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<FONT COLOR="#FFFFFF" SIZE="5">Linux Trace Toolkit Reference Manual</FONT>
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<b><u><font size=6>1. Introduction</font></u></b>
<p><i><font size=4>1.1 Why a Linux trace toolkit?</font></i>
<p> The Linux operating system is a multiprogramming,
multiuser system. This means that it is able to handle multiple running
programs at once. On a uniprocessor system (a computer with only one microprocessor),
this is achieved by sharing this precious resource among multiple tasks,
letting each execute for a certain period of time and then switching to
another. The selection and switching of processes is handled by the Linux
kernel, which also is a program and therefore also needs some time from
the processor. It is also responsible for fulfilling certain requests by
the programs it manages, dealing with error conditions, etc.
<p> One could have the need to know exactly what these
scheduling decisions, process switches and various management tasks are,
how they are handled, how long they take and to which process the processor
is allocated. Spread out over a certain period of time, we call this an
execution trace.
<p> The Linux trace toolkit is a suite of tools designed to
do just that; extract program execution details from the Linux operating
system and interpret them. Specifically, it enables its user to extract
processor utilization and allocation information for a certain period of
time. It is then possible to perform various calculations on this data
and dump this in a text file. the list of probed events can also be included
in this. The integrated environment, based on the GTK+ library, can also
plot these results and perform specific searches.
<p><i><font size=4>1.2 Software requirements</font></i>
<p> In its simplest form, the Linux trace toolkit does
not need any specific libraries. Any properly configured Linux system will
do. By this, we mean a Linux system where the Linux header files, source
code and libraries are present. No, this will NOT work on any other operating
system, although you might be able to compile and use the graphical environment.
This is obviously not very useful if you cannot get any data from the kernel!
<p> As of 12/01/2002, however, the data acquisition components
require version 2.4.16 of the Linux kernel. You may be able to update/downgrade
the patch provided with the package to other kernel versions. As a matter of
fact, many people have succesfully done so and the project's ftp site contains
a directory called "ExtraPatches" which contains the patches submitted by
some of these people.
<p> In any case, you can very well do your data acquisition
with a patched (more about that in the next section) kernel and then
reboot using any other kernel to view the results. The performance hit for a
patched kernel compared to a "normal" one is, however, minimal;
Testing has shown that this is less than 2.5% on a normal use system (measured using
batches of kernel makes). On algorithm
intensive tasks, the impact is close to zero. On file intensive systems (measured using
batches of tar), the impact is less than 5%.
The kernels can be found at <a href="http://www.kernel.org/">www.kernel.org</a>.
<p> This is fine if all you need is to get results and dump
them in a text file. However, you would be missing out on one of LTT's
most powerful features: the plotting of the trace. All of the integrated
environment including the event graph is implemented with the GTK+ library.
You can download it for free from <a href="http://www.gtk.org/">www.gtk.org</a>.
This version on LTT was coded and successfully tested with GTK+ version
1.2.4, although any 1.2.x version will most probably do.
<p><i><font size=4>1.3 Architecture</font></i>
<p> The Linux trace toolkit is made up of the following
components:
<br>
<center>
<table BORDER COLS=2 WIDTH="90%" BGCOLOR="#CCCCCC" NOSAVE >
<tr BGCOLOR="#000000">
<td>
<FONT COLOR="#FFFFFF">
<b>
Component
</b>
</FONT>
</td>
<td>
<FONT COLOR="#FFFFFF">
<b>
Task
</b>
</FONT>
</td>
</tr>
<tr>
<td>Kernel patch</td>
<td>Logs Traced events</td>
</tr>
<tr>
<td>Kernel module</td>
<td>Stores logged events and signals trace daemon when limit is
reached</td>
</tr>
<tr>
<td>Trace daemon</td>
<td>Reads Kernel module's buffer and stores events on disk</td>
</tr>
<tr>
<td>Data decoder</td>
<td>Displays event trace in human readable format and performs analysis</td>
</tr>
</table>
</center>
<p>For more information on the project's architecture, take a peak at the Documentation
section of LTT's web site.
<p><i><font size=4>1.4 Real-Time Linux support</font></i>
<p> Along-side support for Linux, LTT, starting version 0.9.3, has support for the <a href=http://www.rtai.org>
DIAPM RTAI</a> (Dipartimento di Ingegneria Aerospaziale - Politecnico di Milano Real Time Application Interface)
real-time linux extension. Although you could use LTT with an RTAI patched kernel, LTT didn't record
any information about RTAI's decision or the behavior of the tasks it scheduled. Now, LTT is able to
get this information and to analyze it to retrieve important information to the understanding of the
dynamic behavior of an RTAI/Linux system. Most interesting are the event graphs drawn according to
the collected information. They enable a unique view a real-time Linux system. This is most important
when trying to isolate all types of timing problems.
<p> If you do not use RTAI or do not plan to do so, you can skip sections discussing using LTT with
RTAI.
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