It's my htop output:

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For example, I'm confused by this ruby script:

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How many physical memory is it using? 3+1+8+51+51? 51? 51+51?

  • Then the RES column is the one you want to look at. – David Schwartz Jun 21 '13 at 9:02
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    yes, I'm looking at it. But which one is the memory usage by my script? Why are there two 51s? Does it consume 102 megabytes? Or only 51 megabytes? In the outer process RES contains the inner process RES , why the 3, 1, 8 are all fewer than 51? – Lai Yu-Hsuan Jun 21 '13 at 9:03
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    There are two 51's because the process forked. It most likely doesn't consume 102 megabytes because the two processes are each using the same 51 megabytes, but you can't easily tell. The other process RES contains the inner process if that memory is resident in the other process as well and not if it doesn't. The RES fields tells you how much physical memory each process is using, but other processes may (or may not) be using that very same physical memory too. – David Schwartz Jun 21 '13 at 9:16
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    If you want to know if the two processes are using the same memory, use pmap -d <process id> – Sirch Jun 21 '13 at 9:28
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    @LaiYu-Hsuan: Yes, you're wrong. The pages aren't copied, they're shared. They're only copied if either process writes to the page, then they have to be unshared. (The is called "copy on write" or "CoW".) – David Schwartz Jun 21 '13 at 10:58

Hide user threads (shift + H) and close the process tree view (F5), then you can sort out the process of your interest by PID and read the RES column (sort by MEM% by pressing shift + M, or F3 to search in cmd line)

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    Shift-H is what I've been looking for for ages. Thanks! – a paid nerd Jan 19 '15 at 23:24

Memory is a hard thing, you cannot calculate used physical memory by just running ps/htop/top. Memory can be shared between processes.

I recommend you to check usage with this script:


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