Re: [PLUG] memory usage in Linux

On Wed, May 20, 2020 at 1:34 PM Rich Freeman via plug <plug@lists.phillylinux.org> wrote:

On Tue, May 19, 2020 at 2:35 PM Eric Lucas via plug
<plug@lists.phillylinux.org> wrote:
>
> I'm running Kubuntu 18.04 [1] on a Dell Laptop with a 2-core i7 [2] and 16 GB of RAM (the max apparently.)
> I tend to leave lots of windows open on two or three of my four virtual desktops.
> Sometimes things get slow (like participating in a zoom chat with a few dozen fellow homebrewers last night.)
> I installed 'cockpit' to see what's up. All is okay except for RAM. It shows near 16GB of RAM used.
> I seem to recall (from years ago) that Linux "hoovers up" all the RAM and then parcels it out to applications as needed.
> If so, what's the point of telling me how much RAM I have used if it's always 99+%?
> How do others monitor and/or manage RAM usage in Linux?
>

Here is a typical output of the "free" command:
free
total used free shared buff/cache available
Mem: 16373372 8077212 2613968 2766012 5682192 5184788
Swap: 0 0 0

Total is just the amount of RAM in the system - 16GB in this case
(same as with you).

Used is total - free - buffers - cache.
Available is what is available for use, taking into account reclaiming
buffers and cache. It isn't the same as total - used because not all
of this memory can be reclaimed. This is what I would probably focus
on when monitoring.
Free is memory that is completely unused. As you say the kernel tries
to minimize this, but if a big process dies there will be free ram
until it gets filled with cache/etc, which won't happen until there is
IO.

I would just focus on available memory from a memory management standpoint.

I'm sure there are GUI applications that can monitor this as well.
MemAvailable also shows up in /proc/meminfo.

Monitoring swap use is also important, but you really want to see how
much is swapping in and out - a slow memory leak might lead to a bunch
of swap getting used and isn't ideal, but probably won't impact actual
performance.

--
Rich
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Seconding Rich, here.

Memory is fast and disk I/O is slow, so Linux by default tries to cache as many disk pages as possible into memory. The memory consumed by buffers/cache is generally available to be reclaimed for use by applications as it is demanded, so in the output of `free` pay attention primarily to the "available" value.

All that said, you could very much have enough open that you're getting some competition for RAM that's slowing things down. I have similar habits for leaving a lot of $stuff open all the time. One thing that has helped me out a ton is to use some tools to manage my browser's (Chrome, in my case) resource usage. There is a very useful extension available for Chrome called The Great Discarder that automatically kills Chrome tab processes that have been unused for a period of time. As soon as you click that tab again, it is reloaded. There may be something similar for Firefox, but last I remember FF used threads for tabs, not processes, so that could make things more complicated.

HTH!

Dustin Black

Red Hat Performance & Scale Engineering