Re: [gentoo-user] mysqld invoked oom-killer

[Albert Hopkins]

On Friday, July 22 at 11:13 (-0700), Grant said:

> That all makes perfect sense.  So the reason a swap larger than maybe
> 1GB is not usually implemented is because idle processes don't
> normally have more than a few hundred MB of pages in memory?
>
That's not entirely true, either.  For example, My laptop has 4GB of
swap. Why?  Well, because I use hibernate and hibernate works on the
swap partition and I want to make sure that I have enough swap to write
all my memory to swap (actually It's now compressed so actually I
probablldon't really need that much).

> Wouldn't a sufficiently large swap (100GB for example) completely
> prevent out of memory conditions and the oom-killer?

No.  oom killer kicks in when your system is out of virtual memory.
Consider this example:

You have 4GB RAM
You have 0 swap.
Therefore you have a total of 4GB virtual memory.

The second all your processes try to consume more than 4G of virtual
memory, oom killer will kick in*

Consider the next example

You have 4GB RAM
You have 100GB swap.
Therefore you have a total of 104GB virtual memory 

The second all your processes try to consume more than 104GB of virtual
memory, oom killer will kick in.

Oom killer works on virtual memory (RAM + swap).  So it doesn't matter
how much RAM you have or how much swap you have, when the total virtual
memory is consumed, oom killer is called.

The secret is to not run out of virtual memory.

There is no *easy* way not to run out of virtual memory.  You either
don't consume as much VM, or you provide more VM (either through RAM or
swap).

* This is not entirely true, the system also needs memory for the
kernel, buffers, hardware drivers, and other things which simply cannot
be paged out to disk, so the actual number will be less than the amount
of VM.