UNIX for Advanced & Expert Users

Hi Everyone,

I have some misunderstanding about swapping.

Is it true that swap
a) required always (I mean I cannot set size 0 bytes)
and
b) some pages are saved on disk in idle time even free memory is enough
?

Could you please advise some good reading about it?

Whether it's required or not depends on the operating system.

Whether pages are saved to disk in idle times also depends on the operating system but these days, often not. An operating system will often pre-claim space inside the swap though, in case it needs to do so later.

Whether or not it's required, having swap is a good idea. Without it, you may run out of memory and deadlock or die.

Corona688, thank you for your comment.

Could you please be more specific about operating systems?
I mean Solaris, Linux. Is swap must be on these platforms?

Why without swap I may run out of memory but with swap not?
If I have 8Gb RAM and 8Gb swap and an application requests 20Gb will it run out of memory?


And I will try be more specific about the question.
Let's suggest that I have some soft, for example, Oracle database.
And I am worry that with swap my OS will save pages on the disk in background even it is not required.
Am I right?

[duplicate]

These systems respond somewhat differently to swap.

If you ask for 20GB of RAM in Linux it will reserve you that much address space, not that much RAM. Only when you actually start using these individual pages of RAM will it give you actual memory. If programs end up using more RAM than you actually have, pieces will start having to end up in swap. Without swap, or when the swap's full, the kernel will start killing the worst offenders to reclaim some RAM and stop the system from deadlocking. This is known as 'overcommit' and can be disabled if you really want, but is useful for some situations; the google search engine's servers apparently rely heavily on the Linux OOM-killer.

Some other systems(HP-UX if I recall) don't have any sort of overcommit. All memory must be backed by swap of some sort. If you don't have enough, some memory will be used to act like swap for other memory.

I think Solaris is somewhere between these two extremes.

There is a substantial Wikipedia on this subject (and links to more articles):
Paging - Wikipedia, the free encyclopedia


No: The request will be refused.
Btw. The old rule that you must have at least as much swap as you have memory does not apply to modern unix.

Many modern unix systems can be configured to use part of memory for swap. This allows the O/S to defragment main memory quickly and quietly.

If you have Oracle, buy enough memory so that it should never swap and be sure that you understand how to size the SGA and PGA and how to measure these sizes using Oracle itself. It is very common to missize the SGA by a factor of 8 or even 16.

Guys,
Thanks for your comments.

If you have Oracle, buy enough memory so that it should never swap

1) Does this mean that OS will not do unnecessary paging in background?

2) Does this mean that if I have 128Gb of RAM and if I am sure that my soft (for example Oracle) will not use more than 120Gb of RAM (SGA + PGA = 110Gb), I give 8 Gb for OS (kernel, etc) and I can set swap size 0 bytes ?

3) For 8Gb RAM and 8Gb swap how much memory I can eat (really)? As I understand I can 16Gb. Am I right?
Why there was a requirement to have swap the same size as RAM? Why not 3 size of RAM or 4 sizes of RAM?
The requirement to have swap the same size of RAM looks like swap not add space to memory but something like mirror RAM memory on a disk. In this case I would be able to eat 8Gb, not 16Gb.

Please comment

---------- Post updated at 11:42 AM ---------- Previous update was at 11:36 AM ----------

From your comments I can consider that swap is just something as additional slow memory ;-)
But from my current vision concept of swap is more important and indivisible with virtual memory concept.