Linux Swap Table Code Shows The Potential For Huge Performance Gains

A new set of 27 Linux kernel patches introduces a "Swap Tables" mechanism aimed at enhancing virtual memory management. As Phoronix's Michael Larabel reports, "the hope is for lower memory use, higher performance, dynamic swap allocation and growth, greater extensibility, and other improvements over the existing swap code within the Linux kernel." From the report: Engineer Kairui Song with Tencent posted the Swap Table patch series today for implementing the design ideas discussed in recent months by kernel developers. The results are very exciting so let's get straight to it: "With this series, swap subsystem will have a ~20-30% performance gain from basic sequential swap to heavy workloads, for both 4K and mTHP folios. The idle memory usage is already much lower, the average memory consumption is still the same or will also be even lower (with further works). And this enables many more future optimizations, with better defined swap operations." "The patches also clean-up and address various historical issues with the SWAP subsystem," notes Larabel.

Context: In Linux, swap space acts as an overflow for RAM, storing inactive memory pages on disk to free up RAM for active processes. Traditional swap mechanisms are limited in flexibility and performance. The proposed "Swap Tables" aim to address these issues by allowing more efficient and dynamic management of swap space, potentially leading to better system responsiveness and resource utilization.

Huge Performance Gains...

[yababom]

For those unfortunate / foolish enough to be relying on swap...

Re:

[Sethra]

This was going to be my response as well. I can't remember the last time I ran a linux instance with insufficient memory. But I can see use cases on SOC or IoT devices.

Re:Huge Performance Gains...

[dsgrntlxmply]

I have direct experience with this in consumer electronics. Working at minimum hardware cost alongside application flexibility and variation in externally supplied data, one can at times rapidly get into an acute memory shortage. A swap mechanism can give a few extra seconds to recognize the condition and to evict something to resolutely reclaim some space.

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[ChunderDownunder]

I've seen this on the fine print with the marketing of low-end Android tablets on Amazon and ebay.

6GB of RAM - i.e. 4+2. but does anyone really want the wear and tear of writing to storage on media that is soldered in? Admittedly this is throwaway hardware that will end up in landfill/recycling within 4 years but still...

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[gweihir]

Yep, thought the same thing. Faking memory is not really for regular use ...

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[DamnOregonian]

Fuck, I didn't think there were any idiots left with the "hrr hrr VIRTUAL mem0ry is for l0sers" mentality.

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[gweihir]

The problem is on your side. Because that is not what I wrote. Some clue required...

Linux uses Swap even when memory isn't full

[BrightCandle]

Linux uses swap even when RAM isn't full. VM_Swappiness means that it will push pages of programs memory use out if it thinks it can get more performance with the memory for file caching. More information on how this sort of works before this patch: https://www.howtogeek.com/4496... [howtogeek.com]

Re:Linux uses Swap even when memory isn't full

[Sethra]

It's almost never a good idea to leave this value at it's default. For example - the "tuned" profile "throughput-performance" will set this down to 5.

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[drinkypoo]

I always disable swap, unless I am on some pathetically limited hardware. I have 64GB RAM in my desktop and 8GB in my laptop, and no need for any swap on either. Swap just means the system thrashes more before the OOM killer kicks in, plus I don't need a bunch of extra writes on my SSDs.

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[erice]

I always disable swap, unless I am on some pathetically limited hardware. I have 64GB RAM in my desktop and 8GB in my laptop, and no need for any swap on either. Swap just means the system thrashes more before the OOM killer kicks in, plus I don't need a bunch of extra writes on my SSDs.

My desktop also has 64GB of RAM but it does swap occasionally.

• I run gentoo and some compiles can be really big
• My backup regime compresses using lrzip, which will use tons of memory. It is quite a bit more efficient than less memory hungry compressors.

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[DamnOregonian]

This is insane. Without swap, a single process can bring down the entire system if not properly ulimited.

If I use up all of the RAM on the system, it's not necessarily me that dies from an sbrk() failure before the OOM killer kicks in.

There's a reason machines have swap by default- it's because those that design them are smarter than you are.

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[JamesTRexx]

I used a 4GB 2008 laptop until half a year ago when hardware issues forced me to replace it (this one a 2020 Dell with an upgrade to 32GB for the heck of it).

Swap was essential for daily use because the browser already used up loads of RAM (not even using Firefox or Chromium) and I activated zswap some years ago eventually when I read about it.
It did the trick of improving overall peformance and seemed to save on disk read and writes as well from observation, and I still have zswap enabled on this laptop ju

Re: Linux uses Swap even when memory isn't full

[ToasterMonkey]

It's almost never a good idea to leave this value at it's default. For example - the "tuned" profile "throughput-performance" will set this down to 5.

Yah that still swaps. Linux will fill all physical memory with filesystem cache first. Then does one of two things for each cache miss. At a swappiness of 5, for every 100 pages read, 97 pages of old FS cache will be replaced and 3 of some process's idle memory will get swapped out. All you're doing is slowing it down, and over time you'll always have a bunch of process memory swapped out. The actual rate it happens at depends on your workload, how many cache misses you get over time. A daily filesystem bac

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[DamnOregonian]

The fact that it is not set to zero should tell you something important.

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[tlhIngan]

Yes, this can increase performance. Basically if the system is idle enough, Linux can write changed pages to disk ahead of time so if it needs to swap out the pages, they've already been committed to disk and can immediately be reallocated on demand.

Basically instead of writing data to disk when you need to, you write it ahead of time so if it's needed, it's already done. It requires a balance - you don't want to be too aggressive and writing data constantly to disk because you can use up valuable disk I/O

Why

[rossdee]

Why do we need virtual memory these days?

Re:Why

[BrightCandle]

Because it improves performance by saving drive reads and writes. For example we have a program in memory and its doing nothing sat idle consuming half of all our memory. But we are using another program which is reading lots of files in and writing out updates. The second program if given more RAM for caching the file contents its reading would run considerably faster, the NVMe SSD might read at a few 1GB/s but RAM is going to be 100x faster. If the OS pushes the unused programs memory out onto the swap we can that memory for file cache and get better performance for the current program doing work. We can also cache writes into files and only push completed large groups of blocks back to the drive allowing bigger sequential writes which results in less IO overhead and sequential access. If you have ever run CrystalDisk you will know is substantially faster to do this than apparently random writing or individual blocks of 4k.

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[Tony Isaac]

If your memory is so tight that you need all this shuffling, maybe you should invest another $25 for more RAM!

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[Shades72]

Your statement isn't wrong, but only really valid for desktops. Sure, top-class laptops have replaceable RAM, but even on mid-class laptops RAM is being soldered and can't be upgraded anymore. Soldered RAM is pretty much the only option you'll get in low-class laptops. Margins have been very thin for quite a while now in "laptop land". And soldered RAM is cheaper than installing connectors to have upgradeable RAM. As a manufacturer, that choice is easily made.

Pop!_OS 20.04 ran marvelously well in my Lenovo

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[evanh]

Virtual Memory does not require swapping ability to still be VM. Swapping is an optional add-on feature.

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[evanh]

BTW: Swap space is still a physical memory type, just like DRAM, but much slower access.

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[darkain]

I understand the usage of the term "virtual memory" here, as its been incorrectly correlated to swapping memory to disk since its inception really... but i do hope at some point the realization in the broader community that "virtual memory" literally means all memory on a modern operating system's user space. this is how we can get multiple instances of the same application running at once, as they're using the same virtual memory addresses that are then translated over to physical addresses by the memory c

English is an overloaded language Re:Why

[davidwr]

We use the same words or phrases for different things. Sometimes this leads to confusion.

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[evanh]

This is not one of those cases. Virtual memory is not swap. Anyone that conflates the two has just got it wrong.

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[Tony Isaac]

You might find it helpful to read the summary.

Context: In Linux, swap space acts as an overflow for RAM, storing inactive memory pages on disk to free up RAM for active processes

This is indeed talking about swapping memory pages to disk.

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[evanh]

Yes, the question asked "Why do we need virtual memory these days?" looks to be conflating VM with swap. Hence the attempts to clarify.

Re:Why

[Tony Isaac]

According to the summary:

Context: In Linux, swap space acts as an overflow for RAM, storing inactive memory pages on disk to free up RAM for active processes

This is clearly referring to swapping memory to disk, not your proposed concept of "all memory on a modern OS user space."

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[evanh]

The question asked "Why do we need virtual memory these days?" looks to be conflating VM with swap. Hence the attempts to clarify.

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[DamnOregonian]

Not just userspace. Kernel doesn't operate on unmapped memory, either.
Also, the translation happens on the CPU well before the memory controller (or even the root complex).

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[MachineShedFred]

Well, for one specific case; I needed to start up four VMs in my home lab for a test where each one needed 16GB of RAM, my system had 64GB total, and I didn't feel like going and buying several hundred dollars of RAM, throwing away the 64GB I have already in order to do it.

Re: Why

[AcidFnTonic]

That is a different use case where you wanna use KMS memory merging itâ(TM)s built into the kernel.

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[fuzzyfuzzyfungus]

I suspect, in part, the same reasons that we historically did. You are much less likely to be on a 32 bit system and it's a lot easier to afford a lot of RAM in absolute terms; but it remains pretty expensive to have more RAM than you'll ever need, mass storage remains considerably cheaper than RAM, and it's typically still preferable to have things slow down when you run out of RAM rather than having the OOM killer sweep through.

There's also the sheer convenience(and potentially better outcomes, dependi

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[gweihir]

You do not. I have it turned off on most of my Linux boxes. But it can still have benefits.

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[DamnOregonian]

That is a terrible idea, for performance and for stability.

It's alright as long as you never have any kind of memory pressure, but then that just means you're really wasting a good portion of your RAM.