[SOLVED] What is the vm.overcommit_ratio in Linux?

Issue

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 misunderstood anything?

Solution

I believe the default for vm.overcommit_memory is 0 and not 2. Is the overcommit_ratio only relevant to mode 2? I assume yes, but I'm not entirely sure.

From https://www.kernel.org/doc/Documentation/vm/overcommit-accounting

0 - Heuristic overcommit handling. Obvious overcommits of address space are refused. Used for a typical system. It ensures a seriously wild allocation fails while allowing overcommit to reduce swap usage. root is allowed to allocate slightly more memory in this mode. This is the default.

1 - Always overcommit. Appropriate for some scientific applications. Classic example is code using sparse arrays and just relying on the virtual memory consisting almost entirely of zero pages.

2 - Don't overcommit. The total address space commit for the system is not permitted to exceed swap + a configurable amount (default is 50%) of physical RAM. Depending on the amount you use, in most situations this means a process will not be killed while accessing pages but will receive errors on memory allocation as appropriate.

Instead of free -g which I assume rounds down to zero, you might want to use free -m or just free to be more precise.

This might be interesting as well:

cat /proc/meminfo|grep Commit

Answered By - PJ Brunet

Answer Checked By - Dawn Plyler (WPSolving Volunteer)