What is the rule for setting RAM size in general esp in Linux

Linux: Should You Use Twice the Amount of Ram as Swap Space?

Linux and other Unix-like operating systems use the term "swap" to describe both the act of moving memory pages between RAM and disk, and the region of a disk the pages are stored on. It is common to use a whole partition of a hard disk for swapping. However, with the 2.6 Linux kernel, swap files are just as fast as swap partitions. Now, many admins (both Windows and Linux/UNIX) follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. Let us say I've 32GB RAM, should I set swap space to 64 GB? Is 64 GB of swap space really required? How big should your Linux / UNIX swap space be?

Old dumb memory managers

I think the '2x swap space' rule came from Old Solaris and Windows admins. Also, earlier memory mangers were very badly designed. There were not very smart. Today, we have very smart and intelligent memory manager for both Linux and UNIX.

Nonsense rule: Twice the size of your main system RAM for Servers

According to OpenBSD FAQ:

Many people follow an old rule of thumb that your swap partition should be twice the size of your main system RAM. This rule is nonsense. On a modern system, that's a LOT of swap, most people prefer that their systems never swap. You don't want your system to ever run out of RAM+swap, but you usually would rather have enough RAM in the system so it doesn't need to swap.

Select right size for your setup

Here is my rule for normal server (Web / Mail etc):

1. Swap space == Equal RAM size (if RAM < 2GB)
2. Swap space == 2GB size (if RAM > 2GB)

My friend who is a true Oracle GURU recommends something as follows for heavy duty Oracle server with fast storage such as RAID 10:

1. Swap space == Equal RAM size (if RAM < 8GB)
2. Swap space == 0.50 times the size of RAM (if RAM > 8GB)

Swap will just keep running servers...

Swap space will just keep operation running for a while on heavy duty servers by swapping process. You can always find out swap space utilization using any one of the following command:
cat /proc/swaps
swapon -s
free -m
top

See how to find out disk I/O and related information under Linux. In the end, you need toadd more RAM, adjust software (like controlling Apache workers or using lighttpd web server to save RAM) or use some sort of load balancing.

Also, refer Linux kernel documentation for /proc/sys/vm/swappiness. With this you can fine tune swap space.

A note about Desktop and Laptop

If you are going to suspend to disk, then you need swap space more than actual RAM. For example, my laptop has 1GB RAM and swap is setup to 2GB. This only applies to Laptop or desktop but not to servers.

Kernel hackers need more swap space

If you are a kernel hacker (debugging and fixing kernel issues) and generating core dumps, you need twice the RAM swap space.

Conclusion

If Linux kernel is going to use more than 2GiB swap space at a time, all users will feel the heat. Either, you get more RAM (recommend) and move to faster storage to improve disk I/O. There are no rules, each setup and configuration is unique. Adjust values as per your requirements. Select amount of swap that is right for you.