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For free

-l, --lohi Show detailed low and high memory statistics.

Are the statistics calculated over a period of time? How is the period defined?

Why is the high all zero below?

$ free -l
             total       used       free     shared    buffers     cached
Mem:       8067524    7392888     674636     440536      53880     861080
Low:       8067524    7392888     674636
High:            0          0          0
-/+ buffers/cache:    6477928    1589596
Swap:     15625212    7562728    8062484
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2 Answers 2

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Low and High do not refer to whether there is a lot of usage or not. They represent the way it is organized by the system.

According to Wikipedia:

High Memory is the part of physical memory in a computer which is not directly mapped by the page tables of its operating system kernel.

There is no duration for the free command which simply computes a snapshot of the information available.

Most people, including programmers, do not need to understand it more clearly as it is managed in a much simpler form through system calls and compiler/interpreter operations.

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  • thanks. Does the high memory begin to be used only after the low memory is used up? If yes, what is the advantage of that? If not, how do you explain my example, where high memory is zero? Commented Jan 1, 2017 at 3:51
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    That's a good point I hadn't noticed. The 64 bit kernel architecture reorganized memory management so that it all appears as a single block for simplicity. Commented Jan 1, 2017 at 4:06
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    Do you mean in a 64-bit OS, the high memory is always zero, i.e. there is no high memory? Commented Jan 1, 2017 at 4:14
  • Yes, as I said, they reorganized the memory mapping to make it simpler and more efficient. Commented Jan 1, 2017 at 5:18
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Reason that "high is all zero" is because you're using a 64 bit processor:

Using the -l option, you can see how much memory is used in each memory zone. Example 2-8 and Example 2-9 show the example of free -l output of 32 bit and 64 bit systems. Notice that 64-bit systems no longer use high memory.

The worked example in the reference above compares the output of the free -l command using a 32 bit processor with a 64 bit; the later shows all zeros

REF: https://lenovopress.lenovo.com/redp4285.pdf (page 47. The ref is an IBM Red Book that Lenovo published on their own website)

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