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In defence of swap: common misconceptions

chrisdown.name

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Re: In defence of swap: common misconceptions

#111

Post by cdown »

cosarara97 wrote: post #36

In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow. The process of switching to a tty, logging in, and killing whatever the offending process is can easily take a good 15 minutes. Xorg will just freeze. Oh, and hopefully you know what process it is, else good luck running `top`. Until this is fixed, I'll just keep running my systems with very small amounts of sw…

> In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow.

Yeah, this is basically the main drawback of swap. I tried to address this somewhat in the article and the conclusion:

> Swap can make a system slower to OOM kill, since it provides another, slower source of memory to thrash on in out of memory situations – the OOM killer is only used by the kernel as a last resort, after things have already become monumentally screwed. The solutions here depend on your system:

> - You can opportunistically change the system workload depending on cgroup-local or global memory pressure. This prevents getting into these situations in the first place, but solid memory pressure metrics are lacking throughout the history of Unix. Hopefully this should be better soon with the addition of refault detection.

> - You can bias reclaiming (and thus swapping) away from certain processes per-cgroup using memory.low, allowing you to protect critical daemons without disabling swap entirely.

Have a go setting a reasonable memory.low on applications that require low latency/high responsiveness and seeing what the results are -- in this case, that's probably Xorg, your WM, and dbus.

Re: In defence of swap: common misconceptions

#112

Post by cdown »

_9jgl wrote: post #65
quotemstr wrote: post #62

Very few people on this thread read and understood the article. The point isn't working with data sets larger than RAM. The point is making better use of the RAM you do have by taking pages you'll almost never touch and spilling them to disk so that there's more room in RAM for pages you will touch. Banning swap is like making self-storage companies illegal and forcing everyone to hold all possessions in their homes.…

The issue is that the current OOM killer doesn't support this usage at all. To extend the analogy: what do you do if grandma comes and fills your house with stuff? You need space to work, so you go and drop it off at the self storage place, but what if she just keeps filling your house up? The OOM killer will do absolutely nothing until both your house and the whole self storage place are totally full. By that point,…

One problem with relying on the OOM killer in general is that the OOM killer is only invoked in moments of extreme starvation of memory. We really have no ability currently in Linux (or basically any operating system using overcommit) to determine when we're truly "out of memory", so the main metric used is our success or failure to reclaim enough pages to meet new demands.

As for the analogy -- there are metrics you can use today to bat away grandma before she starts hoarding too much. We have metrics for how much grandma is putting in the house (memory.stat), at what rate we kick our own stuff out of the house just to appease grandma, but then we realise we removed stuff we actually need (memory.stat -> workingset_refault), and similar. Using this and Johannes' recent work on memdelay (see https://patchwork.kernel.org/patch/10027103/ for some recent discussion), it's possible to see memory pressure before it actually impacts the system and drives things into swap.

Re: In defence of swap: common misconceptions

#113

Post by viraptor »

Agentlien wrote: post #110
quotemstr wrote: post #100

Earlier quoted context omitted.

> Exactly. Who with any technical experience in this day and age doesn't understand that You're on a site infamous for the comment "I switch to Node when I want to be close to the metal".

Is this a joke or was there actually such a comment? If so, can you link it? To someone like me, who usually lives somewhere between C++ and shader code, it sounds a bit too strange to be true.

I think this was the origin: https://news.ycombinator.com/item?id=2710383

Re: In defence of swap: common misconceptions

#114

Post by mikekchar »

crististm wrote: post #107
cosarara97 wrote: post #36

In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow. The process of switching to a tty, logging in, and killing whatever the offending process is can easily take a good 15 minutes. Xorg will just freeze. Oh, and hopefully you know what process it is, else good luck running `top`. Until this is fixed, I'll just keep running my systems with very small amounts of sw…

In 2005 I was able to run Linux on 512MB RAM _without_ swap (on purpose - every day) without issues. Today it will bark at me on 8GB of RAM for not having swap enabled.

I'm running on an 8GB Linux box without swap and never even come close to running out of memory. If I don't have any VMs running, then it's pretty unusual for me to use much more than 1-2 gigs. It's interesting... one of my colleagues has serious problems with performance because he keeps running out of memory -- and I don't think he's doing anything unusual.

I think there is something wrong with some of the major distros. I got really fed up with Ubuntu because of random junk running without my approval and eventually migrated to Arch simply because I have a lot more control over configuration. I don't mean to trash one distro over another because each one has its strengths and weaknesses, but I'm been surprised at how bloated the average Linux install is these days. I'd love it if there was more attention paid to it.

Re: In defence of swap: common misconceptions

#115

Post by d0mine »

burnte wrote: post #99
quotemstr wrote: post #62

Very few people on this thread read and understood the article. The point isn't working with data sets larger than RAM. The point is making better use of the RAM you do have by taking pages you'll almost never touch and spilling them to disk so that there's more room in RAM for pages you will touch. Banning swap is like making self-storage companies illegal and forcing everyone to hold all possessions in their homes.…

"Very few people on this thread read and understood the article." I started to read the article, and then thought, "I know this, who doesn't know this?" and stopped. "The point is making better use of the RAM you do have by taking pages you'll almost never touch and spilling them to disk so that there's more room in RAM for pages you will touch." Exactly. Who with any technical experience in this day and age doesn't…

In theory swap is useful, in practice it can be less so https://news.ycombinator.com/item?id=16147634

Re: In defence of swap: common misconceptions

#116

Post by _ph_ »

cosarara97 wrote: post #36

In my experience, a misbehaving linux system that's out of RAM and has swap to spare will be unusably slow. The process of switching to a tty, logging in, and killing whatever the offending process is can easily take a good 15 minutes. Xorg will just freeze. Oh, and hopefully you know what process it is, else good luck running `top`. Until this is fixed, I'll just keep running my systems with very small amounts of sw…

The point being is, that a system doesn't have to misbehave to allocate more memory than the total RAM. And in those cases, there is a very good reason to have swap space, and swapping won't impact the performance of the system - rather the opposite.

Re: In defence of swap: common misconceptions

#117

Post by viraptor »

black_puppydog wrote: post #81

Without wanting to impose my method or reasoning here, I run my dev machine without swap, and I'd rather have the same for the cluster machines I access. This is for academic use only. I know how much RAM my machine has, and if I oom, it usually isn't because I tried to squeeze in just a tiny bit too much data, but rather because I made some stupid mistake and keep allocating small chunks of memory very rapidly. On a…

If you're running a single-(or close to)-purpose machine, then seeing an explicit memory usage limit on the main app could give you even better/faster results.

Re: In defence of swap: common misconceptions

#118

Post by cdown »

Aardwolf wrote: post #38

In defence against swap on my personal computer: -PCs have a lot of RAM now -When you allocate that much memory it's usually a bug in your own code like a size_t that overflowed. I never saw programs I would actually want to use try to allocate that much -When using swap instead of ram, everything becomes so slow that you're screwed anyway. The UI doesn't even respond fast enough to kill whatever tries to use all tha…

> I never saw programs I would actually want to use try to allocate that much

What about applications that have memory-bound performance characteristics? In these cases, saving a bit of memory often directly translates into throughput, which translates into $$$.

This isn't a theoretical, a bunch of services which I've run in the past and currently literally make more money because of swap. By using memory more efficiently and monitoring memory pressure metrics instead of just "freeness" (which is not really measurable anyway), we allow more efficient use of the machine overall.

Re: In defence of swap: common misconceptions

#119

Post by cdown »

vardump wrote: post #103
kartickv wrote: post #74

How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps that aren't being used, so that the system remains responsive no matter what? Or, worst case, if two tasks that need 8GB each are running on a machine with 8GB memory, kill one, let the other finish, and restart the first one. Or, less ambitious, freeze one, swap it out to disk, let the other finish, and only then…

> How long before desktop OSs manage memory like mobile ones, automatically shutting down background apps Once desktop systems and applications support required APIs to handle saving state before being shut down.

> Once desktop systems and applications support required APIs to handle saving state before being shut down.

SIGTERM and friends? :-)

If your application is just dropping state on the floor as a result of having an intentionally trappable signal being sent to it or its children, that seems like a bug.

Re: In defence of swap: common misconceptions

#120

Post by viraptor »

I feel like the real metric is like to see from swap usage is: how much time did I spend waiting to swap-in a page, how much extra cache pages it gave me, and what's the cache hit ratio. If the big purpose is to allow those extra few pages to be available, then either it's worth doing or not - there should be an objective way to look at this. Unfortunately only the second and third part is easily available. The first... maybe via systemtap?
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