In defence of swap: common misconceptions

Post by Asooka » Mon, Jan 15, 2018, 2:16 PM UTC

wilun wrote: post #33

slaymaker1907 wrote: post #26

I recently reenabled swap on my Windows machine due to frequent OOM, even with 16GB of RAM while playing Overwatch and browsing on Firefox. It seems like both of these programs allocate vast swaths of memory but then do not actually use that memory very heavily. After I turned swap back on, I did not notice any degradation in performance but my system stability skyrocketed.

Windows has vastly different policies for RAM allocation and commit than Linux. Windows basically does not overcommit while Linux systems not only does it by default but quite depend on it for various loads to work properly. In consequence, the userspace has a tendency to handle RAM differently, but there is no magic: if programs are trying to allocate twice the amount of RAM and then only use half of it, Windows wit…

Post by Anderkent » Mon, Jan 15, 2018, 3:21 PM UTC

clarry wrote: post #128

Anderkent wrote: post #11

Earlier quoted context omitted.

> So, in other words, if you have enough memory for your workload that you won't run out, there's no benefit to having swap space (i.e. you've wasted money on memory you don't need). No, that's the opposite. If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache (instead of keeping unaccessed anonymous pages in real ram). Unless by "won't run out" you mean…

> If you have enough memory for your workload that you won't run out, swapping lets you use more memory for disk cache Except that it doesn't happen in practice, on my systems anyway. If you have plenty of memory, you can keep all your programs in it and as much as the system wants to cache and still not run out. The theory says that swap effectively buys you some memory to spend on more important things (than what t…

Post by tmyklebu » Mon, Jan 15, 2018, 4:48 PM UTC

mikekchar wrote: post #121

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.…

Feel free to explain it to me. " Under no/low memory contention [...] Without swap: We cannot swap out rarely-used anonymous memory, as it’s locked in memory. While this may not immediately present as a problem, on some workloads this may represent a non-trivial drop in performance due to stale, anonymous pages taking space away from more important use." Now imagine that I have no memory contention. In other words I'…

Post by clarry » Mon, Jan 15, 2018, 5:19 PM UTC

Anderkent wrote: post #134

clarry wrote: post #130

Earlier quoted context omitted.

> Sure, but why wouldn't you run some swap with your much bigger ram anyway? If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? > In the end the core idea is: sometimes you have anonymous memory that is accessed so rarely that you'd rather have an extra disk cache page. That's the theory. In practice I always have mor…

>If you don't need it, you don't need it. The other question is: how much swap exactly should I have? And why wouldn't I just add that much RAM instead? Because adding RAM costs money, adding swap space is a config setting. It looks to me that your boxes are idle. Sure, if you're not doing any work, it doesn't matter... >1) Bad assumption 2) it doesn't help you either, so why bother? Actually I might have a use for t…

Post by AstralStorm » Mon, Jan 15, 2018, 5:35 PM UTC

_ph_ wrote: post #116

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.