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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?

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  • you added this to /etc/sysctl.confand run sysctl -p and verified the settings with sysctl -a right? Commented Mar 1, 2017 at 15:38
  • 46 GB out of 94 GB doesn't seem like anything has been committed over. Though I don't think free can show you how much virtual memory has been allocated, anyway. Commented Mar 1, 2017 at 15:46

2 Answers 2

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7

Actually, setting vm.overcommit_memory=2 DOES allow overcommit. If you set overcommit_ratio to (say) 200, then memory can be committed to the extent of swap +(RAM * 200/100).

The kernel documentation is slightly misleading in implying that '2' means don't overcommit - it means commit to this limit, which in the case of overcommit_ratio (and that's a misnomer as it's really a percentage) being greater than 100, does allow for overcommit.

vm.overcommit_memory is more accurately described as setting a limit to overcommit, which by default doesn't allow any overcommit.

You can see the commit limit:

    $free -m | awk '$1 ~/[Mm]em/ {print $2}' ; sysctl -a 2>/dev/null | grep vm.over  ; grep -i commitlimit /proc/meminfo


vm.overcommit_kbytes = 0
vm.overcommit_memory = 2
vm.overcommit_ratio = 800
CommitLimit:    23449596 kB
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1

The vm.overcommit_* settings control userspace memory allocation. They have no effect on the memory the kernel can allocate. Additionally the value you are taking 50% of is physical memory + swap. 47+47=94. So userspace can allocate up to 47gb.

Your free output shows you have 1gb used by userspace, and 45gb used by the kernel for caching. 1gb userspace is well under 50% of 94gb.

Additional correction:

vm.overcommit_memory = 2 will not allow to overcommit memory than 50 % of RAM

This setting will not allow overcommit at all. Combined with vm.overcommit_ratio=50, it will allow userspace to commit up to 50% of total memory. "commit" != "overcommit"

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