Problem: I had a process using 90% of cpu, couldn't understand what was going on.

After seeking the log files and other stuff, I still couldn't undestand why it was taking so much CPU.

I decided to give a look at what files are used by this process.

It's so easy

first, find the PID of your process. My preferred way to do it is with the command:

$ ps -ef | grep <command_name>

you'll see an output like this:

$ ps -ef | grep <command>
root     13141     1 69 10:36 ?        02:52:34 /path/to/<command>
admin    31182 30798  0 14:45 pts/2    00:00:00 grep --color=auto <command>

The pid of this process is 13141.

And now let's get the list of files blocked by this process.

$ ls -al /proc/13141/fd 

The system might ask you to run this command as privileged user, so don't disregard using sudo.

output:

total 0
dr-x------ 2 root root  0 Jun 19 12:12 .
dr-xr-xr-x 8 root root  0 Jun 19 10:36 ..
lrwx------ 1 root root 64 Jun 19 12:12 0 -> /dev/null
lrwx------ 1 root root 64 Jun 19 12:12 1 -> /dev/null
lr-x------ 1 root root 64 Jun 19 14:30 10 -> /run/sdk/user.uid.db
lr-x------ 1 root root 64 Jun 19 14:30 11 -> /run/sdk/user.shadow.db
lrwx------ 1 root root 64 Jun 19 12:12 2 -> /dev/null
lrwx------ 1 root root 64 Jun 19 12:12 3 -> socket:[2342197]
lrwx------ 1 root root 64 Jun 19 12:12 4 -> socket:[2342201]
lr-x------ 1 root root 64 Jun 19 12:12 5 -> anon_inode:inotify
lr-x------ 1 root root 64 Jun 19 12:12 6 -> /backup_rsync/backup/20170825-1225/lib/azure-cli/lib/python2.7/site-packages/azure/mgmt/network/v2017_03_01/models
lr-x------ 1 root root 64 Jun 19 12:12 7 -> /run/sdk/lock/lock_pw
lr-x------ 1 root root 64 Jun 19 14:30 8 -> /run/sdk/user.name.db
lr-x------ 1 root root 64 Jun 19 14:30 9 -> /run/sdk/user.misc.db

Apart from a lot of stuff that seems creepy (/dev/null, socket:\\\[...\\\]), I can clearly spot a file used by my process: the one starting with /backup_rsync.

Running this command other two-three times, I understood that it is not hung, it is just scanning every file in my system and checking against a pattern. Very poor design.

Hope this helps!