Jukka Larja (roskakori2006.delete@this.gmail.com) on January 11, 2020 7:20 am wrote:
> Adrian (a.delete@this.acm.org) on January 11, 2020 12:05 am wrote:
>
> > I have said that I believe that 2 of the others are not really useful, the virtual memory,
> > which I have never used on hundreds of computers, from large servers to small laptops,
> > during 20 years
>
> Just because you don't need it doesn't make it useless. I have dozen servers (and my workstation,
> probably also workstations of several of my co-workers) which would need about double the
> amount of memory just because every once in a while (maybe once a day, once a week or once
> a month) the load their running just happens to use more than usual. In many cases that
> would have required buying a second server, as what we had were maxed out.
>
> The worst thing in my opinion is the failure mode when running without page file is that things just break
> when you run out of physical memory. With page file, you first get slowdowns. Total failure happens only
> once swap space is also exhausted. Much easier to manage in real world where resources are limited.

There's mostly 4 possibilities:

a) Without over-commit, with swap space. In this case there's no real problem; and it's possible to end up with the perfect scenario - RAM full of "virtual memory that's actually used" and swap space just existing for "virtual memory committed to but not actually used".

b) Without over-commit, without swap space. In this case most of the RAM ends up being wasted for "virtual memory committed to but not actually used".

c) With over-commit, with swap space. In this case you the OS can commit to more virtual memory than it can provide (from RAM and swap) and you can end up screwed ("OOM killer").

d) With over-commit, without swap space. In this case you the OS can commit to more virtual memory than it can provide (from RAM alone) and you can end up screwed ("OOM killer").

- Brendan