Free RAM in Linux .

Free RAM in Linux .

[Neel chakraborty]

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I was studying the Linux Kernel Source , and came across this doubt .

Does Linux use all of the physical memory (RAM) I have ? In both the
outputs of /proc/meminfo and free -h , shows that 1.4 gigs is used and 1.6
gigs is cached , and the rest is "free" out of 32 Gigs . The available ram
is the cached ram + reclaimable ram + free ram , right ?

I went through the source code of /fs/proc/meminfo.c and it's polling ram
usage from the sysinfo struct , so I browsed through
/linux/include/uapi/Linux/sysinfo.h , over there freeram is available
memory size .

But again in meminfo.c , we are seeing MemFree as i.freeram ( here i is
assigned to the structure of type sysinfo ) .

so is free ram = available memory ? But then , I have free ram of 27 Gi ,
and available memory of 28 Gi , why is that ?

And also , does the linux kernel use the amount of ram which is not used by
applications as paging cache ? Say I have 4 gigs of ram , and Firefox is
using 1 gig of it , the rest of RAM is used for disk/page caching or is it
just unused and left there ?

Thank you

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Re: Free RAM in Linux .

[Valdis Klētnieks]

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On Tue, 17 Dec 2019 10:39:08 +0530, Neel chakraborty said:

> Does Linux use all of the physical memory (RAM) I have ? In both the
> outputs of /proc/meminfo and free -h , shows that 1.4 gigs is used and 1.6
> gigs is cached , and the rest is "free" out of 32 Gigs . The available ram
> is the cached ram + reclaimable ram + free ram , right ?

That probably means that the processes you have running use a total
of 1.4G of ram, and you've referenced 1.6G of files on disk.

The rest is free because you've not done anything to give the system even
a hint of what to do with the other 27G of RAM.

If you reference a whole bunch of files (find /usr -type f | xargs cat) > /dev/null
or other similar), you'll see more gigs used for cache.

If you run a few large processes, like a Chrome with 90 tabs open, you'll
see the other number go up.

> And also , does the linux kernel use the amount of ram which is not used by
> applications as paging cache ? Say I have 4 gigs of ram , and Firefox is
> using 1 gig of it , the rest of RAM is used for disk/page caching or is it
> just unused and left there ?

The kernel itself will use some of it, other processes will use some of it, and
if there's any left, it will be used for disk caching - but not until a process
has actually referenced data off the disk.

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