By Default, Linux is borrowing unused memory for disk caching. Sometimes it makes it looks like your memory is low, practically, it’s not. Disk caching makes the system much faster and more responsive. It makes sure there are no downsides. It does not take memory away from applications in any way, you think.
Not always we run few applications, but sometime we have run more applications or some of our 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 and this make sure it doesn’t feel like its running out of memory.
And disk caching only borrows the ram that applications doesn’t need for time being. It never uses swap memory. If applications want more memory, they just take it back from the disk cache. They will not start swapping.
Can we disable Disk Caching?
No, 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 do that. Disk cache makes only applications load faster and run smoother, it never takes memory away from it, so there is no point to disable it.
Output Comparison of TOP and FREE
This is just a difference in terminology. 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 can still be made available to applications?
You might count that memory as “free” and/or “available”. Linux instead counts it as “used”, but also “available”:
| Memory that is | You’d call it | Linux calls it |
| used by applications | Used | Used |
| used, but can be made available | Free (or Available) | Used (and Available) |
| not used for anything | Free | Free |
This “something” is (roughly) 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.
Free Command Explained.!!
To see how much ram your applications could use without swapping, run free -m and look at the “available” column:
$ free -m
total used free shared buff/cache available
Mem: 4951 392 2392 2 2165 4193
Swap: 2043 184 1859
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 47%!
Read More: /proc/meminfo: provide estimated available memory
Memory Thresholds
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
Read More: How to increase SWAP partition on Linux