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I found this page because I was pretty confused about these issues, and still am...

Question for the crowd: In this site the example given says that in reality there are 869MB of used RAM. I'm comparing this with my VPS values, and would like to know if this is the sum of some column in top. Is it? It looks like it's pretty close to the sum of the SHR column. Does this make sense? Thanks in advance.

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You can't really do sums of top columns, because some memory is shared and you'll end up double-counting it.

And you can't just subtract the shared memory numbers, because different sets of pages are shared between different sets of processes, and top doesn't give enough information to figure out what's actually happening where.

Running the pmaps tool on all pids and summing the Pss number is perhaps the closest you can get to the actual memory use.

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