REMI : Let's see how to manage the memory -- not management
to see how much memory is used for all the programs running on the Linux
system.
So first command to see that is free.
Let's see the help with dash dash help, and free
indeed displays the amount of free and used system memory.
And you can use it with some options minus b, k, m, or g.
b for byte, and then k for kilobyte, megabyte, and gigabyte.
So let's try it with free minus megabyte. (free -m)
Here you have a table with the total memory that you have, the used and the free.
Let's ignore here the shared and buffers.
So the line Mem will show you the total memory that you have,
25 megabytes, 3 megabytes are used, 22 megabytes is the free memory,
and if we use free minus k (free -k), you would have the same numbers, more precise
because they are in kilobyte here.
And, of course, free minus b (free -b) would be in bytes.
The second option to see how many bytes are used is by using top.
Let's go to the help of the top command, and here you
can see that you can hit some keys to show the memory, so that would be the S key
or to show and to sort by memory usage here by hitting the M key.
Let's try that.
Let's try to go to top.
First at the very top of the top command,
you have a little bit of statistics here.
You can see that 3,256 kilobytes of memory
is used, and here is the number that is free,
and here we have a lot of programs running on the system,
and we can sort them by memory.
But here you cannot see the column that says memory.
Yes, we can see percent of CPU, but where is present memory doesn't exist.
Well, here it is VSZ that stands for Virtual Size,
and the Virtual Size is indeed the virtual memory
for each program running on the system.
We won't go into the details of why it is called virtual memory, but sometimes
on some versions of the top that are installed on some Linux,
you will have VIRT instead of the VSZ like this.
So let's try to push and M key.
Type the M key, and here it just started all the programs,
all the commands running, by VSZ.
From top to bottom, you can see that the first one is using
1,512 kilobytes of memory, and so forth.
So that is sorted by memory, and now if we hit the S key,
we go into a special visualization of the top command that
is only showing the memory.
And indeed we have only one program running here,
one user program that is named top, that is the actual program running.
And we can see that it is using 1,496 kilobytes of memory,
and this is the same column as VSZ.
All right, let's quit stop by hitting Q, and the last thing I want to show you
is how to see the memory consumption using htop.
So it's a visual representation of top, a little bit more advanced as you know.
Here you have little visualization of the memory.
We can see that we are using 2 out of 25 megabytes,
so here are the used megabytes, and here the free ones.
And here we can see that it is sorted by CPU percent,
but we can change that by pressing F6, which means sort by, as you
can see in the menu at the bottom.
So on my keyboard, I have to hit the function activate
button key, which is fn.
And then F6, and here you have a menu on the left : Sort by,
with the up and down arrows, you can select an option here,
and we want to select M_SIZE, which stands for memory size.
I select that and I press the Enter key, and indeed, you
can see that now it is ordered by the VIRT column.
Virtual memory, which is indeed the quantity of memory that
is used by each command.
So the first line here is using 1,512 kilobytes
and htop, as you can see here, is using 1,496 kilobytes of memory.
That's it, so you know how to use free, top,
and htop to see how much memory is used by your program.
And indeed, to control if a program is using too much memory on the system
and putting too much pressure, maybe you can kill it if it is too big in memory.
So here is how to manage the memory on the Linux system.