Delving the depths of Linux with sysdig

Big Dig

© Lead Image © Stanislav Komogorov,

© Lead Image © Stanislav Komogorov,

Article from Issue 168/2014

Many Linux diagnostic tools require knowledge of a special syntax, which complicates handling and confuses the output. Sysdig groups several important tools into a single interface.

On a modern Linux system, numerous processes often run simultaneously. Several applications might be running at once, and each application opens files, writes data, reads data, closes files, and so on. All this activity stresses the CPU, which can lead to bottlenecks that can slow down the entire system.

System administrators use tools such as top, ps, vmstat, strace, and lsof to find and fix these bottlenecks. The output of the tools often serves as input for other tools, which often leads to complex and confusing situations.

Sysdig [1] cleans up some of that confusion. The sysdig developers grouped the commands they used most frequently and equipped the tool with a programmable interface. Sysdig understands a large number of options that control specific properties. (You can try the sysdig --help command for a list of options.)

Sysdig's line-by-line output consists of several parts, or fields. The first two fields, evt.num and evt.time clearly identify the described event with a number and the date on which the software registered it. Additionally, evt.cpu describes the involved CPU for systems with multiple CPUs.

The field stands for the process, thread.tid for the thread. The software uses evt.dir to tell the user how the event works: < stands for incoming data and > for outgoing. The evt.type field classifies the results themselves as, for example, read or open. Last, but not least, the evt.args field summarizes the event arguments.


You can install the current 1.82 version of the program from the repository under Arch Linux. For other distributions, the manufacturer offers a somewhat unorthodox method for setting up the software: You use a script downloaded by curl that you run in Bash:

curl -s | sudo bash

The script automatically detects the operating system and sets up the appropriate version. It currently supports Debian from version 6.0, Ubuntu 10.04 onward, CentOS from v6, RHEL from v6, Fedora from v13, and Linux Mint from v9. If it does not work with older versions of Ubuntu, install Sysdig with the following commands:

curl -s | sudo apt-key add -
curl -s -o /etc/apt/sources.list.d/draios.list
sudo apt-get update

The string in the first line downloads the repository's public key and installs it; line 2 registers the repository in a file in /etc/apt/sources.list.d/. After the obligatory update (line 3), the packages are ready for selection in Synaptic. You then need to generate and install a special kernel module. The kernel headers can be installed with the following command:

apt-get -y install

For more details on installation, refer to the online documentation [2]. Matching versions for Windows and Mac OS X also are available to download.


If you start sysdig as root without options, you will instantly see output in the form shown in Listing 1. To exit this mode, type Ctrl+C.

Listing 1

sysdig Output


The first recorded event is labeled number 3 and comes from the systemd-journal process It follows one of many processes signified as plugin-containe (the output truncates the "r"), the embedded Flash player in Firefox, and then Firefox itself, which generates an array of events. Of interest, among other things, is that Firefox uses a different CPU than the container. In the last row of the listing (with the data= string), you can see a number of dots. Sysdig writes them to represent non-printable characters in the output. If necessary, you can change this behavior using options like -A, which tells the program to output only ASCII characters.

Because sysdig can register almost 20,000 events a second, it is evident that meaningful use of the software requires a powerful filter to restrict output to the desired events. You append the details for filtering to the command as options. Listing 2 shows how to reduce the output to the read event.

Listing 2

Filtering Events



The system does not directly reveal very many complex details (e.g., processes with the most inputs or outputs), so you will need to determine this information by aggregating data and using statistical methods. This, and much more, is done through what are known as chisels, which are 2KB Lua scripts that sysdig activates via the -c <chisel name> option.

This is how the example in Listing 3 analyzes the slowest system calls. To begin, sysdig collects data and doesn't stop until you stop the program, when the collected data and its output are analyzed. Again, Flash player stands out: Besides the Java program, it consumes the most resources and slows down the system the most.

Listing 3

Finding Bottlenecks


Many chisels require additional arguments – perhaps a monitored IP address or a port – specified directly after the chisel, say

sysdig -c spy_ip <IP address>

During the installation, sysdig copies the chisels into the /usr/share/sysdig/chisels/ directory. Thanks to the relatively simple structure, they are suitable as templates for your own development.The sysdig-cl selection shows existing chisels organized in six categories, allowing you to:

  • examine CPU and network workload,
  • determine throughput,
  • analyze performance of the entire system,
  • make security checks, and
  • drive error analysis.

Most categories include several different variants of chisels, which allow special predictions.


As mentioned, sysdig lets you restrict output to the most relevant information. To do this, specify the events relevant for you at the command line:


Out of the many events, Table 1 summarizes a few of the most common. Typing

sysdig -l

shows all supported events.

Table 1

Key Fields




Number of file descriptors


Type of file descriptors

Path or connection (for sockets)



File name without path

PID of the producing process


Name and path of the producing process


Command line of the producing process


Thread ID of the producing thread


Total CPU time of the active threads


Number of events


Event timestamp


Event timestamp (absolute, in nanoseconds)


Type of event


All the arguments in one string


Array with arguments


Binary buffers


Return value of the event


Various I/O events


User ID to which the generating process belongs

Username to which the generating process belongs


Associated user's home directory

Shell of the producing process


Various latency values

If you combine multiple fields with the logical expression and, they act as variables for the outputs, or you can limit them using contains <pattern>. In this case, the specified pattern must occur in the data of the field for sysdig to output them. The focus is often only a certain range of values, especially with numeric data. You can limit these more accurately, if necessary (see Table 2).

Table 2





Equal to


Not equal to


Less than the specified threshold


More than the specified threshold


Same or less than the specified threshold


More or less than the specified threshold

Another possibility is grouping the operators using parentheses, and negating them using not. There is also a logical OR; however, this presupposes that you double quote the expressions in the shell. An example from the documentation shows this:

sysdig "not ( contains /proc or contains /dev)"

It is not always necessary or sensible to analyze the data collected by sysdig directly. Sometimes, it makes sense to cache the data obtained initially and analyze it subsequently in different ways.

You can save the unfiltered output in a file using the -w <file> option. Developers recommend the .scap suffix for these files. Selecting sysdig -r <file> reads all collected data; you can append the desired filter options to the command line.

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