If you use sudo to run commands as root, you've probably run into “permission denied” problems when only part of a pipeline or part of a command is running with root permissions.
This fails with “permission denied” because the file is writable only by root:
$ echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
But, this fails too:
$ sudo echo 12000 > /proc/sys/vm/dirty_writeback_centisecs
Why? The /bin/echo program is running as root, because of sudo, but the shell that's redirecting echo's output to the root-only file is still running as you. Your current shell does the redirection before sudo starts.
The solution is to run the whole pipeline under sudo. There are a couple ways to do it, but I prefer:
echo "echo 12000 > /proc/sys/vm/dirty_writeback_centisecs" | sudo sh
That way, I can type everything before the pipe character, and see what I'm about to run as root, then press the up arrow and add the | sudo sh to do it for real. This is not a big deal for short, obvious pipelines, but when you're building up a more complicated command as root, it's safer to look at it first before you run it.
To see the interrupts occurring on your system, run the command:
# watch -n1 "cat /proc/interrupts" CPU0 CPU1 0: 330 0 IO-APIC-edge timer 1: 11336 0 IO-APIC-edge i8042 4: 2 0 IO-APIC-edge 6: 3 0 IO-APIC-edge floppy ... NMI: 0 0 Non-maskable interrupts LOC: 5806923 6239132 Local timer interrupts ...
The -n1 option passed to watch causes the specified command to be re-run every second.