words of wisdom from a systems engineer

Tap into your Linux system with SystemTap

One of the most interesting topics I’ve seen so far during my RHCA training at Rackspace this week is SystemTap. In short, SystemTap allows you to dig out a bunch of details about your running system relatively easily. It takes scripts, converts them to C, builds a kernel module, and then runs the code within your script.

HOLD IT: The steps below are definitely not meant for those who are new to Linux. Utilizing SystemTap on a production system is a bad idea — it can chew up significant resources while it runs and it can also cause a running system to kernel panic if you’re not careful with the packages you install.

These instructions will work well with Fedora, CentOS and Red Hat Enterprise Linux. Luckily, the SystemTap folks put together some instructions for Debian and Ubuntu as well.

Before you can start working with SystemTap on your RPM-based distribution, you’ll need to get some prerequisites together:

yum install gcc systemtap systemtap-runtime systemtap-testsuite kernel-devel
yum --enablerepo=*-debuginfo install kernel-debuginfo kernel-debuginfo-common

WHOA THERE: Ensure that the kernel-devel and kernel-debuginfo* packages that you install via yum match up with your running kernel. If there’s a newer kernel available from your yum repo, yum will pull that one. If it’s been a while since you updated, you’ll either need to upgrade your current kernel to the latest and reboot or you’ll need to hunt down the corresponding kernel-devel and kernel-debuginfo* packages from a repository. Installing the wrong package version can lead to kernel panics. Also, bear in mind that the debuginfo packages are quite large: almost 200MB in Red Hat/CentOS and almost 300MB in Fedora.

You can’t write the script in just any language. SystemTap uses an odd syntax to get things going:

#! /usr/bin/env stap
probe begin { println("hello world") exit () }

Just run the script with stap:

# stap -v helloworld.stp
Pass 1: parsed user script and 73 library script(s) using 94380virt/21988res/2628shr kb, in 140usr/30sys/167real ms.
Pass 2: analyzed script: 1 probe(s), 1 function(s), 0 embed(s), 0 global(s) using 94776virt/22516res/2692shr kb, in 10usr/0sys/5real ms.
Pass 3: using cached /root/.systemtap/cache/bc/stap_bc368822da380b943d4e845ee15ed047_773.c
Pass 4: using cached /root/.systemtap/cache/bc/stap_bc368822da380b943d4e845ee15ed047_773.ko
Pass 5: starting run.
hello world
Pass 5: run completed in 0usr/20sys/285real ms.

The systemtap-testsuite package gives you a tubload of extremely handy SystemTap scripts. For example:

# cd /usr/share/systemtap/testsuite/systemtap.examples/io/
# stap iotime.stp
15138470 6351 (httpd) access /usr/share/cacti/index.php read: 0 write: 0
15142243 6351 (httpd) access /usr/share/cacti/include/auth.php read: 0 write: 0
15143780 6351 (httpd) access /usr/share/cacti/include/global.php read: 0 write: 0
15144099 6351 (httpd) access /etc/cacti/db.php read: 0 write: 0
15187641 6351 (httpd) access /usr/share/cacti/lib/adodb/ read: 106486 write: 0
15187664 6351 (httpd) iotime /usr/share/cacti/lib/adodb/ time: 218
15194965 6351 (httpd) access /usr/share/cacti/lib/adodb/ read: 0 write: 0
15195692 6351 (httpd) access /usr/share/cacti/lib/adodb/ read: 0 write: 0
   ... output continues ...

The iotime.stp script dumps out the reads and writes occurring on the system in real time. After starting the script above, I accessed my cacti instance on the server and immediately started seeing some reads as apache began picking up PHP files to parse.

Consider a situation in which you need to decrease interrupts on a Linux machine. This is vital for laptops and systems that need to remain in low power states. Some might suggest powertop for that, but why not give SystemTap a try?

# cd /usr/share/systemtap/testsuite/systemtap.examples/interrupt/
# stap interrupts-by-dev.stp
        ohci_hcd:usb3 :      1
        ohci_hcd:usb4 :      1
            hda_intel :      1
                 eth0 :      2
                 eth0 :      2
                 eth0 :      2
                 eth0 :      2
                 eth0 :      2
                 eth0 :      2

On this particular system, it’s pretty obvious that the ethernet interface is causing a lot of interrupts.

If you want more examples, keep hunting around in the systemtap-testsuite package (remember rpm -ql systemtap-testsuite) or review the giant list of examples on SystemTap’s site.

Thanks again to Phil Hopkins at Rackspace for giving us a detailed explanation of system profiling during training.