If I have been logged into the Ubuntu 18.04 graphical console for a while and run either:
echo 2 >| /proc/sys/vm/drop_caches
echo 3 >| /proc/sys/vm/drop_caches
the process takes almost 2 minutes, whereas it is supposed to be nearly instantaneous. This in turn causes 'logout' to hang for more than a minute. At the same time the CPU pegs at 100%.
This happens even if run the commands repeatedly. However, echoing
1 is almost instantaneous.
If I log out of the graphical console, then
dump_cache becomes almost instantaneous. If I log back in to the graphical console,
dump_cache starts taking a couple of seconds but over time will revert to 1-2 minutes again -- not sure though what triggers that change.
I don't have anything running in the console itself other than 2 xterm windows and whatever background processes launch automatically with Ubuntu 18.04
More generally, I am not anything other than a few screen sessions, ssh client sessions, and emacs.
Free shows very little memory or cache usage:
total used free shared buff/cache available
Mem: 39G 4.7G 33G 209M 1.3G 33G
Swap: 0B 0B 0B
Root and system files are stored on SSD so writing cached files should be super-quick.
CPU usage as shown by
htop is < 10% on all 4 processors.
- Any idea what could be causing this?
- And even if the first dump_cache takes a while due to accumulated 'stuff', why would a second dump_cache that follows immediately take the same amount of time?
- What could possible be taking almost 2 minutes to dump given that there are <2GB of cache and my disk is a fast Samsung SSD?