High System CPU usage

After upgrading a webserver running Apache to Debian Jessie (from Wheezy), I noticed that the system CPU usage was higher.  Running an strace on one of the Apache processes was giving me very little info:

strace -c -p 10112
Process 10112 attached - interrupt to quit
^CProcess 10112 detached
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
95.71 28.549784 3172198 9 4 futex

I had previously tried to spread interrupts across cores and also limited network activity – as they are something one can do to work out why System CPU is high – but they didn’t bring down the CPU usage.

One culprit remained: Futex/Mutex locks. I changed the default Mutex [1] to file and magically the System CPU usage went down.

[1] Apache 2.4 Mutex doc

Checking for the existence of domains from commandline in Bash

while read line; do wget --quiet --spider --timeout=10 $line; if [ $? -ne 0 ]; then echo $line; fi; done < "/tmp/domains.txt" 
/tmp/domains.txt would hold the list of domain names separated by newline.
Note this is just a quick hack and not entirely reliable.

Unix find, xargs and basename.

Xargs is great. If you want to know why use xargs over exec, have a read of this: xargs versus exec.

But, there is one problem. If the command that you are trying to execute on the results from find cannot take more than one file as param (basename for e.g, can take only one file), then use the -n 1 param with xargs (to process one file at a time) . In that case, xargs is probably going to be as quick as using exec.

find . -name "*.rar" | xargs -n 1 unrar e
find . -name "*.rar" | xargs -n 1 basename

And this will get the basename without using the basename function (I go it from this page).

find . -name "*.rar" -printf "%fn"

Munin: Could not open required defaults file: /root/.my.cnf

For some reason I kept getting this error (even when the file was readable by all) in one of the munin (mysql slave) plugins that I had written.

So this was the solution that worked for me;

under the [mysql*] section in /etc/munin/plugin-conf.d/munin-node 
I added this: 
env.mysqlopts -u{user} -p{password}

and in my plugin

I did something like this:

mysql ${mysqlopts} -e'show slave status G'

and that seemed to have picked up the user and password from env.mysqlopts and the plugin started working (ofcourse after I restarted munin).