Slashdot Mirror
Is Swap Necessary?
johnnyb writes "Kernel Trap has a great conversation on swap, whether it's necessary, why swapless systems might seem faster, and an overall discussion of swap issues in modern computing. This is often an issue for system administrators, and this is a great set of posts about the issue."
but today's production, heavily loaded system will still need the ability to swap to/from disk.
Already, there are systems that minimize that need, set-top boxes, embedded systems in general. But each of those is seriously modified (kernel-wise, mostly) to achieve the responsiveness, the frugality of resource treatment that a general purpose desktop computer can't expect to enjoy.
That doesn't mean that developers should stay in the same rut, assuming that hardware that confined system design in the '60s, '70s... '00s will perpetually assign similar constraints.
IMO, desktops still need to swap... for now. but let's not paint ourselves into a performance corner.
Maybe you have enough memory to run your program, but you don't have enough memory to keep enough directory structures into RAM, so you keep needing to read the disk. If there are unused pages in that program that were only used once during startup, for example, it makes sense to get them out or memory, so that memory can be used for disk caching instead.
Now, you have to understand how Linux handles paging, too. Unmodified pages from executables that are running may be discarded by the kernel at any time, because it knows where to get them. They won't be thrown into swap because it's not necessary. On the other hand, if that particular page has been modified (and some are modified as they are loaded by ld.so, for example), then the page must be copied into swap before it's discarded.
I used up all my sick days, so I'm calling in dead.
The "swap=2x RAM" thing is obsolete admin trivia that simply refuses the die. It comes from the days when physical RAM was mapped into swap to simplify the swapping algorithm. If you didn't have at least a 1:1 correspondence between RAM and swap performance would suffer immensly. Starting with Linux 2.4 and up this is simply no longer true, there is no benefit from using excessively large swap partitions. Same goes for Sun OS and the BSDs these days.
Instead, the swap needed depends on the sort of usage pattern your machine has. If it's a desktop with 1-3GB of RAM, a swap partition of 1GB is completely adequate. Want the machine to swap as little as possible and utilize all the RAM, so turn down swappiness a bit to avoid Mozilla/Firefox from being paged out when you leave for 15 minutes.
On a server you need a whole lot more swap, the more the better. Not because it's necessarily any faster, it might be slower in fact with a high swappiness setting the system decides you don't really need that 2GB DB in memory if it's been unused for a month. But when you do run out of memory in legitimate use, the shit will really hit the fan if there isn't enough swap to pick up the slack.
I forgot to explain swappiness. This is a entry in proc, /proc/sys/vm/swappiness, that you can plug a numerical value between 0 and 100 into. The higher the number, the more eager Linux will be to swap out applications from RAM to disk. There's a lot of conflicting opinions on what values you should use. Kerneltrap had a good article on it recently.
Personally I use a value of around 20 or less for desktop machines. This keeps Mozilla being paged out after a short while, that really shouldn't be happening on modern hardware. Too bad you can't achieve the same effect in Windows 2000. Some people swear that a swappiness of 0 is ideal for their desktops, your mileage may vary. It's fun to play with in any case, any changes you make take effect instantaneously.
There are two "theorems" quoted: The first says that no matter what, if you have a size X of RAM used by the OS, and you add a size Y swap disk, you get better OS performance than if you only had X RAM.
The second "theorem" says: if you have X RAM + Y swap disk, then add Y RAM and use that instead as the swap disk, then you get *faster* performance.
The naysaysers now say that the second statement is misleading. Why? Because with X+Y RAM and Z swap disk, you'd get better performance again.
I think this betrays an underlying assumption which I'm not sure is true, namely: X+Y RAM managed by the OS any way it likes is always better managed then X RAM managed by the OS any way it likes and Y RAM reserved for swap operations.
In fact, let us suppose that the OS memory management is not optimal, ie when the OS manages X+Y amount of RAM, it does so suboptimally. Then it is possible that a different memory management scheme, e.g. X RAM used normally + Y RAM used exclusively for swap, may turn out to better use the available total RAM.
So the theoretical question is this: is Linux's memory management sufficiently optimal that with an ordinary set of applications running, it can always make better use of X+Y amount of RAM than if it always reserved Y for swap? Alternatively, under what kind of running application mix is it true that reserving Y amount for swap yields a better memory management algorithm than using X+Y fully?