[manuel_w]

The pro-swap stance has never made sense to me because it feels like a logical loop.

There’s a common rule of thumb that says you should have swap space equal to some multiple of your RAM.

For instance, if I have 8 GB of RAM, people recommend adding 8 GB of swap. But since I like having plenty of memory, I install 16 GB of RAM instead—and yet, people still tell me to use swap. Why? At that point, I already have the same total memory as those with 8 GB of RAM and 8 GB of swap combined.

Then, if I upgrade to 24 GB of RAM, the advice doesn’t change—they still insist on enabling swap. I could install an absurd amount of RAM, and people would still tell me to set up swap space.

It seems that for some, using swap has become dogma. I just don’t see the reasoning. Memory is limited either way; whether it’s RAM or RAM + swap, the total available space is what really matters. So why insist on swap for its own sake?

[viraptor]

You're mashing together two groups. One claims having swap is good actually. The other claims you need N times ram for swap. They're not the same group.

> Memory is limited either way; whether it’s RAM or RAM + swap

For two reasons: usage spikes and actually having more usable memory. There's lots of unused pages on a typical system. You get free ram for the price of cheap storage, so why wouldn't you?

[man8alexd]

This rule of thumb is outdated by two decades.

The proper rule of thumb is to make the swap large enough to keep all inactive anonymous pages after the workload has stabilized, but not too large to cause swap thrashing and a delayed OOM kill if a fast memory leak happens.

[tremon]

That's not useful as a rule of thumb, since you can't know the size of "all inactive anonymous pages" without doing extensive runtime analysis of the system under consideration. That's pretty much the opposite of what a rule of thumb is for.

[dspillett]

> There’s a common rule of thumb that says you should have swap space equal to some multiple of your RAM.

That rule came about when RAM was measured in a couple of MB rather than GB, and hasn't made sense for a long time in most circumstances (if you are paging our a few GB of stuff on spinning drives your system is likely to be stalling so hard due to disk thrashing that you hit the power switch, and on SSDs you are not-so-slowly killing them due to the excess writing).

That doesn't mean it isn't still a good idea to have a little allocated just-in-case. And as RAM prices soar while IO throughput & latency are low, we may see larger Swap/RAM ratios being useful again as RAM sizes are constrained by working-sets aren't getting any smaller.

In a theoretical ideal computer, which the actual designs we have are leaky-abstraction laden implementations of, things are the other way around: all the online storage is your active memory and RAM is just the first level of cache. That ideal hasn't historically ended up being what we have because the disparities in speed & latency between other online storage and RAM have been so high (several orders of magnitude), fast RAM has been volatile, and hardware & software designs or not stable & correct enough such that regular complete state resets are necessary.

> Why? At that point, I already have the same total memory as those with 8 GB of RAM and 8 GB of swap combined.

Because your need for fast immediate storage has increased, so 8-quick-8-slow is no longer sufficient. You are right in that this doesn't mean you need 16-quick-16-slow is sensible, and 128-quick-128-slow would be ridiculous. But no swap at all doesn't make sense either: on your machine imbued with silly amounts of RAM are you really going to miss a few GB of space allocated just-in-case? When it could be the difference between slower operation for a short while and some thing(s) getting OOM-killed?

[man8alexd]

Swap is not a replacement for RAM. It is not just slow. It is very-very-very slow. Even SSDs are 10^3 slower at random access with small 4K blocks. Swap is for allocated but unused memory. If the system tries to use swap as active memory, it is going to become unresponsive very quickly - 0.1% memory excess causes a 2x degradation, 1% - 10x degradation, 10% - 100x degradation.

[xorcist]

There is too much focus in this discussion about low memory situations. You want to avoid those as much as possible. Set reasonable ulimit for your applications.

The reason you want swap is because everything in the Linux (and all of UNIX really) is written with virtual memory in mind. Everything from applications to schedulers will have that use case in mind. That's the short answer.

Memory is expensive and storage is cheap. Even if you have 16 GB RAM in your box, and perhaps especially then, you will have some unused pages. Paging out those and utilizing more memory to buffer I/O will give you higher performance under most normal circumstances. So having a little bit of swap should help performance.

For laptops hibernation can be useful too.

[rstuart4133]

It's true that if you always have free RAM, you don't need swap. But most people don't have that it can always be used as a disk cache. Even if you are just web browsing, the browser is writing to disk stuff fetched from the internet in the hopes it won't change, the OS is will be keeping all of that in RAM until no more will fit.

Once the system has used all available RAM if has for disk cache it has a choice if it has swap. It can write write modified RAM to swap, and use the space it freed for disk cache. There is invariably some RAM where that tradeoff works - RAM use by login programs, and other servers that haven't been accessed in hours. Assuming the system is tuned well, that is all that goes to swap. The freed RAM is then used for disk cache, and your system runs faster - merely because you added swap.

There is no penalty for giving a system too much swap (apart from disk space), as the OS will just use it up until the tradeoff doesn't make sense. If your system is running slow because swap is being overused the fix isn't removing swap (if you did you system may die because of lack of RAM), it's to add RAM until swap usage goes down.

So, the swap recipe is: give your system so much swap you are sure it exceeds the size of stuff that's running but not used. 4Gb is probably fine for a desktop. Monitor it occasionally, particularly if your system slows down. If swap usage ever goes above 1Gb, you probably need to add RAM.

On servers swap can be used to handle DDOS from malicious logins. I've seen 1000's of ssh attempts happen at once, in an attempt to break in. Eventually the system will notice and firewall the IP's doing it. If you don't have swap, those login's will kill the system unless you have huge amounts of RAM that isn't normally used. With swap it slows to a crawl, but then recovers when the firewall kicks in. So both provisioning swap and having loads of RAM prevent DDOS's from killing your system, but this is in a VM, one costs me far more per month than the other, and I'm trying fix to a problem that happens very rarely.

[man8alexd]

> There is no penalty for giving a system too much swap (apart from disk space)

There is a huge penalty for having too much swap - swap thrashing. When the active working set exceeds physical memory, performance degrades so much that the system becomes unresponsive instead of triggering OOM.

> Monitor it occasionally, particularly if your system slows down.

Swap doesn't slow down the system. It either improves performance by freeing unused memory, or it is a completely unresponsive system when you run out of memory. Gradual performance degradation never happens.

> give your system so much swap you are sure it exceeds the size of stuff that's running but not used. 4Gb is probably fine for a desktop.

Don't do this. Unless hibernation is used, you only need a few hundred megabytes of free swap space.

[Balinares]

Another factor other commenters haven't mentioned, although the article does bring it up: you may disable swap and you will still get paging behavior regardless, because in a pinch the kernel will reclaim pages that are mmapped to files. Most typically binaries and librairies. Which means the process in question will incur a map page read next time it schedules. But of course you're out of memory, so the kernel will need to page out another process's code page to make room, and when that process next schedules... Etc.

This has far worse degradation behavior than normal swapping of regular data pages. That at least gives you the breathing space to still schedule processes when under memory pressure, such as whichever OOM killer you favor.

[man8alexd]

Binaries and libraries are not paged out. Being read-only, they are simply discarded from the memory. And I'll repeat, actively used executable pages are explicitly excluded from reclaim and never discarded.

[t-3]

The reason you're supposed to have swap equal in size to your RAM is so that you can hibernate, not to make things faster. You can easily get away with far less than that because swap is rarely needed.

[dspillett]

> so that you can hibernate

The “paging space needs to be X*RAM” and “paging space needs to be RAM+Y” predate hibernate being a common thing (even a thing at all), with hibernate being an extra use for that paging space not the reason it is there in the first place. Some OSs have hibernate space allocated separately from paging/swap space.

[Balinares]

I do wish there was a way to reserve swap spaces for hibernation that don't contribute to the virtual memory. Else by construction the hibernation space is not sufficient for the entire virtual memory space, and hibernation will fail when the virtual memory is getting full.

[ch_123]

You're implying that people are telling you to set up swap without any reason, when in fact there are good reasons - namely dealing with memory pressure. Maybe you could fit so much RAM into your computer that you never hit pressure - but why would you do that vs allocating a few GB of disk space for swap?

Also, as has been pointed out by another commenter, 8GB of swap for a system with 8GB of physical memory is overkill.

[tremon]

I'm also in the GP's camp; RAM is for volatile data, disk is for data persistence. The first "why would you do that" that needs to be addressed is why volatile data should be written to disk. And "it's just a few % of your disk" is not a sufficient answer to that question.