Re: master 5c532fe303: Recommend that the user turn off memory overcommit

[Phil Sainty]

I keep seeing the choice of process to kill being described as
"random", but it is not random.

The kernel may or may not choose the best process to kill, but
it does choose it for a reason, and I would suggest that the
described criteria is fairly unlikely to describe an Emacs
process (although it's obviously still possible).

The selection process is described in the article that I linked
to previously in a related thread:

https://www.kernel.org/doc/gorman/html/understand/understand016.html

> 13.3 Selecting a Process
>
> The function select_bad_process() is responsible for choosing a
> process to kill. It decides by stepping through each running task
> and calculating how suitable it is for killing with the function
> badness(). The badness is calculated as follows, note that the
> square roots are integer approximations calculated with
> int_sqrt();
>
> badness_for_task = total_vm_for_task /
>   (sqrt(cpu_time_in_seconds) * sqrt(sqrt(cpu_time_in_minutes)))
>
> This has been chosen to select a process that is using a large
> amount of memory but is not that long lived. Processes which have
> been running a long time are unlikely to be the cause of memory
> shortage so this calculation is likely to select a process that
> uses a lot of memory but has not been running long. If the
> process is a root process or has CAP_SYS_ADMIN capabilities, the
> points are divided by four as it is assumed that root privilege
> processes are well behaved. Similarly, if it has CAP_SYS_RAWIO
> capabilities (access to raw devices) privileges, the points are
> further divided by 4 as it is undesirable to kill a process that
> has direct access to hardware.