overcommit_memory and overcommit_ratio

Question

Here is my current setting:

vm.overcommit_ratio = 50 (default)
vm.overcommit_memory = 2

And Current Memory Usage:

[localhost~]$ free -g
             total       used       free     shared    buffers     cached
Mem:            47         46          0          0          0         45
-/+ buffers/cache:          1         45
Swap:           47          0         47

As per the documentation what I understood is:

vm.overcommit_memory = 2 will not allow to overcommit memory than 50 % of RAM (as vm.overcommit_ratio is 50) but still I can see that current memory usage is 46 GB out of 47 GB.

Did I misunderstand something?

Answer 1

From your documentation link:

2 — The kernel fails requests for memory that add up to all of swap plus the percent of physical RAM specified in /proc/sys/vm/overcommit_ratio

In your case, you have 47gB RAM, 47gB swap and overcommit_ratio is 50 so a the total memory used can go up to 47+47*.5=70.5gB.

Your currently used memory is 46gB in RAM and 0gB in swap so a total of 46gB which is still under the limit.