Linux – What Happens When Out of Swap Memory?

memoryout of memoryswap

On my Debian VM machine with 512 MB RAM and 348 MB swap, what will happen if I open a 1 GB file in an editor and get out of memory?

Will it crash the system? Or if not, how will Linux handle this?

Wouldn't it be wise to install Swapspace so if needed there will be created enough swap automatically and dynamically?

sudo apt-get install swapspace

Best Answer

It depends on the settings you're running with, in particular memory overcommit (/proc/sys/vm/overcommit_memory; see man 5 proc for details).

If memory overcommit is disabled, the editor's (and possibly other programs attempting at the same time) attempt to allocate memory will fail. They'll get a failure result from the system call. Its up to each program to handle this, though an unfortunately common result is for the program to crash. The editor may also, for example, just refuse to open the file.

If memory overcommit is enabled, then the system call requesting memory may well succeed. In that case, when the memory is actually accessed, the kernel will notice its out of memory, and kill a process to reclaim memory. That process may or may not be the editor. The choice is governed by the oom_score (the result of several kernel heuristics) and oom_score_adj (configured) of each process on the system. Those are also in that proc(5) manpage.

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Linux – Will swap file engage automatically when I write too much to /dev/shm

I just went ahead and see what happens, when you write too many files to /dev/shm.

OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device
inFile: RC_2018-01-02
outDir: tmp
outputToScreenOnly: 0
OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device

It “ran out of space” after 33 days of input files, as shown above.

>>> 36.*33/12
99.0

So an estimated 99GB -- which is very close to 100GB -- of files, is where the “no space left” error happened. This is exactly the size of the /dev/shm device which I had set to be 100G. This means swap was not really able to be used when it was needed. This has demonstrated that unfortunately, /dev/shm simply runs out, and does not let you start to use swap as virtual memory to keep going, when there’s a shortage. So now we know thanks to this experiment. You just need real physical RAM for /dev/shm to work, and virtual RAM aka swap is not helpful for /dev/shm at all.

I have posted this answer to share with others who may benefit from the information gained.

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