What's
going on?
Linux is borrowing unused memory for
disk caching. This makes it looks like you are low on memory, but you are not!
Everything is fine!
Why
is it doing this?
Disk caching makes the system much
faster and more responsive! There are no downsides, except for confusing
newbies. It does not take memory away from applications in any way, ever!
What
if I want to run more applications?
If your applications want more memory,
they just take back a chunk that the disk cache borrowed. Disk cache can always
be given back to applications immediately! You are not low on ram!
Do
I need more swap?
No, disk caching only borrows the
ram that applications don't currently want. It will not use swap. If
applications want more memory, they just take it back from the disk cache. They
will not start swapping.
How
do I stop Linux from doing this?
You can't disable disk caching. The
only reason anyone ever wants to disable disk caching is because they think it
takes memory away from their applications, which it doesn't! Disk cache makes
applications load faster and run smoother, but it NEVER EVER takes memory away
from them! Therefore, there's absolutely no reason to disable it!
Why
does top and free say all my ram is used if it isn't?
This is just a differing
understanding of terms. Both you and Linux agree that memory taken by
applications is "used", while memory that isn't used for anything is
"free".
But how do you count memory that is
currently used for something, but still available for applications?
You might consider that memory
"free" or at least "available". Linux instead counts it as
both "used" and "available", but not "free":
|
Memory
that is
|
You'd
call it
|
Linux
calls it
|
|
taken by applications
|
Used
|
Used
|
|
available for applications, and
used for something
|
Free (or Available)
|
Used (and Available)
|
|
not used for anything
|
Free
|
Free
|
This "something" is what
top and free calls "buffers" and "cached". Since your and
Linux's terminology differs, you might think you are low on ram when you're
not.
How
do I see how much free ram I really have?
To see how much ram is free to use
for your applications, run free -m and look at the "available" column:
$
free -m
total used free shared
buff/cache available
Mem: 1504 1491 13 0 855 869
Swap: 2047 6
2041
If you are using an older
distribution that doesn't show an "available" column, look at the
"free" column in the row that says "-/+ buffers/cache":
$
free -m
total used free
shared buffers cached
Mem: 1504 1491 13 0 91 764
-/+
buffers/cache: 635 869
Swap: 2047 6 2041
This is your answer in megabytes. If
you just naively look at "used" and "free", you'll think
your RAM is 99% full when it's really just 42%!
When
should I start to worry?
A healthy Linux system with
more than enough memory will, after running for a while, show the following
expected and harmless behavior:
- free memory is close to 0
- used memory is close to total
- available memory (or "free + buffers/cache") has enough room (let's say, 20%+ of total)
- swap used does not change
Warning signs of a genuine low memory situation that you may want to look
into:
- available memory (or "free + buffers/cache") is close to zero
- swap used increases or fluctuates
- dmesg | grep oom-killer shows the OutOfMemory-killer at work
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