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-%%
-%% paper.tex = template for real-time Linux workshop papers
-%%
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\documentclass[10pt,a4paper]{article}
-\usepackage[english]{babel}
-\usepackage{multicol}
-\usepackage{listings}
-\usepackage{epsfig}
-\newcounter{figcounter}
-\def\epsin #1#2#3#4{
- \refstepcounter{figcounter} \label{#3}
- \[
- \mbox{
- \epsfxsize=#2mm
- \epsffile{./img/#1.eps}
- }
- \]
- \begin{center}
- \parbox{7cm}{{\bf FIGURE \arabic{figcounter}:}\quad {\it #4 } } \\
- \end{center}
-}
-
-\setlength{\paperheight}{297mm}
-\setlength{\paperwidth}{210mm}
-\setlength{\voffset}{-12mm}
-\setlength{\topmargin}{0mm}
-\setlength{\headsep}{8mm}
-\setlength{\headheight}{10mm}
-\setlength{\textheight}{235mm}
-\setlength{\hoffset}{-4mm}
-\setlength{\textwidth}{166mm}
-\setlength{\oddsidemargin}{0mm}
-\setlength{\evensidemargin}{0mm}
-\setlength{\marginparwidth}{0mm}
-\setlength{\marginparpush}{0mm}
-\setlength{\columnsep}{6mm}
-\setlength{\parindent}{6mm}
-
-%% insert eps pictures
-%% use as \epsin{epsfile}{width_in_mm}{label}{caption}
-
-%% insert table
-%% use as \tabin{size_in_mm}{label}{caption}{table_data}
-\newcounter{tabcounter}
-\def\tabin #1#2#3#4{
-\refstepcounter{tabcounter} \label{#2}
-\[ \makebox[#1mm][c]{#4} \]
-%\vspace{0mm}
-\begin{center}
- \parbox{7cm}{{\bf TABLE \arabic{tabcounter}:}\quad {\it #3 } } \\
-\end{center}
-}
-
-\title{\LARGE
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% TITLE OF PAPER (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-Real-Time CORBA performance on Linux-RT\_PREEMPT
-}
-
-\author{\large
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% AUTHOR (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-{\bf Manuel Traut }\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% AFFILIATION (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-Linutronix GmbH\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% STREET ADDRESS (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-Auf dem Berg 3, 88690 Uhldingen, Germany\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% E-MAIL (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-manut$@$linutronix.de \\
-\vspace{8mm}
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% AUTHOR (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%{\bf Li Su}\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% AFFILIATION (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%CASC Long March Launch Vehicle Technology CO. LTD\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% STREET ADDRESS (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%7 Building, 15 Block, NO.188 West Road, the forth South Round, BeiJing\\
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% E-MAIL (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%sudanlina$@$yahoo.com.cn \\
-}
-\date{}
-
-
-\begin{document}
-
-\maketitle
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% ABSTRACT (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\begin{abstract}
-Automation technology lacks an established platform independent, high-level, object oriented real-time capable communication standard, which is based on standard Ethernet hardware and drivers. ACE/TAO is an Open Source implementation of the OMG Real-Time CORBA Specification and might fill this gap. It is designed platform independent, implemented in C++ and provides a standardized communication framework. Real-Time CORBA is already used in industrial environments, e.g. aircraft, naval equipment and others. This paper explains the basics of the ACE/TAO framework and its usage in industrial communication. On the basis of a real-world example - transmission of an 1 KiB data frame - two communication methods are evaluated: the RT-CORBA Remote Procedure Call and the TAO Real-Time Event-Channel. The performance measurement methods are explained in detail. Measurement results under various system loads and a comparison of ACE/TAO on top of a vanilla Linux kernel and a RT\_PREEMPT enabled Linux kernel provide a meaningful insight in the capabilities of RT-CORBA. Finally, the paper provides an analysis of functionality which needs to be improved in the operating system to provide real deterministic communication through a standardized framework.
-\end{abstract}
-
-\vspace{10mm}
-
-\begin{multicols}{2}
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% SECTION (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\section{Introduction}
-
-Currently communication in automation is mainly realized with a huge number of different fieldbuses. However since 2005 there is an increasing trend, for using ethernet based communication methods. Another trend is, using PCs, real-time operating systems and software for controlling machines. But why not combine these two trends, controlling machines in real-time over an object-orientated middleware? The efforts would be: faster development cycles, easier maintaining and extending of the software modules\dots
-
-And with ACE/TAO a Real-Time CORBA Implementation and the RT\_PREEMPT patch, the infrastructure for doing this with Linux, is already given!
-
-\subsection{CORBA}
-CORBA is a Middleware, which allows RPC\footnote{Remote Procedure Call}-based IPC\footnote{Inter Process Communication} between different operating systems and different programming languages (Figure 1).
-\epsin{orb}{80}{fig1:f1}{CORBA Architecture}
-The communication interfaces are defined in IDL\footnote{Interface Definition Language}. The IDL files are compiled into, e.g. c++, java, \dots, code which does the (de)serialization of the datatypes. The interface implementations (CORBA objects) are registered at language specific ORB\footnote{Object Request Broker}s. Each CORBA process owns one ORB, which handles the function requests and returns the calculated values.
-
-\subsection{Real-Time CORBA}
-\epsin{rtcorbaext}{70}{fig1:f2}{Real-time CORBA \\(source: [1])}
-As shown in figure 2, a real-time capable ORB extends a standard ORB with the following features: locating objects in constant time, preallocation of resources, operating system independent priority handling, priority based scheduling.
-
-\subsection{ACE/TAO}
-ACE is an open-source c++ framework for platform-independent system- and network-programming. TAO is a Real-Time CORBA implementation build on top of ACE (Figure 3).
-
-\epsin{ace}{80}{fig1:f3}{ACE/TAO framework \\(source: [2])}
-
-The ACE/TAO package is available for all important operating systems. The hole framework is trimmed for embedded systems: Each application described in this paper consumes less than 1 MByte of RAM. Also the consumed CPU time is suprisingly low.
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% NEXT SECTION (OPTIONAL)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\section{Performance Measurements}
-
-The measurements were made on embedded systems (Intel Mobile CPU 600 MHz, 512 MB RAM, Intel e100 NIC) with digital I/0 ports. A square-pulse generator is connected with the digital input on the first embedded system (supplier) and channel 1 of the oscilloscope. The digital outputs of the embedded systems 2 and 3 (receivers) are connected with channel 2 and 3 of the oscilloscope.
-
-\epsin{versuch4}{80}{fig1:f4}{Measurement environment}
-
-\subsection{RPC}
-
-Each receiver hosts an object, for writting values to its digital output:
-
-\begin{lstlisting}
-module benchmark{
- interface Put{
- void Port( in short portNo,
- in short value,
- in string data );
- };
-};
-\end{lstlisting}
-
-The \textit{Port} function is called by the \textit{sender} as soon as the state of one of its digital inputs changes. The real end to end latency is meassured with the oscilloscope.
-
-\epsin{sequenzV1}{80}{fig1:f5}{RPC measurement: sequence diagramm}
-
-Figure 6a shows a histogramm over the latency, a process data image, including \textit{portNo} and \textit{value} (the \textit{data} string has length 0), needs from the digital input of the supplier system to the digital output of receiver system I. These results are measured on a RT\_PREEMPT enabled kernel, during heavy disk i/o, network traffic on none priorized NICs and 5 low priorized CPU burners running.
-
-\epsin{v1Last}{80}{fig1:f6a}{Latency histogramm RT\_PREEMPT, system load}
-
-\epsin{vanilla-ohne-last}{80}{fig1:f6b}{Latency histogramm Vanilla, without system load}
-
-The results of the same measurement on a none RT\_PREEMPT enabled kernel are shown in figure 6b (without system load) and 6c.
-
-\epsin{vanilla-mit-last}{80}{fig1:f6c}{Latency histogramm Vanilla, system load}
-
-Basicly the same measurement under identical system load, was made once again. Only change: Both receiving systems are used. Receiving system I hosts the object for setting port values, with lower priorization, than receiving system II. On the supplier system, the rightangle waveform is connected with two digital input ports. Changes on one port are commited two the higher priorized \textit{receiver} on system II; changes on the other port are sent to the lower priorized receiving system I. Figure 7 shows, that no priority inversion occours.
-
-\epsin{v3Last}{80}{fig1:f7}{Latency histogramm, two priorizations}
-
-To simulate the transmission of bigger process data images, the parameter \textit{in string data} is read in from a text file, so his length, could be changed after compile time, by editing the text file. Figure 8 shows, the bigger the process data image, the bigger the difference between immediate and worst case.
-
-\epsin{v4plot}{80}{fig1:f8}{dependency between process data image siye and latency}
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% SUB SECTION (OPTIONAL)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\subsection{TAO Real-Time Event-Channel}
-
-The TAO Real-Time Event-Channel is a Messaging Service. Suppliers are sending messages to the Messaging Service. A client can subscribe for messages at the Messaging Service.
-
-As soon as the value of the digital input of the Supplier changes, the new value of the digital input port is send to the Messaging Service. The Messaging Service sends this value, to all subscribed clients. A client writes the values from the messages, to their digital output.
-
-\epsin{sequenzV2}{80}{fig1:f9}{Event-Channel measurement: sequence diagramm}
-
-In case, the TAO Real-Time Event-Channel is a CORBA application working between Receiver and Supplier, the latency should be approximately two times the latency of the 1:1 RPC measurement. Figure 10 shows, that this is a correct assumption.
-
-\epsin{v2Last}{80}{fig1:f10}{Latency histogramm, TAO RT Event-Channel}
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% NEXT SECTION (OPTIONAL)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\section{Needed operating system functionalities}
-
-A TAO ORB can only schedule its CORBA requests correctly, if the underlying operating system has an priorization based scheduler, with a defined maximum scheduling latency. The timer and scheduling latency of a Linux system can be determined with cyclictest. For example, the worst case latency on a RT\_PREEMPT enabled box is $26 \mu s$. The same test, on the same box, but with RT\_PREEMPT disabled is $39.6 ms$.
-
-The operating system should support priorization of device i/o data. The real-time application must not beeing interrupted by incomming traffic on any device, excepted data the real-time application must react on (e.g. digital inputs). On a RT\_PREEMPT enabled Linux, the most part of all ISR\footnote{interrupt service routine}s are done in kernel threads, which can be priorized from userspace. This solution has two little cons: IRQ sharing and the softirqs for networking. IRQ sharing prevents from a real per device priorization. The softirqs for networking handle the traffic of all NICs in the same thread. So the priorization of network data from a specific device is not possible. But in case real-time networking gets more and more important, the community works on a solution for this issue.
-
-Not all NICs (especially their firmware and driver) are good for real-time networking. Many modern NICs don't request an interrupt for each received package. They only request an interrupt if a defined amount of data is received, or a defined timeout is over. Esspecially for real-time network communications with little data transfer, these cards causes high latencies.
-
-\section{Conclusion}
-
-The pros of an object-orientated middleware is, that the applications can be easily extended, also debugging is much more easier, than working with bits and bytes on sockets. A further effort is, that the hole connection handling is done by the middleware.
-
-This paper shows, that using an object-orientated middleware for real-time communication over standard ethernet hardware, is especially in the automation industrie, not a trivial thing. But with ACE/TAO and RT\_PREEMPT enabled Linux it is no longer science-fiction.
-
-[3] is a good resource for further informations.
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% REFERNCES (REQUIRED)
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-\begin{thebibliography}{9}%use this if you have <=9 bib refs
-%\begin{thebibliography}{99}%use this if you have >9 bib refs
-\bibitem{paper1},{\it Real-time CORBA Specification},2005, {\sc OMG}
-\bibitem{paper2},{\it Overview of ACE},2007\\{\it http://www.cs.wustl.edu/schmidt/ACE-overview.html}
-\bibitem{paper3},{\it TAO technical documents},2007\\{\it http://www.cs.wustl.edu/schmidt/corba-research-realtime.html}
-\end{thebibliography}
-
-\end{multicols}
-\end{document}
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% +%% paper.tex = template for real-time Linux workshop papers +%% +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\documentclass[10pt,a4paper]{article} +\usepackage[english]{babel} +\usepackage{multicol} +\usepackage{listings} +\usepackage{epsfig} +\newcounter{figcounter} +\def\epsin #1#2#3#4{ + \refstepcounter{figcounter} \label{#3} + \[ + \mbox{ + \epsfxsize=#2mm + \epsffile{./img/#1.eps} + } + \] + \begin{center} + \parbox{7cm}{{\bf FIGURE \arabic{figcounter}:}\quad {\it #4 } } \\ + \end{center} +} + +\setlength{\paperheight}{297mm} +\setlength{\paperwidth}{210mm} +\setlength{\voffset}{-12mm} +\setlength{\topmargin}{0mm} +\setlength{\headsep}{8mm} +\setlength{\headheight}{10mm} +\setlength{\textheight}{235mm} +\setlength{\hoffset}{-4mm} +\setlength{\textwidth}{166mm} +\setlength{\oddsidemargin}{0mm} +\setlength{\evensidemargin}{0mm} +\setlength{\marginparwidth}{0mm} +\setlength{\marginparpush}{0mm} +\setlength{\columnsep}{6mm} +\setlength{\parindent}{6mm} + +%% insert eps pictures +%% use as \epsin{epsfile}{width_in_mm}{label}{caption} + +%% insert table +%% use as \tabin{size_in_mm}{label}{caption}{table_data} +\newcounter{tabcounter} +\def\tabin #1#2#3#4{ +\refstepcounter{tabcounter} \label{#2} +\[ \makebox[#1mm][c]{#4} \] +%\vspace{0mm} +\begin{center} + \parbox{7cm}{{\bf TABLE \arabic{tabcounter}:}\quad {\it #3 } } \\ +\end{center} +} + +\title{\LARGE +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% TITLE OF PAPER (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Real-time CORBA performance on Linux-RT\_PREEMPT +} + +\author{\large +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% AUTHOR (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +{\bf Manuel Traut }\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% AFFILIATION (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Linutronix GmbH\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% STREET ADDRESS (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +Auf dem Berg 3, 88690 Uhldingen, Germany\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% E-MAIL (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +manut$@$linutronix.de \\ +\vspace{8mm} +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% AUTHOR (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%{\bf Li Su}\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% AFFILIATION (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%CASC Long March Launch Vehicle Technology CO. LTD\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% STREET ADDRESS (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%7 Building, 15 Block, NO.188 West Road, the forth South Round, BeiJing\\ +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% E-MAIL (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%sudanlina$@$yahoo.com.cn \\ +} +\date{} + + +\begin{document} + +\maketitle + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% ABSTRACT (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\begin{abstract} +Automation technology lacks an established platform independent, high-level, object oriented real-time capable communication standard, which is based on standard Ethernet hardware and drivers. ACE/TAO is an Open Source implementation of the OMG Real-Time CORBA Specification and might fill this gap. It is designed platform independent, implemented in C++ and provides a standardized communication framework. Real-Time CORBA is already used in industrial environments, e.g. aircraft, naval equipment and others. This paper explains the basics of the ACE/TAO framework and its usage in industrial communication. On the basis of a real-world example - transmission of an 1 KiB data frame - two communication methods are evaluated: the RT-CORBA Remote Procedure Call and the TAO Real-Time Event-Channel. The performance measurement methods are explained in detail. Measurement results under various system loads and a comparison of ACE/TAO on top of a vanilla Linux kernel and a RT\_PREEMPT enabled Linux kernel provide a meaningful insight in the capabilities of RT-CORBA. Finally, the paper provides an analysis of functionality which needs to be improved in the operating system to provide real deterministic communication through a standardized framework. +\end{abstract} + +\vspace{10mm} + +\begin{multicols}{2} + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% SECTION (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\section{Introduction} + +The main communication infrastructure in industrial automation is +based on a broad variety of industrial fieldbusses based on CAN, RS422 +and RS485 physical layers. Since 2005 ethernet based communication is +gaining popularity. Aside of this the usage of PC based hardware with +realtime operating systems becomes more wide spread. The systems +provide the usual set of interfaces and capabilities known from the IT +world with specialized software for machine controls. + +Combining PC based hardware and ethernet based real-time capable +communication methods utilizing object-oriented middleware could +provide a couple of advantages: +\begin{itemize} +\item fast development cycles +\item ease of maintenance +\item flexible reusability of software modules +\item connection handling is done by the middleware +\item \dots +\end{itemize} +ACE/TAO, a Real-time CORBA implementation, and the RT\_PREEMPT +patch for the Linux kernel provide such a base environment today. + +\subsection{CORBA} +CORBA is a middleware, which allows RPC\footnote{Remote Procedure + Call}-based IPC\footnote{Inter Process Communication} between +different operating systems and different programming languages +(Figure 1). + +The communication interfaces are defined in IDL\footnote{Interface Definition + Language}. The IDL files are compiled into, e.g. c++, java, \dots, +code which does the (de)serialization of the datatypes. The interface +implementations (CORBA objects) are registered with language +specific ORB\footnote{Object Request Broker}s. Each CORBA process owns +one ORB, which handles the function requests and returns the +calculated values. + +\epsin{orb}{80}{fig1:f1}{CORBA Architecture} + +\subsection{Real-Time CORBA} +\epsin{rtcorbaext}{70}{fig1:f2}{Real-time CORBA \\(source: [1])} As +shown in figure 2, a real-time capable ORB extends a standard ORB with +the following features: locating objects in constant time, +preallocation of resources, operating system independent priority +handling, priority based scheduling. + +\subsection{ACE/TAO} +ACE is an open-source c++ framework for platform-independent system- +and network-programming. TAO is a Real-time CORBA implementation build +on top of ACE (Figure 3). + +\epsin{ace}{80}{fig1:f3}{ACE/TAO framework \\(source: [2])} + +The ACE/TAO package is available for all important operating +systems. The framework can be trimmed for embedded systems: Each +application described in this paper consumes less than 1 MByte of +RAM. Also the consumed CPU time is suprisingly low. + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% NEXT SECTION (OPTIONAL) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\section{Performance Measurements} + +The measurements were made on embedded systems (Intel Mobile CPU 600 +MHz, 512 MB RAM, Intel e100 NIC) with digital I/0 ports. A +square-pulse generator is connected to a digital input of the +\textit{supplier system} and to channel 1 of the +oscilloscope. The digital outputs of the \textit{receiver systems} +\#I and \#II are connected to channel 2 and 3 of the oscilloscope. + +\epsin{versuch4}{80}{fig1:f4}{Measurement environment} + +\subsection{RPC} + +Each receiver hosts an object, for writing values to its digital +output: + +\begin{lstlisting} +module benchmark{ + interface Put{ + void Port( in short portNo, + in short value, + in string data ); + }; +}; +\end{lstlisting} + +The \textit{Port} function is called by the \textit{supplier} as soon as +the state of one of its digital inputs changes. The real end to end +latency is measured with the oscilloscope. + +\epsin{sequenzV1}{80}{fig1:f5}{RPC measurement: sequence diagramm} + +Figure 6a shows a latency histogram measured on system running a +RT\_PREEMPT enabled kernel. Extra system load is generate by heavy disk +I/O, network traffic on non-prioritized NICs and five low priority CPU +hogs. The RPC \textit{data} string has zero length. + +The latency is the total time of the RPC execution. The RPC execution +is triggered by the rising and the falling edge of the square-wave +generator connected to the digital input of the supplier system. The +RPC results in toggling the output on the receiver system. The input +and the output are monitored by a digital oscilloscope, which provides +histogram generation functionalities. + +\epsin{v1Last}{80}{fig1:f6a}{Latency histogram RT\_PREEMPT, system load} + +\epsin{vanilla-ohne-last}{80}{fig1:f6b}{Latency histogram Vanilla, without system load} + +The results of the same measurement on a none RT\_PREEMPT enabled +kernel are shown in figure 6b (without system load) and 6c (with +system load). + +\epsin{vanilla-mit-last}{80}{fig1:f6c}{Latency histogramm Vanilla, system load} + +The same measurement under identical system load was made once again +with both receiving systems active. Receiving system \#I hosts the +object for setting port values with lower priorization than receiving +system \#II. On the supplier system the square-waveform generator is +connected to two digital input ports. Changes on the first port are +commited to the higher prioritized \textit{receiver} on system \#II; +changes on the second port are sent to the lower prioritized receiving +system \#I. Figure 7 shows, that no priority inversion occours. + +\epsin{v3Last}{80}{fig1:f7}{Latency histogramm, two priorizations} + +To simulate a higher data transmission rate, the parameter +\textit{in string data} is read in from a text file, so its +length can be changed after compile time by editing the text +file. Figure 8 shows the larger the process data image is the larger +is the difference between immediate and worst case latency. + +\epsin{v4plot}{80}{fig1:f8}{dependency between process data image size and latency} + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% SUB SECTION (OPTIONAL) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\subsection{TAO Real-time Event-Channel} + +The TAO Real-time Event-Channel is a Messaging Service. Suppliers are +sending messages to the Messaging Service. A client can subscribe for +messages at the Messaging Service. + +As soon as the value of the digital input of the Supplier changes, the +new value of the digital input port is send to the Messaging +Service. The Messaging Service sends this value, to all subscribed +clients. The clients write the values from the messages to the digital +outputs. + +\epsin{sequenzV2}{80}{fig1:f9}{Event-Channel measurement: sequence diagramm} + +The TAO Real-time Event-Channel is an additional CORBA application the +data has to pass, so the latency should be approximately two times the +latency of the RPC measurement. Figure 10 shows, that this is a +correct assumption. + +\epsin{v2Last}{80}{fig1:f10}{Latency histogramm, TAO RT Event-Channel} + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% NEXT SECTION (OPTIONAL) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\section{Needed operating system functionalities} + +A TAO ORB can only schedule its CORBA requests correctly when the +underlying operating system has real-time capabilities with a +deterministic scheduling latency. The programming model expects a +priority based scheduling algorithm. The operating system latencies of +Linux can be tested with cyclictest. cyclictest is an utiliy which +determines the deviation of the expected time line of a periodic +timer. This takes the full chain of timer interrupt, scheduler +invocation, context switch and return to the user space application +into account. On a given test system the maximum latency with a +RT\_PREEMPT enabled kernel was $26 \mu s$, with a vanilla kernel the +maximum latency increased to $39.6 ms$. + +Another requirement for deterministic communication is the +priorization of device I/O. Right now this is not fully implemented in +the RT\_PREEMPT kernel. The priority of interrupt service handlers is +only configurable per interrupt line, which causes problems if the +interrupt line is shared between a high priority and a low priority +device. The same applies for the networking soft interrupt which +handles all network interfaces in the same queue. + +The kernel community has already recognized the importance of real-time +networking capabilities and solutions for this problem are already +discussed. + +Not all NICs (especially their firmware and drivers) are suitable for +real-time networking. Many modern NICs don't request an interrupt for +each received package. They only request an interrupt if a defined +amount of data is received, or a defined timeout is over. These cards +cause high latencies especially when the transfered data packages are +small. + +\section{Conclusion} + +Real-time communicaion based on object-orientated middleware over +standard ethernet hardware is a promising solution. Various problems +have been identified, but resolving those is not trivial. + +Interestingly enough of these problems are not restricted to the +communication requirements of the automation industry. The increasing +demands on deterministic networking for other application areas e.g. +telecommunication provide additional momentum for improving the +deterministic behaviour of network communication in the Linux kernel. + +ACE/TAO and RT\_PREEMPT enabled Linux are providing a useful and solid +environment today with further improvements in the foreseeable future. + +[3] is a good resource for further informations. + +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +%% REFERNCES (REQUIRED) +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% +\begin{thebibliography}{9}%use this if you have <=9 bib refs +%\begin{thebibliography}{99}%use this if you have >9 bib refs +\bibitem{paper1},{\it Real-time CORBA Specification},2005, {\sc OMG} +\bibitem{paper2},{\it Overview of ACE},2007\\{\it http://www.cs.wustl.edu/schmidt/ACE-overview.html} +\bibitem{paper3},{\it TAO technical documents},2007\\{\it http://www.cs.wustl.edu/schmidt/corba-research-realtime.html} +\end{thebibliography} + +\end{multicols} +\end{document} |