Origin of the rule that swap size should be 2x of the physical memory | Hacker News Reader

Origin of the rule that swap size should be 2x of the physical memory

retrocomputing.stackexchange.com

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Early BSD VM pre-allocated swap backing for every anonymous page — you couldn't allocate virtual memory without a swap slot reserved for it, even if the page was never paged out.

When a process forks, the child needed swap reservations for the parent's entire address space (before exec replaces it). A large process forking temporarily needs double its swap allocation. If your working set is roughly equal to physical RAM, fork alone gets you to 2x.

This was the practical bottleneck people actually hit. Your system had enough RAM, swap wasn't full, but fork() failed because there wasn't enough contiguous swap to reserve. 2x was the number that made fork() stop failing on a reasonably loaded system.

The later overcommit/copy-on-write changes made this less relevant, but the rule of thumb outlived the technical reason. Most people repeating "2x RAM" today are running systems where anonymous pages aren't swap-backed until actually paged out.

Today swap is no longer about extending your address space, it's about giving the kernel room to page out cold anonymous pages so that RAM can be used for disk cache.

A little swap makes the system faster even when you're nowhere near running out of memory, because the kernel can evict pages it hasn't touched in hours and use that RAM for hot file data instead.

The exception is hibernation — you need swap >= RAM for that, which is why Ubuntu's recommendations are higher than RedHat's 20% of RAM.

TBF, I think overcommit was and remains an ugliness in how we manage memory. I wish we'd solved the fork commit-charge-spike issue by encouraging vfork (and later, posix_spawn) more heavily, not by making the OS lie about the availability of memory.

The ship's long sailed though, so even I run with overcommit enabled and only grumble about what might have been.

It's not just fork. The operating system overcommits memory all over the place. For example, when you map memory, that can/will succeed without actually mapping physical pages. Even "available" memory is put to some use and freed in an asynchronous way behind the scene, a process that is not always successful.

Honestly, I think overcommit is a good thing. If you want to give a process an isolated address space, then you have to allow that process to lay out memory as it sees fit, without having to worry too much about what else happens to be on the system. If you immediately "charge" the process for this, you will end up nit-picking every process on the system, even though with overcommit you would have been fine.

Overcommit allows much better memory utilization. People disable overcommit all the time and get surprised when their programs start failing mallocs while there are still tons of discardable page cache in the system.

https://unix.stackexchange.com/q/797835/1027

https://unix.stackexchange.com/q/797841/1027

Today's swap also is not preallocated by the user. It is entirely handled by the OS itself. If it needs swap space to hibernate it will go ahead and allocate it itself.

It does? Last I checked Linux doesn't do dynamic swap sizes, and while Windows has dynamic swap sizes it has a separate big non-dynamic file for hibernation. I have no idea what MacOS does.

You guys are getting Linux to hibernate!?

I managed to get mine to wake up from sleep, haven't recovered enough to attempt hibernation yet.

on my thinkpad with fedora hibernate works fine. i use it frequently. i even use it when for an unknown reason my usb-c headphones stop working. i don't know why. but after hibernate they work again. somehow waking up from hibernation fixes the problem

The OP here. I think this is the correct answer. I'm aware of the strict swap reservation, even mentioned it in the question, but I didn't realize that it doubles memory on fork even with copy-on-write, so I assumed that it implies a minimum of 1x, not 2x. This is specific to BSD and SunOS, but with them being popular in the 80s, the 2x rule became widespread.

The OP clearly states that he wants to know the earliest origin of the rule, and the only answers he gets are people giving their own opinions on how much swap space you should have.

Too bad because it's an interesting question that I would also like to know the answer to.

Nope. Those are not the only answers I am seeing. I’m still curious though. 2x was nice because nobody really questioned it. Now that we have there doesn’t seem to be one “answer”. This is a fun/interesting question that comes up every now and then here and elsewhere :-) I suspect someone smarter than me about system tuning will have a much smarter and nuanced answer than “just use 2x”

I thought the modern advice was you don't need it at all. No more spinning disks, so the there's no speed gain using the inner-most ring, and modern OSes manage memory in more advanced, and dynamic ways. That's what I choose to believe anyway, I don't need anymore hard choices when setting up Linux :)

The main downside to not having swap is that Linux may start discarding clean file backed pages under memory pressure, when if you had swap available it could go after anonymous pages that are actually cold.

On a related note, your program code is very likely (mostly) clean file backed pages.

Of course, in the modern era of SSDs this isn't as big of a problem, but in the late days of running serious systems with OS/programs on spinning rust I regularly saw full blown collapse this way, like processes getting stuck for tens of seconds as every process on the system was contending on a single disk pagefaulting as they execute code.

I don't think that's correct. Having swap still allows you to page out rarely-used pages from RAM, and letting that RAM be used for things that positively impact performance, like caching actually used filesystem objects. Pages that are backed by disk (e.g. files) don't need that, but anonymous memory that e.g. has only been touched once and then never even read afterwards should have a place to go as well. Also, without swap space you have to write out file backed pages, instead of including anonymous memory in that choice.

For that reason, I always set up swap space.

Nowadays, some systems also have compression in the virtual memory layer, i.e. rarely used pages get compressed in RAM to use up less space there, without necessarily being paged out (= written to swap). Note that I don't know much about modern virtual memory and how exactly compression interacts with paging out.

Every time I've ran out of physical memory on Linux I've had to just reboot the machine, being unable to issue any kind of commands by input devices. I don't know what it is, but Linux just doesn't seem to be able to deal with that situation cleanly.

The mentioned situation is not running out of memory, but being able to use memory more efficiently.

Running out of memory is a hard problem, because in some ways we still assume that computers are turing machines with an infinite tape. (And in some ways, theoretically, we have to.) But it's not clear at all which memory to free up (by killing processes).

If you are lucky, there's one giant with tens of GB of resident memory usage to kill to put your system back into a usable state, but that's not the only case.

Windows doesn't do that, though. If a process starts thrashing the performance goes to shit, but you can still operate the machine to kill it manually. Linux though? Utterly impossible. Usually even the desktop environment dies and I'm left with a blinking cursor.

What good is it to get marginally better performance under low memory pressure at the cost of having to reboot the machine under extremely high memory pressure?

In my experience the situations where you run into thrashing are rather rare nowadays. I personally wouldn't give up a good optimization for the rare worst case. (There's probably some knobs to turn as well, but I haven't had the need to figure that out.)

Try doing cargo build on a large Rust codebase with a matching number of CPU cores and GBs of RAM.

I believe that it's not very hard to intentionally get into that situation, but... if you notice it doesn't work, won't you just not? (It's not that this will work without swap after all, just OOM-kill without thrashing-pain.)

I don't intentionally configure crash-prone VMs. I have multiple concerns to juggle and can't always predict with certainty the best memory configuration. My point is that Linux should be able to deal with this situation without shitting the bed. It sucks to have some unsaved work in one window while another has decided that now would be a good time to turn the computer unusable. Like I said before, trading instability for marginal performance gains is foolish.

No argument there. I also always had the impression that Linux fails less gracefully than other systems.

That only helps if you don't have much free RAM. If you've got more free RAM than you need cache (including disk cache), swap only slows things down. With RAM prices these days, getting enough RAM is not worth it to avoid swap. IME on a desktop with 128GiB of RAM & Zswap I've never hit the backing store but have gone over 64GiB a few times. I wouldn't want to have pay to rebuild my desktop these days, 128GiB of ECC RAM was pricey enough in 2023!

It’s still beneficial so that unused data pages are evicted in favor of more disk cache

I'm the OP. I got myself into collecting falsehoods people believe about Linux swap and OOM[1]. There is an entry about this 2x rule in this collection, with my answer on how to select swap size.

My question on Retrocomputing.StackExchange is my attempt to add some historical background to this entry.

[1]:https://alexeydemidov.com/2025/05/15/falsehoods-people-and-L...

Managed over 50k servers with zero swap. Set overcommit ratio to 0, min_free configured based on a Redhat formula and had application teams keep some memory free. Adjust oom scores at application startup especially for database servers where panic is set to 0.

Servers ranged from 144GB ram to 3TB ram and that memory is heavily utilized. On servers meant to be stateless app and web servers panic was set to 2 to reboot on oom which mostly occurred in the performance team that were constantly load testing hardware and apps and a few dev machines were developers were not sharing nicely. Engineered correctly OOM will be very rare and this only gets better with time as applications have more controls over memory allocation and other tools like namespaces/cgroups. Java will always leak, just leave more room for it.

There's a chance that those servers might run more efficiently with some swap space, for the reasons mentioned many times in this thread. Swap space is not just for overcommitting.

The theories are repeated a often but I have never seen any empirical data to back it up assuming one is setting the options I mentioned. These anecdotes usually come from servers with default settings and no attempt to tune them for the intended workloads and no capacity planning for application resources. Even OS maintainers are starting to recognize this and have created daemons such as tuned for the people that never touch settings. The next evolution will be dynamic adjustments from continuous bpf traces. I just keep it simple and avoid the circular arguments all together.

Oh sure, it might or might not make a significant difference at all. Chances are, if you do a lot of I/O on a large (or very large) amount of data, and you also have a lot of rarely used but resident anonymous memory, then swap space should help, as that anonymous memory can get paged out in favor of disk cache, but I have no idea how common that is.

Yeah I mean, I know what you mean but this is where it gets into circular reasoning. I will always have operations groups move the workload to a node that has more memory if that is what is needed. In my case having swap on disk would require it to be encrypted due to contracts requiring any customer data touching a disk to be encrypted but I just avoid that all together and just add more memory. If 2TB or RAM isn't enough then they get 3TB and so on. We pushed vendors and OEM's to grow their motherboard capacity. At some point application groups just get more servers.

Yeah, that seems like a reasonable approach for your case!

None of the answers are satisfying to me, tbh.

I install more RAM so I can swap less. If I have 8 GB, then the 2x rule means I should have a 16 GB swap file, giving me 24 GB of total memory to work with. If I then stumble upon a good deal on RAM and upgrade to 32 GB, then if I never had memory problems with 24 GB, then I should be able to completely disable paging and not have a problem. But instead, the advice would be to increase my paging file to 64 GB!?

It doesn't make any sense. At all.

It's not meant for that kind of comparison. It's a variant of Simpson's paradox. Any individual system with a fixed set of tasks needs less swap when it gets more RAM. But when you look at the aggregate of systems, the systems that have more tasks to run get more RAM to run them, and systems with fewer tasks get less RAM. And since more tasks need more swap, everything scales together (though often not linearly).

I'm getting tired of typing this, but swap space is not just to increase available virtual memory. If you upgrade from 8 GB to 24 GB, then with proper swap space usage, you have 16 GB that could be used for additional disk cache.

Sure, you're still better off with 24 GB overall compared to 8GB+swap whether you add swap to your 24 GB or not, but swap can still make things more better.

(That says nothing about whether the 2x rule is still useful though, I have no idea.)

The key parameter for swap size is "how memory-hungry things you want to run", which isn't easy to measure, but paying for installed RAM is a somewhat usable proxy metric for that. If you were happy with 8gb, it's some evidence that your apps don't need much memory (and swap), but if you needed to pay for an upgrade to 32gb, that's some evidence that you're the kind of user who needs much more swap than those with 8gb of RAM.

I've had arguments with people about this for 20 years now and the most compelling case I heard involved the price of storage vs the price of RAM in the mid to late 1990s, and that this 2x represented an optimal use of money in designing a system at that point in time.

Also I still put my swap partition at the end of the drive out of habit because you used to want to do that for performance reasons when it was stored on a spinning magnet.

in 1997 people talking about it on the Slackware Usenet group:

    >Question: Why do you need 500MB of swap space? You would be better of
    >spending your money on more RAM than wasting it on so much swap space,
    >considering that it would most likely never be used anyways.

    I work with systems that have between 256MB and 1GB of RAM and
    between 4GB and 16GB available for Linux. My experience with other
    operating systems is that swap should be 2X to 3X RAM

    ...

    The info that I have read about Linux is that the 2x for swap space is
    only for those running less than 16mb of ram. Your swap space could be
    equal to your ram

    ...

    I know there are broken OSes out there where it's recomended to
    have 2x RAM swapspace, but Linux is not broken in that way.
    With Linux you should have <Max needed memory> - <RAM> swapspace,
    and depending on your needs that might range from 0 to infinity
    MBs of swap.

    ...

    THIS IS CRAZY!!!! YOU DON'T KNOW WHAT THE F--K YOU'RE TALKING ABOUT.

It goes downhill from there..

https://groups.google.com/g/alt.os.linux.slackware/c/hWy0h_S...

As has been mentioned a few times in other comments here, I don't believe that's correct. Swap space is not just for "using more memory than you have RAM".

I think more people should know about the existence of ZRAM on modern Linux distributions. It's really changed the way I look at swap configs.

ZRAM is a compressed block device that is stored in RAM. It's great!

Previously, if I ever had high memory pressure situations, I really dreaded the slowdowns. Now, with swap sitting on top of /dev/zram0 it's a completely different experience.

I have ZRAM enabled on all of my personal machines, both laptops with limited memory, and desktops with 64 or 128GB of RAM. It's rarely used, but it is nice to have that extra room sometimes.

The performance of a zram device is so much faster than even the latest NVMe drives.

One interesting thing with zram (which OS X also does by default) is that certain memory leaks... effectively don't. I have a little raspberry pi where I have zram enabled. If I make a string in python and keep appending 'a's to it, eventually zram just soaks it up:

  $ zramctl
  NAME       ALGORITHM DISKSIZE  DATA COMPR TOTAL STREAMS MOUNTPOINT
  /dev/zram0 zstd          3.8G  2.3G 13.2M 17.2M       4 [SWAP]

2.3GB of 'a's that gets compressed down to 20MB.

macOS also uses compression in the virtual memory layer.

(It's fun to note that I try to type out "virtual memory" in this thread, because I don't want people to think I talk about virtual machines.)

Curious, How much swap have you personally allocated on your personal setup?

Why was this downvoted? I’m generally curious what current recommendations for swap are too!

Edit: oh and I don’t have an actual personal system with swap configuration on it anymore to give my own answer anymore either.

same, completely fair thing to ask.

people are too negative these days :|

Zero. My office workstation has 48 GB of RAM, my home computer has 64 (I went a bit overboard). I have very bad memories of swap thrashing and the computer becoming totally unresponsive until I forced a reset; if I manage to fill up so much RAM, I very much prefer the offending process to die instead of killing the whole computer.

It's funny how people think they're disabling swapping just because they don't have a swap file. Where do you think mmap()-ed file pages go? Your machine can still reclaim resident file-backed pages (either by discarding them if they're clear or writing them to their backing file if dirty) and reload them later. That's.... swap.

Instead of achieving responsiveness by disabling swap entirely (which is silly, because everyone has some very cold pages that don't deserve to be stuck in memory), people should mlockall essential processes, adjust the kernel's VM swap propensity, and so on.

Also, I wish we'd just do away with the separation between the anonymous-memory and file-backed memory subsystems entirely. The only special about MAP_ANONYMOUS should be that its backing file is the swap file.

The ghost of Multics walks yet the page tables awaiting recorporealization.

mmap is not swap. It's using the same virtual memory mechanisms to load/dump pages to disk. The policy for when to read and write those pages is completely different.

When the room for memory mapped files gets low enough you get bad thrashing anyway, so the policy difference isn't that important.

Having no swap limits how much you can overburden your computer, but you also hit problems earlier. Here's some example numbers for 64GB of memory: With swap you can go up to 62GB of active program data (85GB allocated and used) before you have performance issues. Without swap you can go up to 45GB of active program data (63GB allocated and used) before you hit a brick wall of either thrashing or killing processes. The no-swap version is better at maintaining snappiness within its happy range, but it's a tradeoff.

It is doing exactly what swap is doing. That it's swap with a different policy doesn't make it not-swap.

Also, that separate policy shouldn't even exist. For LRU/active-list/inactive-list purposes, why does it matter whether a page is anonymous or file-backed? If you need it, you need it, and if you don't, you don't. No reason for anonymous and file-backed memory to be separate sub-sub-systems under vm.

Anonymous memory and files have different access patterns. Files are frequently read sequentially and only once, so there is no need to keep them in memory. When files are read, cached memory pages are put into inactive file LRU first and promoted to active LRU only on the second access.

It is possible to unify things behind a single mechanism, yet apply different policies to different instances of this mechanism depending on circumstances and heuristics. We do not need almost entirely disjoint paging systems in the Linux kernel to notice that some kinds of memory have access patterns different from other kinds of memory. Instead of guessing based on whether someone used MAP_ANONYMOUS, we should observe what a program is actually doing.

All LRUs, file-backend and anonymous are handled by the same code. There are some conditionals here and there - executable pages are promoted to active LRU on the first access, locked pages (if I remember correctly) too, but all pages go through the same cycle. See this Linux Plumbers conference presentation https://www.youtube.com/watch?v=0bnWQF7WQP0 with the following slides https://d3s.mff.cuni.cz/files/teaching/nswi161/2023_24/08_li...

I did similar with my 32GB laptop, but it was fairly flaky for ~4 years and I just recently put 48GB of swap on and it's been so much better. It's using over 20GB of the swap. The are cases in Linux where running without swap results in situations very similar to swapping too much.

What kind of workload do you run that consumes 20 GB of swap???

Chrome and Slack. :-(

Oh god... My company uses Teams and it is one of the main reasons why I installed so much RAM on my workstation. Part of me wishes that the recent increase in RAM prices will force companies to reduce the ridiculous memory footprint of their software.

I'm not an expert, but aren't you just reducing the choice of what pages can be offloaded from RAM? Without swap space, only file-backed pages can be written out to reclaim RAM for other uses (including caching). With swap space, rarely used anonymous memory can be written out as well.

Swap space is not just for overcommitting memory (in fact, I suspect nowadays it rarely ever is), but also for improving performance by maximizing efficient usage of RAM.

With 48GB, you're probably fine, but run a few VMs or large programs, and you're backing your kernel into a corner in terms of making RAM available for efficient caching.

The point is to have so much RAM that you don't need to offload anything.

I have 64GB of RAM and 16GB of swap. Swap is small enough it can't get really out of hand.

I have memories from like 20 years ago that even when I had plenty of RAM, and plenty of it was free, I would get random OOM killer events relatively regularly. Adding just a tiny bit of swap made that stop happening.

I'm like 90% sure at this point it's just a stupid superstition I carry. But I'm not gonna stop doing it even though it is stupid.

Same here, though I settled on 32GB of swap because I have a 4TB SSD (caught a good sale on a Samsung EVO SSD at Newegg). But whenever I run `top`, I constantly see:

    MiB Swap:  32768.0 total,  32768.0 free,      0.0 used.

I could safely get away with 4GB of swap, and see no difference.

Luckily we're not in the spinning HDDs thrashing a working set in and out of 128 MB of primary memory days anymore. We have laptops that ship with SSDs that read/write at 6 GB/s.

I was experimenting with some graphics algorithm and had a memory leak where it would leak the uncompressed 12 MP image with every iteration. I was browsing the web when waiting for it to process when I wondered why it was taking so long. That's when I noticed it was using 80+ GB of swap just holding onto all those dead frames. It finished and meanwhile it had no noticeable performance impact on whatever else I was doing.

I ran with a setup like this for a bit, but I experienced far worse thrashing (and far more sudden onset) than I did with swap enabled. You need to take some extra steps to get a quick and graceful failure on RAM exhaustion.

My work laptop currently has 96GB of RAM. 32 of it is allocated to the graphics portion of the APU. I have 128GB (2x) of SWAP allocated, since I sometimes do big FPGA Synthesizations, which take up 50GB of RAM on its own. Add another two IDEs and a browser, and my 64GB or remaining RAM is full.

64GB RAM, zero swap. Until recently RAM was cheap so swap made little sense when you could simply buy more RAM.

On systems with 32/64/128 GB of ram, I'll typically have a 1GB or 2GB swap. Just so that the system can page out here and there to run optimally. Depending on the system, swap is typically either empty or just has a couple hundred MB kicking around.

On what OS are you using these settings? I found that Windows will refuse to allocate more virtual memory when the commit charge hit the max RAM size even if there is plenty of physical memory left to use.

I have 64 GiB of RAM and programs would start to crash at only 25 GiB of physical memory usage in some workloads because of high commit charge. I had to re-enable a 64 GiB SWAP file again just to be able to actually use my RAM.

My understanding is that Linux will not crash on the allocation and instead crash when too much virtual memory becomes active instead. Not sure how Mac handles it.

> On what OS are you using these settings?

Linux. I also don't typically run any individual workload that would consume all system ram. Not loading a giant scientific model into memory all at once for fast processing or anything like that, so mileage will certainly vary based on requirements.

Windows: I set min size to whatever is necessary to make RAM+swap add up to ~2 GBytes per CPU thread, to avoid problems with parallel Visual Studio builds. (See, e.g., https://devblogs.microsoft.com/cppblog/precompiled-header-pc...) Performance is typically fine with ~0.75+ GBytes RAM per job, but if the swapfile isn't preconfigured then Windows can seemingly end up sometimes refusing to grow it fast enough. Safest to configure it first

macOS: never found a reason not to just let it do whatever it does. There's a hard limit of ~100 GBytes swap anyway, for some reason, so, either you'll never run out, or macOS is not for you

Linux: I've always gone for 1x physical RAM, though with modern RAM sizes I don't really know why any more

16gb.

Fwiw you’ll see technical reasons for swap being a bad idea on servers. These are valid. Virtualised servers don’t really have great ways to make swap work.

On a personal setup though there’s no reason not to have swap space. Your main ram gets to cache more files if you let the os have some space to place allocated but never actually used objects.

As in ‘I don’t use swap because i don’t use all my ram’ isn’t valid since free ram caches files on all major OS’s. You pretty much always end up using all your ram. Having swap is purely a win, it lets you cache even more.

Having everything in RAM might also be better for security, unless you're encrypting the swap partition.

The contents of swap could be read after a power cut.

But then you're putting data that used to be on RAM on storage, in order to keep copies of stored data on RAM. Without any advance knowledge of access patterns, it doesn't seem like it buys you anything.

0. I've got 128GiB of RAM in my desktop, and Zram. I have a backing swapfile, but it's never been used so I count it as 0.

It's old enough that I'd put money on DEC. Any takers on that.

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