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In defence of swap: common misconceptions

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Re: In defence of swap: common misconceptions

#81

Post by black_puppydog »

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 system with even a moderate amount of swap, this makes everything grind to a halt, and it is usually much faster to just reboot the machine and deal with the problems later in the unlikely event that rebooting actually causes problems.

Re: In defence of swap: common misconceptions

#82

Post by leoc »

I'd be more convinced by the argument that swap shouldn't be thought of as slow RAM if the author addressed the fact that it's generally known as 'virtual memory'—and it has been since at least System 370, so it's not simply a later misconception: http://pages.cs.wisc.edu/~stjones/proj/vm_reading/ibmrd2505M... . Instead the article just omits the term 'virtual memory' completely, and pretty conspicuously.

I also think that a convincing case for swap would have to discuss the concepts of latency, interactivity, and (soft) real-time performance, things that largely weren't to the fore in the salad days of the 370 family or the VAX. Virtual memory is the TCP of local storage.

Re: In defence of swap: common misconceptions

#83

Post by kuschku »

kam wrote: post #77
vanni wrote: post #59

Earlier quoted context omitted.

You can use Alt+SysRq+f to manually call oom_kill.

On many distros, this is disabled by default because there's a chance that the OOM killer will hit something important, like the screen lock. For Ubuntu, enable it in /etc/sysctl.d/10-magic-sysrq.conf.

If you’re using logind, this isn’t a problem – if the screenlocker dies, it will be restarted up to 3 times (without revealing screen content), and if it is killed yet once more, your screen just won’t unlock (you can then unlock via a tty by logging into that and using loginctl session-unlock).

If the daemon responsible for this is killed, all your sessions will simply be killed.

In no situation will your screen unlock due to a process being killed (in contrast to the pre-logind world, where, if the screenlocker dies, your screen is free for all, as there the locker is just a fullscreen window)

Re: In defence of swap: common misconceptions

#84

Post by carver »

loeg wrote: post #5

All of this boils down to "because buying more disk is cheaper than buying more RAM" and "avoid memory contention." The author discusses the situation as if the quantity of RAM is fixed and swap can be added (or not). But that isn't the only possibility — you can also add more RAM (it's just expensive). For the same number of GB of RAM+swap vs just RAM, there is no reason to prefer the option with swap.

Agreed, in most situations.

One counter example:

If your processes in sum tend to A) access many disk locations, at large total disk space B) hold a lot of underused data in ram

This isn't totally impossible. Maybe an Ethereum node with a script doing a bunch of data reads, running side by side with hundreds of Chrome tabs, few of which are regularly accessed. (Totally hypothetical, of course...)

Swapping some rarely used ram out so the OS can buffer disk into ram seems like a reasonable approach (although maybe even more reasonable is: close some tabs).

Your point still stands that more ram is strictly as good or better performance in this scenario, but you might be able to get an equivalent performance boost much more cheaply with some swap space for the underused ram. Also, upgrading past 32 GB ram starts to veer from expensive to impossible on a laptop.

Re: In defence of swap: common misconceptions

#85

Post by b4lancesh33t »

mort96 wrote: post #39
CoconutPilot wrote: post #28

Swap was a great idea, but its time is gone. Swap doesn't make sense anymore, hard drives have not scaled and kept up with the improvements in RAM. In the Pentium 1 era EDO RAM maxed out at 256MB/s and hard disk xfer was 10MB/s. Common RAM size was 16MB. In today's era DDR4 maxes out at 32GB/s and hard disk xfer is 500 MB/s. Common RAM size is 16GB. RAM xfer rate has grown is 320x. RAM capacity has grown 100x. Disk x…

There have been times when I have needed to do something which requires more memory than is physically available on the machine. Without swap, those tasks would have been literally impossible to do without upgrading the machine. With swap, it just takes a little longer - but still much shorter than it would take to order more RAM sticks. One of those times was on a 512MB RAM VPS, where I needed to compile something -…

Well, they wouldn't have been literally impossible. Anything that can be done with swap can be done by writing the data to disk and then reading it back. That's all swap is doing.

Re: In defence of swap: common misconceptions

#86

Post by Karunamon »

petecox wrote: post #53
kzrdude wrote: post #50

Earlier quoted context omitted.

What function does the 0.5GB swap have?

I just wish Linux distro installers would make opting out the default option; no, I don't want to swap on my SSD. The last time I installed a distro, I still had to select the manual option for partitioning. With an 8 Gig stick in my NUC, for normal desktop usage it never goes above 3.

And even then; they provision an absurd amount of it. I just did a fresh Ubuntu install. On a machine with 32 gigs ram, it creates a 32 gig swap partition by default!

Re: In defence of swap: common misconceptions

#87

Post by quotemstr »

WalterBright wrote: post #79
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.…

> self storage is plenty useful With self-storage rising to over $300 per month, it's more cost effective to take the stuff to the dump and buy it again if it is ever needed.

Well, it depends where you live. In Buffalo, you can get self-storage for around $25/month, if internets are to be believed.

Re: In defence of swap: common misconceptions

#88

Post by phire »

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…

I actually did this on a old laptop, setup 200mb of swap for 4gb of ram.

And it caused huge problems for me, would run out of swap while having plenty of free memory and then go cripplingly slow.

Re: In defence of swap: common misconceptions

#89

Post by codesnik »

perlgeek wrote: post #34

> Under temporary spikes in memory usage > With swap: We’re more resilient to temporary spikes, but in cases of severe memory starvation, the period from memory thrashing beginning to the OOM killer may be prolonged. We have more visibility into the instigators of memory pressure and can act on them more reasonably, and can perform a controlled intervention. Somehow that doesn't resonate with my experience. I tend to…

I remember not being able to SSH on the box too, but this stopped be the case (for me at least) some five years ago - I was able to login to heavy swap trashing boxes without any problem. I thought that something was changed in contemporary linux distros. Either login shell was given much more priority or something like that.

Re: In defence of swap: common misconceptions

#90

Post by deathanatos »

sigstoat wrote: post #68
tmyklebu wrote: post #52

Earlier quoted context omitted.

If I may ask, which disks are you using?

the highest end NVMe SSDs can sustain those multi-gigabyte/second reads. https://smile.amazon.com/Samsung-950-PRO-Internal-MZ-V5P512B... for instance. not sure even they could pull off those speeds with random reads, though.

I think when most people say "hard disk", they usually mean rotating discs that use magnetism to store data. That is what I took tmyklebu's question to mean, since I too have never heard of a HDD reaching anywhere near 2 GB/s.
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