Low RAM condition makes SSD run like crazy, even when swapping is switched off?
by browny_amiga from LinuxQuestions.org on (#4XDVS)
This has stumped me for years, I notice that all my Linux systems, servers and desktops, enter into a quasi denial of service condition when RAM is almost full: The harddisk / SSD runs like crazy, the system gets slower and slower and is basically unusable, the loadavg goes up to 20 or higher (on a quad core system) and it is not even possible to terminate apps anymore to free up some RAM. Everything becomes EXTREMELY slow.
Now I have had this more than 10 years ago and noticed that swap memory was totally useless in a situation like this, but made things much worse: An app with a memory leak takes more and more RAM, more and more RAM is swapped, which makes the system slower, slower and slooooooower, which in the end defacto crashes the whole system (denial of service), i.e. makes it unusable, where you can't even restart anymore (you wait 10 minutes for a login, starting any tool in CLI takes 30 mins)
So I switched off swap, and things got better. There is a useful Kernel functionality called the OOM, the out of memory killer, which will forcefully free up memory by killing (which?) an app.
This kicks in much earlier than with swap, because Swap gives the system the false illusion that there is still plenty of RAM.
BUT the OOM, in the condition I describe here (without swap) STILL takes a looooooooooooooooong time to kick in, I have left the SSD trashing for 2 hours before it finally took action. WHAT is using the SSD? What is being written / read in this crazy condition? It is a big mystery for me, so far I have not been able to glimpse it.
Anybody has some insights what is happening here?
Now I have had this more than 10 years ago and noticed that swap memory was totally useless in a situation like this, but made things much worse: An app with a memory leak takes more and more RAM, more and more RAM is swapped, which makes the system slower, slower and slooooooower, which in the end defacto crashes the whole system (denial of service), i.e. makes it unusable, where you can't even restart anymore (you wait 10 minutes for a login, starting any tool in CLI takes 30 mins)
So I switched off swap, and things got better. There is a useful Kernel functionality called the OOM, the out of memory killer, which will forcefully free up memory by killing (which?) an app.
This kicks in much earlier than with swap, because Swap gives the system the false illusion that there is still plenty of RAM.
BUT the OOM, in the condition I describe here (without swap) STILL takes a looooooooooooooooong time to kick in, I have left the SSD trashing for 2 hours before it finally took action. WHAT is using the SSD? What is being written / read in this crazy condition? It is a big mystery for me, so far I have not been able to glimpse it.
Anybody has some insights what is happening here?