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Episode: 1002
Title: HPR1002: LiTS 008: free: Understanding Linux Memory Usage
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1002/hpr1002.mp3
Transcribed: 2025-10-17 17:11:40
---
Welcome to Linux in the Shell episode 8, my name is Dan Washgroo, I will be your host
today.
Today we are going to be talking about the free command and understanding Linux memory
usage.
First I want to thank Hacker Public Radio for hosting the audio and the website for
the show and to remind you to get a full right up of this episode head on over to Linux
in the Shell.org, look up episode 8, check out the free command entry and also you can
watch the video of utilizing the free command.
Alright, let's talk about the free command, the free command is a snapshot into how your
system's memory usage is, how much memory is being utilized, how much is free and how
much swap is actually being utilized and how much is free.
Very basic information but can be alarming if you are not familiar with how Linux utilizes
the memory of your system.
So by default if you issue the free command and hit enter, you will see a line says
mem colon and followed by six columns, total used free shared buffers and cashed.
Now the output of the values here will be in bytes by default and of course in today's
systems having four or more gigs of RAM, displaying it in bytes may not be the most
ideal way that you want to see it.
Well free has a bunch of options to how you can display those values and depending on
how your system is at a version of free, you have available to you the following options.
Again dash B shows it in bytes and that's the default, dash K shows it in kilobytes, dash
M shows it in megabytes, dash G shows it in gigabytes, dash dash Tara, that's TRA shows
it in terabytes.
So if you have terabytes of RAM available so you can display that.
Now I prefer to use the dash H as much as possible and that's human readable mode.
What that basically does is it takes the largest value of three to be displayed.
So if you have a gig of RAM in your system and you do the dash H, more than likely it
will report to used and free in the largest human readable value of three.
So that would be more than likely megabytes being utilized.
Now if you only have 512 megabytes, it may show the values in megs or in kilobytes.
Again, it's all dynamically determined by the largest value it can show in that three
digit format.
So when I issue free dash H on my system, I have 1.5 gig of RAM, it shows me total and
the first column used it shows me in megabytes and free it shows me in megabytes and it pretty
much shows everything in megabytes.
Instead of saying if I were to show it free in gigabytes, dash G, it would show me a total
of one gigabyte being total and zero for used and zero for free because it can't, it doesn't
show percentage values or decimal values, it only show full integer values.
So if I do free M, it shows me 1,501 megabytes available, 858 megs use 8643 free.
And again, if I were to try and show it in the dash Terra, because I don't have a
terabyte of RAM, it reports all values is zero because again, it can't show it in decimals
or whatever.
Now continuing forward, when I issue the free command, free dash H, I shows me, for instance,
I have 1.5 gigabytes of memory available to me, used 862 megabytes and 639 megabytes
free.
Because those values in the first row under M, may not be what you would expect to see
and you may be alarmed, should you see that over 90% of your RAM is being utilized and
you have 10% free or less, don't be alarmed right off the bat because you have to understand
how Linux kernel is using memory.
What the Linux kernel does with your free memory is that it allocates to what's called
the buffer cache.
And the buffer cache is this area of memory the kernel utilizes or secures, so to speak,
to hold data.
Data like recent information pulled from disk by an application or information that needs
to be written out to disk by an application.
The thought behind this is reading and writing from a disk is a lot slower than reading and
writing from RAM.
So if you take some information off a disk, you call information to be utilized by a program,
for instance, like the LS command, it'll pull it into memory or a value of a find search,
it'll pull it into memory.
And the idea is, chances are you're going to want to do something again with that memory,
that information that you pulled off of the disk.
So it puts it into the buffer cache space allocated in the free RAM so that if it needs
to do something with that data, it pulls it from RAM instead of having to go back out
to the disk again.
So if it's writing to disk, if an application wants to write to disk, the kernel will take
and put it into the buffer cache and then write it out to the disk, in some cases, so
that the program can continue doing whatever it's doing and not have to wait for the information
to be written to the disk.
Just like a program might be running, pull some data from disk and it might need to get
that data again.
Instead of having to pull the data from the disk again, it pulls it right at the kernel
gives it to it from memory and if the program continues on, it doesn't have to wait to get
the information from the disk.
It's a method to reduce disk utilization and to speed everything up because accessing
information from RAM is a lot faster than accessing information from disk.
Now the amount of memory used by the kernel for buffer cache can be fairly small depending
on what the system is doing to almost 90 to 95% of your free RAM that's available to
be utilized.
So understand that the way that the kernel then handles that, to you, it's technically
still free RAM, to applications, it's free RAM, to the kernel, it is being used by the
kernel for something.
It should an application need RAM or need to use memory, the kernel will start jettising
the data in the buffer cache and give it to the application as needed.
The data being thrown out will be the least used data or the least access data in the buffer
cache.
So if there's something being accessed currently, it's not going to, it's not necessarily
first in first out basis.
It's based upon usage, if it's not, it hasn't been used for a long time, it's been stale,
it doesn't keep it in the buffer cache.
It might even jettison, you know, you'll see that that buffer cache, your usage, you
don't grow and shrink as the system does different things.
But should your system need, should an application need that RAM, it's still considered quote
free.
So looking at the used and free values in the first row, taking a consideration that
you also have to need no, the amount of buffer, the amount of memory being used for the
buffer cache.
And you can see that a more accurate picture of that on the second line that says minus
slash plus buffer slash cache.
What that shows you is the amount of used memory, not including the buffer cache and then
the amount of free memory, not including the buffer cache.
So it's the values without the buffer cache taken into consideration.
Now if you look at the first column or the first row of free on the memory line, there
are three columns after buffer, after they used and free, they're shared, there's buffers
and there's cache.
Essentially, shared is not used anymore, it's obsolete.
But buffers and cache, if you add those two values together, you get the amount of RAM
being utilized by the Linux kernel's buffer cache.
And you subtract that value from the amount of used RAM on the first line, you should get
an approximate value for the amount of RAM used reported in the second line, the second
row, the buffer cache row.
And if you add it to the free, you should get a value roughly equal to what's in the
free value RAM available to you.
So you can see all that information on the first line, second line breaks it down a little
easier for you.
And then the third line is your swap and free.
That tells you how much you may have a swap file or swap partition, how much of your
swap available system swap is available, how much is being used and how much is free.
So that's pretty straightforward.
So just take into consideration that even if you are showing a 90 plus percent RAM utilized,
pay attention to how much of that is RAM is being used by the buffer's cache.
Look at those last two columns of buffers in the cache and realize that technically to
the kernel, it's being used, but to you and to applications, that memory is available
to be utilized.
So don't be too alarmed.
Now if the amount of physical RAM applications are using starts to exceed 90 percent and
don't know what the exact threshold performance threshold would be on this.
But if you see your RAM usage growing and it's actually RAM utilized by applications, at
some point you're going to hit that threshold where the amount of memory being utilized exceeds
the amount that the system would like to have for that buffer's cache and therefore applications
are going to be calling data directly from disk and you won't have that busher buffer
cache to be able to store that information.
So performance is going to be a little degraded because you don't have enough buffer cache
available to you.
But then again, if you're exceeding 90 to 95 percent of your RAM, better watch out as to
what you're doing.
You might have to stop a few processes because you have something heavy due to going on
or you have a runaway program that's consuming too much system resources.
So just be aware of that.
Now finishing out the free command, there are a few other options that are of value to
you.
The one is dash t or total, which will show you the totals on a fourth line of how much
RAM and swap your systems is utilizing.
Totals will not, it will add up the first and the third line and provide those totals
so you get the memory and the swap and it will not take into account any buffer cache
utilization.
So that's total.
There is the dash s, which stands for seconds and what that will do is run the free command
every and seconds and display the output of free.
So if you did free dash s five, it would show the output of free every five seconds continuously
until you killed it.
You can use the dash s with the dash c or dash dash count option, which allows you to
display the output of free every and seconds for a number of iterations specified by dash
c.
So free dash s five dash c ten would display the output of free every five seconds for
ten iterations after which it will return control back to the shell.
You can only use the dash c option in conjunction with dash s.
That's free in a nutshell.
Just remember the output of free, pay close attention to the buffer's cache and don't
be alarmed if the first row shows a used and free that exceeds your expectations.
I want to thank the last known god for reporting to me a website called Linux8MyRAM.com and
the all-cast planet channel on IRC and free node.
That's a great website for explaining Linux memory usage and this whole thing I just
talked about in the buffer cache.
Head on over to linuxinachial.org for the full write-up of this episode and in the bibliography
I have the links to that website along with a few other websites for your information.
Thank you very much.
Have a great week.
Remember, contribute to HackRepublicRadio and enjoy.