[Numerlor]

The swap/memory situation in linux has surprised me quite a bit coming from Windows.

Windows remains mostly fully responsive even when memory is being pushed to the limits and swapping gigabytes per second, while on linux when I ran a stress test that ate all the memory I had trouble even terminating the script

[dlcarrier]

There's two things that cause this. First, Windows has a variable swap file size, whereas Linux has a fixed size, so Windows can just fill up your drive, instead of running out of swap space. Second, the default behavior for the out-of-memory killer in Linux isn't very aggressive, with the default behavior being to over-commit memory instead of killing processes.

As far as I know, Linux still doesn't support a variable-sized swap file, but it is possible to change how aggressively it over-commits memory or kills processes to free memory.

As to why there differences are there, they're more historical than technical. My best guess is that Windows figured it out sooner, because it has always existed in an environment where multiple programs are memory hogs, whereas it wasn't common in Linux until the proliferation of web-based everything requiring hundreds of megabytes to gigabytes of memory for each process running in a Chrome tab or Electron instance, even if it's something as simple as a news article or chat client.

Check out this series of blog posts. for more information on Linux memory management: https://dev.to/fritshooglandyugabyte/series/16577

[Joker_vD]

Windows "figured it out sooner" because it never really had to seriously deal with overcommitting memory: there is no fork(), so the memory usage figures of the processes are accurate. On Linux, however, the un-negotiable existence of fork() really leaves one with no truly good solution (and this has been debated for decades).

[p_ing]

NT has been able to overcommit since it's inception.

[goodpoint]

fork is a massive feature, not a bug.

[tliltocatl]

fork() is a misfeature, as is SIGCHILD/wait and most of Unix process management. It worked fine on PDP-11 and that's it.

But Linux also overcommits mmap-anonymous/sbrk, while Windows leaves the decision to the user space, which is significantly slower.

[rwmj]

Not really. It elegantly solves the "create a process, letting it inherit these settings and reset these other settings", where "settings" is an ever changing and expanding list of things that you wouldn't want to bake into the API. Thus (omitting error checks and simplifying many details):

  pipe (fd[2]);      // create a pipe to share with the child
  if (fork () == 0) { // child
    close (...);     // close some stuff
    setrlimit (...); // add a ulimit to the child
    sigaction (...); // change signal masks
    // also: clean the environment, set cgroups
    execvp (...);    // run the child
  }

It's also enormously flexible. I don't know any other API that as well as the above, also lets you change the relationship of parent and child, and create duplicate worker processes.

Comparing it to Windows is hilarious because Linux can create processes vastly more efficiently and quickly than Windows.

[Joker_vD]

> It elegantly solves the "create a process, letting it inherit these settings and reset these other settings", where "settings" is an ever changing and expanding list of things that you wouldn't want to bake into the API.

Or, to quote a paper on deficiencies of fork, "fork() tremendously simplifies the task of writing a shell. But most programs are not shells".

Next. A first solution is trivial: make (almost) all syscalls to accept the target process's pidfd as an argument (and introduce a new syscall to create an empty process in suspended state) — which Windows almost (but not quite) can do already. A second solution would be to push all the insides of the "if (fork () == 0) { ... }" into a eBPF program and pass that to fork() — that will also tremendously cut on the syscall costs of setting up the new process's state as opposed to Windows (which has posix_spawn()-like API).

> create duplicate worker processes.

We have threads for this. Of course, Linux (and POSIX) threads are quite a sad sight, especially with all the unavoidable signalling nonsense and O_CLOFORK/O_CLOEXEC shenanigans.

[tliltocatl]

Yes, but at what cost? 99% of fork calls are immediately followed by exec(), but now every kernel object need to handle being forked. And a great deal of memory-management housekeeping is done only to be discarded afterward. And it doesn't work at all for AMP systems (which we will have to deal with, sooner or latter).

In 1970 it might have been the only way to provide a flexible API, but nowadays we have a great variety of extensible serialization formats better than "struct".

[ChocolateGod]

Windows will also prioritise to keep the desktop and current focussed application running smoothly, the Linux kernel has no idea what's currently focused or what not to kill, your desktop shell is up there on the menu in oom situations.

[p_ing]

The same behavior exists as far back as NT4 Server, which does not provide a foreground priority boost by default.

[LargoLasskhyfv]

> As far as I know, Linux still doesn't support a variable-sized swap file...

You can add (and remove) additional swapfiles during runtime, or rather on demand. I'm just unaware of any mechanism doing that automagically, though.

Could probably done in eBPF and some shell scripts, I guess?

[dsr_]

swapspace (https://github.com/Tookmund/Swapspace) does this. Available in Debian stable.

[LargoLasskhyfv]

Wow. Since 20 years. And I'm rambling about eBPF...

[dlcarrier]

Linux's ePBF has its issues, too.

I once was trying to set up a VPN that needed to adjust the TTL to keep its presence transparent, only to discover that I'd have to recompile the kernel to do so. How did packet filtering end up privileged, let alone running inside the kernel?

I recently started using SSHFS, which I can run as an unprivileged user, and suspending with a drive mounted reliably crashes the entire system. Back on the topic of swap space, any user that's in a cgroup, which is rarely the case, can also crash the system by allocating a bunch of RAM.

Linux is one of the most advanced operating systems in existence, with new capabilities regularly being added in, but it feels like it's skipped over several basics.

[kalaksi]

There are daemons (not installed by default) that monitor memory usage and can increase swap size or kill processes accordingly (you can ofc also configure OOM killer).

[quotemstr]

Huh? What does swap area size have to do with responsiveness under load? Linux has a long history of being unusable under memory pressure. systemd-oomd helps a little bit (killing processes before direct reclaim makes everything seize up), but there's still no general solution. The relevance to history is that Windows got is basically right ever and Linux never did.

Nothing to do with overcommit either. Why would that make a difference either? We're talking about interactivity under load. How we got to the loaded state doesn't matter.

[man8alexd]

There are some mistakes in these blog posts, especially the one about overcommit.

[01HNNWZ0MV43FF]

I've had that same experience. On new systems I install earlyoom. I'd rather have one app die than the whole system.

You'd think after 30 years of GUIs and multi-tasking, we'd have this figured out, but then again we don't even have a good GUI framework.

[LtWorf]

I used to use it but it's too aggressive. It kills stuff too quickly.

[jauntywundrkind]

Like Linux / open source often, it depends on what you do with it!

The kernel is very slow to kill stuff. Very very very very slow. It will try and try and try to prevent having to kill anything. It will be absolutely certain it can reclaim nothing more, and it will be at an absolute crawl trying to make every little kilobyte it can free, swapping like mad to try options to free stuff.

But there are a number of daemons you can use if you want to be more proactive! Systemd now has systemd-oomd. It's pretty good! There's others, with other strategies for what to kill first, based on other indicators!

The flexibility is a feature, not a bug. What distro are you on? I'm kind of surprised it didn't ship with something on?

[nasretdinov]

In Linux the default swap behaviour is to also swap out the memory mapped to the executable file, not just memory allocated by the process. This is a relatively sane approach on servers, but not so much on desktops. I believe both Windows and macOS don't swap out code pages, so the applications remain responsive, at the of (potentially) lower swap efficiency

[superjan]

Don’t know about Macs, but on Windows executable code is treated like a readonly memory mapped file that can be loaded and restored as the kernel sees fit. It could also be shared between processes, though that is not happening that much anymore due to ASLR.

[inkyoto]

> In Linux the default swap behaviour is to also swap out the memory mapped to the executable file, not just memory allocated by the process […] I believe both Windows and macOS don't swap out code pages, so the applications remain responsive, at the of (potentially) lower swap efficiency

Linux does not page out code pages into the swap. You might be conflating page reclamation with swapping instead.

In Linux, executable «.text» pages are mapped[0] as file-backed pages, not anonymous memory, so when the kernel needs to reclaim RAM it normally drops those pages and reloads them from the executable file on the next page fault once they are accessed again (i.e. on demand) rather than writing them to swap.

In this particular regard, Linux is no different from any other modern UNIX[1] kernel (*BSD, Solaris, AIX and may others).

[0] Via mmap(2) in argv[0], essentially.

[1] Modern UNIX is mid-1990's and onwards.

[nasretdinov]

Yes, you are correct, I wasn't precise enough. It doesn't make sense to swap the existing code pages, they are just unmapped. (And that's the reason why you get "text file busy" when doing scp over the file: since the OS relies on the fact that the .text pages can be safely unmapped it needs to guarantee that they stay read-only)

[man8alexd]

It seems to be a persistent myth. The Linux kernel explicitly excludes active VM_EXEC pages from reclaim.

[Onavo]

> Windows remains mostly fully responsive even when memory is being pushed to the limits and swapping gigabytes per second

In my experience this is only on later versions of the NT Kernel and only on NVME (mostly the latter I think).

[robinsonb5]

Yeah I think SSD / NVME makes all the difference here - I certainly remember XP / Vista / Win 7 boxes that became unusable and more-or-less unrecoverable (just like Linux) once a swap storm starts.

[p_ing]

NT4 exhibits the same behavior under extreme load.

[demosito666]

Oh yeah. Bug 12309 was reported now what, 20 years ago? It’s fair to say that at this point arrival of GNU Mach will happen sooner than Linux will be able to properly work under memory pressure.

[rwmj]

The annoying thing I've found with Linux under memory stress (and still haven't found a nice way to solve) is I want it to always always always kill firefox first. Instead it tends to either kill nothing (causing the system to hang) or kill some vital service.

[0xbadcafebee]

Linux being... Linux, it's not easy to use, but it can do what you want.

1. Use `choom` to give your Firefox PIDs a score of +1000, so they always get reaped first

2. Use systemd to create a Control Group to limit firefox and reap it first (https://dev.to/msugakov/taking-firefox-memory-usage-under-co...)

3. Enable vm.oom_kill_allocating_task to kill the task that asked for too much memory

4. Nuclear option: change how all overcommiting works (https://www.kernel.org/doc/html/v5.1/vm/overcommit-accountin...)

[man8alexd]

3. vm.oom_kill_allocating_task is a footgun. It kills the last task that asked for memory and it could be any random task in the system.

4. disabling overcommit is another footgun, it makes malloc fail long before the memory is exhausted. See for a detailed explanation https://unix.stackexchange.com/a/797888/1027

[jauntywundrkind]

If using systemd-oomd, you can launch Firefox into it's own cgroup / systemd.scope, that has memory pressure control settings set to not kill it. ManagedOOMPreference=avoid.

https://www.freedesktop.org/software/systemd/man/latest/syst...

There's a variety of oom daemons. bustd is very lightweight & new. earlyoom has been around a long time, and has an --avoid flag. https://github.com/rfjakob/earlyoom?tab=readme-ov-file#prefe...

Your concerns are very addressable.

[ajb]

Yeah sytemd-oomd seems tuned for server workloads, I couldn't get it to stop killing my session instead of whichever app had eaten the memory.

Honestly on the desktop I'd rather a popup that allowed me to select which app to kill. But the kernel doesn't seem to even be able to prioritize the display server memory.

[delamon]

You can bump /proc/$firefox_pid/oom_score_adj to make it likely target. The easiest way is to make wrapper script that bumps the score and then starts firefox. All children will inherit the score.

[pmontra]

I'm not sure that I'd want the OS to kill my browser while I'm working within it.

Of course the browser is the largest process in my system, so when I notice that memory is running low I restart it and I gain some 15 GB.

Basically I am the memory manager of my system and I've been able to run my 32 GB Linux laptop with no swap since 2014. I read that a system with no swap is suboptimal but the only tradeoff I notice is that manual OOM vs less writes on my SSD. I'm happy with it.

[robinsonb5]

There are two pillars to managing RAM with virtual memory: the obvious one is is writing one program's working set to disk, so that another program can use that memory. The other one - which isn't prevented by disabling swap - is flushing parts of a program which were loaded from disk, and reloading them from disk when next needed.

That second pillar is actually worse for interactivity than swapping the working set, which is why disabling swap entirely isn't considered optimal.

By far the best approach is just to have an absurd amount of RAM - which of course is a much less accessible option now than it was a year ago.

[TacticalCoder]

> Windows remains mostly fully responsive even when memory is being pushed to the limits and swapping gigabytes per second ...

The problem problem though is all the times when Windows is totally unusable even though it's doing exactly jack shit. An example would be when it's doing all its pointless updates/upgrades.

I don't know what people are smoking in this world when they somehow believe that Windows 11 is an acceptable user experience and a better OS than Linux.

Somehow though it's Linux that's powering billions if not tens of billions of devices worldwide and only about 12% of all new devices sold worldwide are running that piece of turd that Windows is.