Another useless feature into Linux kernel. Who uses swap space nowadays?! Last time I used swap on Linux device was around Pentium 2 era but in reality closer to 486DX era
I rely on it heavily. Have you tried zram swap?
Modernizing Linux swapping: introducing the swap table
41–50 of 132 posts
Re: Modernizing Linux swapping: introducing the swap table
#42Earlier quoted context omitted.
This rule of thumb is outdated by two decades.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.
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.
You are right, it is not a rule of thumb, and you can't determine optimal swap size right away. But you don't need "extensive runtime analysis". Start with a small swap - a few hundred megabytes (assuming the system has GBs of RAM). Check its utilization periodically. If it is full, add a few hundred megabytes more. That's all.
Re: Modernizing Linux swapping: introducing the swap table
#43Another useless feature into Linux kernel. Who uses swap space nowadays?! Last time I used swap on Linux device was around Pentium 2 era but in reality closer to 486DX era
It's unloved on Linux because using Linux under memory pressure sucks. But that's not a good reason to abandon improvements. Even more so with the direction RAM prices are headed
It sucks for interactive use only. It could be solved in user space (see the other comment with cgroups), it just isn't.
Re: Modernizing Linux swapping: introducing the swap table
#44Earlier quoted context omitted.
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 over…
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.
> RAM is for volatile data, disk is for data persistence.
Genuinely curious where this idea has come from. Is it something being taught currently?
Re: Modernizing Linux swapping: introducing the swap table
#45Earlier quoted context omitted.
It sucks without proper cgroup limits because swap makes OOM slower to trigger. Either set the cgroup limits or make the swap small.
This requires additional setup from the user, the default setup should just "work".
There are different definitions of "just work".
Re: Modernizing Linux swapping: introducing the swap table
#46Earlier quoted context omitted.
I dont get why anti-swap is so prevalent in Linux discussions. Like, what does it hurt to stick 8-16-32gb extra "oh fuck" space on your drive. Either you're going to never exhaust your system ram, so it doesn't matter, minimally exhaust it and swap in some peak load but at least nothing goes down, or exhaust it all and start having things get OOM'd which feels bad to me. Am I out of touch? Surely it's the children wh…
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 s…
> 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?
Re: Modernizing Linux swapping: introducing the swap table
#47Earlier quoted context omitted.
I dont get why anti-swap is so prevalent in Linux discussions. Like, what does it hurt to stick 8-16-32gb extra "oh fuck" space on your drive. Either you're going to never exhaust your system ram, so it doesn't matter, minimally exhaust it and swap in some peak load but at least nothing goes down, or exhaust it all and start having things get OOM'd which feels bad to me. Am I out of touch? Surely it's the children wh…
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 s…
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.
Re: Modernizing Linux swapping: introducing the swap table
#48Both Canonical and Microsoft recommend enabling swap file for Ubuntu cloud images, even if you allocate plenty of RAM to the VM. Any thoughts on that?
Yeah because Linux's memory management is quite poor and running out of RAM without swap will often mean a hard reboot. Swap definitely helps a lot, even if it doesn't fully solve the problem.
To be honest I don't know why it's such an issue on Linux. Mac and Windows don't have this issue at all. Windows presumably because it doesn't over-commit memory. I'm not sure why Mac is so much better than Linux at memory management.
My eventual solution was to just buy a PC with a ton of RAM (128 GB). Haven't had any hard reboots due to OOM since then!
Re: Modernizing Linux swapping: introducing the swap table
#49I'd like to see Linux gain support for actual memory compression, without the need to go through zram, similar to macOS/Windows.
zswap?
Re: Modernizing Linux swapping: introducing the swap table
#50Earlier quoted context omitted.
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 s…
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.
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.