Comment by Dayshine
7 years ago
I'm on Linux right now and can cause this to happen by using more than 60% of my RAM. Afaik no distros correctly handle swapping onto a HDD.
If I hit 80%, I get 10-20 second lock ups.
If I hit >95%, I get 1-2 minute lock ups.
Using Ubuntu 18.04.
What is the "correct" handling of swap on an HDD supposed to be like? It is going to be slow no matter what you do. Windows also locks up for long periods of time if you use up almost all the RAM and it has to swap to HDD.
When I say lock up I mean the UI completely stops. As in, my i3 bar stops updating for several minutes. Not even the linux magic commands let me recover.
On windows things may be unresponsive, but at least ctrl-alt-del responds, and at least the mouse moves!
The main difficulty is I can't tell if my machine has crashed vs is overloaded if the UI doesn't do anything for several minutes.
> Not even the linux magic commands let me recover.
Are you sure you've set up the magic sysctls correctly? Ubuntu ships with magic oomkill disabled, by default.
On all machines I've tried, whenever I've needed it, magic oomkill has always worked, and I've been thankful of the fact that it's implemented down in the kernel.
If you renice the UI processes to a higher-than-normal priority, it should work more like it does in Windows. (This used to be somewhat risky, but today Linux UI is not going to hog your system resources.) The underlying problem is that when memory is tight, Linux starts evicting "clean" pages from the page cache that actually will need to be accessed shortly afterwards (i.e. they're part of the working set of some running process), and thrashing occurs. There's no easy solution to this issue, other than making user programs more responsive to memory pressure in the first place. (This could extend as far as some sort of seamless checkpoint+resume support, like what you see in mobile OS's today.)
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"Correct" handling of swap would mean mostly leaving the window manager and its dependencies in memory. Individual application windows may stop responding, but everything else should be pretty quick. And small things like terminal emulators should get priority to stay in ram too.
Not that I'd recommend this, but my work MBP has 16GB of ram, and my typical software development setup (JVM, IntelliJ, Xcode, gradle) easily uses up 30GB. It swaps a lot but generally OSX does a good job of keeping the window manager and foreground applications at priority so I can still use my machine while this is happening.
I attribute this to the fact that the darwin kernel has a keen awareness of what threads directly affect the user interface and which do not (even including the handling of XPC calls across process boundaries... if your work drives the UI, you get scheduling/RAM priority). I don't think the linux kernel has nearly this level of awareness.
> ... the darwin kernel has a keen awareness of what threads directly affect the user interface and which do not (even including the handling of XPC calls across process boundaries... if your work drives the UI, you get scheduling/RAM priority). I don't think the linux kernel has nearly this level of awareness.
You're talking about priority inheritance in the kernel. In Linux, this is in development as part of the PREEMPT_RT ("real-time") patches, already available experimentally in a number of distributions.
Install earlyoom, it will kill processes before a lock up occurs. Greatly helped me (at the expense of random chrome tabs killed)
I usually set vm.swappiness to about 20.