How does vm.overcommit_memory work?

When I use the default settings:

vm.overcommit_memory = 0
vm.overcommit_ratio = 50

I can read these values from /proc/meminfo file:

CommitLimit:     2609604 kB
Committed_AS:    1579976 kB

But when I change vm.overcommit_memory from 0 to 2, I'm unable to start the same set of applications that I could start before the change, especially amarok. I had to change vm.overcommit_ratio to 300, so the limit could be increased. Now when I start amarok, /proc/meminfo shows the following:

CommitLimit:     5171884 kB
Committed_AS:    3929668 kB

This machine has only 1GiB of RAM, but amarok works without problems when vm.overcommit_memory is set to 0. But in the case of setting it to 2, amarok needs to allocate over 2GiB of memory. Is it a normal behavior? If so, could anyone explain why, for instance, firefox (which consumes 4-6x more memory than amarok) works in the same way before and after the change?

Solution 1:

You can find the documentation in man 5 proc (or at kernel.org):

/proc/sys/vm/overcommit_memory
       This file contains the kernel virtual memory accounting mode.
       Values are:

              0: heuristic overcommit (this is the default)
              1: always overcommit, never check
              2: always check, never overcommit

       In mode 0, calls of mmap(2) with MAP_NORESERVE are not
       checked, and the default check is very weak, leading to the
       risk of getting a process "OOM-killed".

       In mode 2 (available since Linux 2.6), the total virtual
       address space that can be allocated (CommitLimit in /proc/mem‐
       info) is calculated as

           CommitLimit = (total_RAM - total_huge_TLB) *
                         overcommit_ratio / 100 + total_swap

The simple answer is that setting overcommit to 1, will set the stage so that when a program calls something like malloc() to allocate a chunk of memory (man 3 malloc), it will always succeed regardless if the system knows it will not have all the memory that is being asked for.

The underlying concept to understand is the idea of virtual memory. Programs see a virtual address space that may, or may not, be mapped to actual physical memory. By disabling overcommit checking, you tell the OS to just assume that there is always enough physical memory to backup the virtual space.

Example

To highlight why this can sometimes matter, take a look at the Redis guidances on why vm.overcommit_memory should be set to 1 for it.

Solution 2:

This is an old question with a well-established answer, but I think there's more to add.

First of all, when vm.overcommit_memory = 0, the vm.overcommit_ratio value is irrelevant. The kernel will use a heuristic algorithm to overcommit memory, so that your amarok process can be allocated more memory than is available.

When you set vm.overcommit_memory to 2, the vm.overcommit_ratio value becomes relevant. By default, this value is set to 50, which means the system would only allocate up to 50% of your RAM (plus swap). This explains why you are unable to start programs that was fine when vm.overcommit_memory = 0 - because there's less than 500MB of allocatable memory (assuming no swap).

When you set it to 300, you are allowing the system to allocate up to 300% of your RAM (plus swap, if any), which is why the CommitLimit value in /proc/meminfo is so high.

Although vm.overcommit_memory = 2 is usually used to prevent overcommitment, here, you're using it to cap the amount that can be overcommitted. Setting it to 300 is dangerous as your system don't have 5171884 kB of memory, and so, depending on how much swap space you have, the system will use swap (which is slow), or will run out of memory altogether.

As to why amarok uses more memory when vm.overcommit_memory = 2 - this is probably because amarok works best with more memory, but is fine with less as well. So the logic of the program may try to allocate 2GB of memory initially, but if it fails, try 1GB.