conclusion

Memory usage is a complex domain, and you will need more than one tool to see in details which piece of memory is used by what. free, ps, slabtop, /proc/meminfo content, top, vmstat, htop, ... all they can bring a piece of information helpfull to build the full picture on your particular memory usage.

Best Answer

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