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"free": what is hilo?

asked 2017-11-11 00:37:27 -0500

toddandmargo gravatar image

Hi All,

In the manual page for "free"

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

Does this tell me what the historic high and history low have been?

If so, how does it know how far back to go? From the last reboot?

Does it mean something else?

Why is my "High" "0B" below?

Many thanks, -T

$ free -lh
              total        used        free      shared  buff/cache available
Mem:            11G        2.9G        1.7G        104M        6.9G   8.1G
Low:            11G        9.8G        1.7G
High:            0B          0B          0B
Swap:          5.9G        504K        5.9G
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answered 2017-11-11 13:57:00 -0500

fcomida gravatar image

In 32 bits systems with more than 4GB using PAE linux kernel divides memory in HIMEM and LOWMEM. For 64 bits system things are somehow simpler for the kernel (basically no high mem). You can read more here: hi-low memory

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That explains it. Thank you!

toddandmargo gravatar imagetoddandmargo ( 2017-11-11 14:28:37 -0500 )edit

Thank you. Thank explains it. I am a 64 bit system.

Follow up questions:

1) why are my "Mem" and my "Low" number different?

2) does "--lohi" tag of "free" give any useful information on a 64 bit system? If so, what?

Many thanks, -T

toddandmargo gravatar imagetoddandmargo ( 2017-11-11 14:56:00 -0500 )edit

Mem: is memory available to user processes (but buff/cache is shown there) Low: memory addressable by the kernel so Low: used is Mem: used (by user space) + Mem: buff/cache In 64 bits systems the virtual address space is so big there's no need for the kernel to do any sort of clever machinations as before. Memory management is a complex topic.

fcomida gravatar imagefcomida ( 2017-11-11 16:21:01 -0500 )edit

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Asked: 2017-11-11 00:37:27 -0500

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Last updated: Nov 11 '17