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I have a CentOS 7 machine and there's a question that bothers me.

When I use lsmem --summary, I get:

Memory block size:       128M
Total online memory:       8G
Total offline memory:      0B

When I use free -h, I get:

              total        used        free      shared  buff/cache   available
Mem:           7.6G        850M        1.7G        361M        5.1G        6.1G
Swap:          7.7G          0B        7.7G

Why is the total memory I get different in the same machine?

What's the difference between these two commands?

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lsmem lists memory blocks and their state; these reflect physical memory and are counted in units of memory blocks, i.e. 128MiB on your system. To do this, lsmem reads information made available by the kernel in /sys/devices/system/memory. On your system, the kernel tracks 64 memory blocks for a total of 8GiB.

free lists memory that’s usable by the system; “total” is the amount of physical memory, minus memory reserved by the system (for the firmware’s purposes mostly) and the kernel’s executable code. free reads this information from /proc/meminfo.

The difference in output is explained by this difference in what is measured. In all cases, free’s total memory will be smaller than lsmem’s total online memory.

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