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804 points jryio | 2 comments | 21 Oct 25 20:28 UTC | HN request time: 0s | source

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speedgoose ◴[21 Oct 25 21:12 UTC] No.45661785[source]▶

Looking at the htop screenshot, I notice the lack of swap. You may want to enable earlyoom, so your whole server doesn't go down when a service goes bananas. The Linux Kernel OOM killer is often a bit too late to trigger.

You can also enable zram to compress ram, so you can over-provision like the pros'. A lot of long-running software leaks memory that compresses pretty well.

Here is how I do it on my Hetzner bare-metal servers using Ansible: https://gist.github.com/fungiboletus/794a265cc186e79cd5eb2fe... It also works on VMs.

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levkk ◴[21 Oct 25 21:51 UTC] No.45662183[source]▶

>>45661785 #

Yeah, no way. As soon as you hit swap, _most_ apps are going to have a bad, bad time. This is well known, so much so that all EC2 instances in AWS disable it by default. Sure, they want to sell you more RAM, but it's also just true that swap doesn't work for today's expectations.

Maybe back in the 90s, it was okay to wait 2-3 seconds for a button click, but today we just assume the thing is dead and reboot.

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KaiserPro ◴[21 Oct 25 22:22 UTC] No.45662472[source]▶

>>45662183 #

Yeahna, thats just memory exhaustion.

Swap helps you use ram more efficiently, as you put the hot stuff in swap and let the rest fester on disk.

Sure if you overwhelm it, then you're gonna have a bad day, but thats the same without swap.

Seriously, swap is good, don't believe the noise.

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adastra22 ◴[21 Oct 25 22:43 UTC] No.45662672[source]▶

>>45662472 #

I don’t understand. If you provision the system with enough RAM, then you can for every page in RAM, hot or not.

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akvadrako ◴[21 Oct 25 23:16 UTC] No.45663000[source]▶

>>45662672 #

Only if you have more RAM than disk space, which is wasteful for many applications.

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adastra22 ◴[21 Oct 25 23:36 UTC] No.45663147[source]▶

>>45663000 #

Running out of memory kills performance. It is better to kill the VM and restart it so that any active VM remains low latency.

That is my interpretation of what people are saying upthread, at least. To which posters such as yourself are saying “you still need swap.” Why?

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eru ◴[22 Oct 25 00:03 UTC] No.45663366[source]▶

>>45663147 #

RAM costs money, disk space costs less money.

It's a bit wasteful to provision your computers so that all the cold data lives in expensive RAM.

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fluoridation ◴[22 Oct 25 00:08 UTC] No.45663411[source]▶

>>45663366 #

>It's a bit wasteful to provision your computers so that all the cold data lives in expensive RAM.

But that's a job applications are already doing. They put data that's being actively worked on in RAM they leave all the rest in storage. Why would you need swap once you can already fit the entire working set in RAM?

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vlovich123 ◴[22 Oct 25 00:40 UTC] No.45663646[source]▶

>>45663411 #

Because then you have more active working memory as infrequently used pages are moved to compressed swap and can be used for more page cache or just normal resident memory.

Swap ram by itself would be stupid but no one doing this isn’t also turning on compression.

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1. fluoridation ◴[22 Oct 25 16:54 UTC] No.45671937{8}[source]▶

>>45663646 #

>Because then you have more active working memory as infrequently used pages are moved to compressed swap and can be used for more page cache or just normal resident memory.

Uhh... A VMM that swaps out to disk an allocated page to make room for more disk cache would be braindead. The process has allocated that memory to use it. The kernel doesn't have enough information to deem disk cache a higher priority. The only thing that should cause it to be swapped out is either another process or the kernel requesting memory.

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2. vlovich123 ◴[23 Oct 25 02:37 UTC] No.45677588[source]▶

>>45671937 (TP) #

> A VMM that swaps out to disk an allocated page to make room for more disk cache would be braindead

Claiming any decision is “brain dead” in something as heuristic heavy and impossible to compute optimally as resident memory pages is quite the statement to make; this is a form of the knapsack problem (NP-complete at least) with the added benefit of time where the items are needed in some specific indeterminate order in the future and there’s a whole bunch of different workloads and workload permutations that alter this.

To drive this point home in case you disagree, what’s dumber? Swapping out to disk an allocated page (from the kernel’s perspective) that’s just sitting in the free list of the userspace allocator for that process or a page of some frequently accessed page of data?

Now, I agree that VMMs may not do this because it’s difficult to come up with these kinds of scenarios that don’t penalize the general case, more importantly than performance this has to be a mechanism that is explainable to others and understandable for them. But claiming it’s a braindead option to even consider is IMHO a bridge too far.

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