A Comparison of Hard Real-Time Linux Alternatives – Interrupt Latency

We’ll be covering the interrupt latency of the hard real-time Linux alternatives. The interrupt latency is the time that elapses when an interrupt begins to where the interrupt source is serviced.

Depending on the microprocessor design, interrupt masking, interrupt controllers and the OS’ interrupt handling, the latency can be affected.

This work was done as a feature of an examination to decide the appropriateness of utilizing Linux in a hard constant advanced control framework. The control circles right now executed at a pace of 100 Hz. A key prerequisite of the framework is that the entirety of the control counts must be finished inside the 10 ms window accessible — paying little mind to some other stacking on the framework. So as to accomplish this, it was felt that a most extreme deferral from the time the start of a control interim is flagged (the occasion) to the time that the control task is begun ought to be under 0.5 ms.

There are various Linux setups that are accessible for constant. A portion of these depends on Linux alone, some utilization Linux with a sub-bit, and some utilization piece patches to improve the continuous conduct of Linux. This paper centers around four of the potential arrangements. The alternatives chose are for the most part uninhibitedly accessible for download from legitimate sites.

Right now, and any sub-pieces utilized were treated as secret elements. The methodology followed was to arrange Linux and program the framework for the different choices, and afterward measure results. No endeavors were made to explore the piece particulars to decide why the designs reacted the manner in which they did, nor were any endeavors made to improve the presentation utilizing custom patches or custom arrangements.

Tests were done on a delicately stacked framework and on a framework under moderately overwhelming correspondences stacking. Correspondences stacking was utilized since it was anything but difficult to set up, it gave an approach to practice interface equipment and the gadget driver for the equipment, and it gave a reasonable circumstance where hard ongoing and interchanges informing must be conceivable all the while.

This assessment was done on an Intel x86 processor. A portion of the choices that were tried may not be upheld on all processor structures.

The four Linux options assessed include:

probably the most up-to-date 2.4 Linux distribution downloaded from kernels.org (2.4.27)
a recently available 2.6 Linux circulation downloaded from kernels.org (
a 2.4 Linux distribution with RTAI downloaded from rtai.org (3.1-test5)
a 2.4 Linux distribution with LXRT distributed with RTAI (3.1-test5) All of these options are defined further below:

Linux 2.4

Linux 2.4.27 had been the latest 2.4 launch offered by the period that is right of investigation. The 2.4 kernel is just a well balanced kernel who has well documented deficiencies with regards to real-time that is difficult. Different approaches have been utilized to create 2.4 with real-time abilities. Linux 2.4 was indeed contained in the scholarly study as being set up a baseline against which to compare one other options.

Linux 2.6

Linux 2.6 has a amount that is true of improvements over Linux 2.4. A majority of these improvements were built to enhance the real-time abilities of Linux 2.6. Some of those include:

New scheduler algorithm — O(1) algorithm must-have performance that is superior under higher lots and on multiprocessor systems
Kernel preemption spot — before 2.6, a person application could not preempt a task operating in kernel mode. With 2.6, preemption has become possible leading to reduce latencies for user applications that are interactive.
The enhanced model that is threading in-kernel help for Native Posix Threading Library (NPTL) escalates the performance of threading operations and provides support for more threads.
Merging of much of the uClinux project (Linux for microcontrollers). This gives help for processors that do perhaps not feature an MMU (Memory Management Unit)
Improvements to your module subsystem.

Please consult the resources listed in the resource section for additional information on Linux 2.6.

  • Linux 2.4 with RTAI
  • RTAI — Real-Time Application Interface — is a hard expansion that is real-time the Linux kernel. The RTAI project is a Free Software project that was created by the Department of Aerospace Engineering of Politecnico di Milano (DIAPM). It has evolved into a community that is grouped coordinated by Professor Paulo Mantegazza of DIAPM.

RTAI is a sub-kernel that runs under Linux. It provides difficult real-time response by running Linux as the job that is idle. Interrupts are intercepted by RTAI where they might be processed by an RTAI interrupt handler or passed up to Linux. RTAI runs only in kernel-space, and so, all tasks which can be RTAI need certainly to run in kernel-space. Versions of RTAI are available that is utilized utilizing the Linux 2.4 kernel and the Linux 2.6 kernel.

Launch 3.1 of RTAI will depend on the Adeos nano-kernel — a migration far from the Real-Time Hardware Abstraction Layer (RTHAL) that is the centre of a patent infringement claim by FSMLabs. Future releases of RTAI should include fusion. RTAI/Fusion is the purpose of convergence of Adeos, LXRT, the Linux that is preemptable kernel low latency improvements. Version 0.6.2 of Fusion is actually released but was perhaps not tested as an element of the focus that is ongoing this paper, since it ended up being detailed to be experimental.

All hard tasks that are real-time implemented in kernel-space as RTAI tasks for this option.

Linux 2.4 with LXRT
LXRT is an expansion of RTAI. It allows real-time and hard that is soft to perform in user-space utilising the RTAI API. The benefits of involved in user-space with LXRT comprise of:

  • tasks execute under Linux memory security
  • assistance for IPC calls with standard Linux processes
  • user-space tasks is debugged with standard debug tools
  • LXRT and RTAI work together to produce performance that is real-time user-space tasks. RTAI kernel-space modules are nevertheless necessary for LXRT, and any execution will include LXRT tasks generally that run in user-space and tasks that are RTAI run in kernel-space.

All hard real-time tasks had been implemented in user-space utilizing support that is LXRT this technique.

Hardware Platform

The GE Fanuc, VMIVME-7700 CPU board ended up being chosen for use in the control system. The board is a VMEbus board that is single considering the Intel Celeron CPU running at 650 MHz. The board is populated with 512 MB SDRAM and 128 MB CompactFlash. Other options that come with the board that were found in the assessment connected with options that are real-time:

  • 10/100 BaseT user interface,
  • Video controller that is graphic
  • keyboard software,
  • remote Ethernet booting
  • programmable timer

The target had been allowed by the Ethernet interface to be linked to the host PC employing a LAN. The host PC was A pc that is x86-based the Fedora Core 2 Linux circulation. The target had its root filesystem NFS mounted to the host PC.

The movie controller that is a graphic keyboard display provided a convenient interface to monitor and control the perspective.

The Ethernet that is remote booting was in fact utilized to permit the kernel image to become downloaded to your target from a host that is tftp on the host PC.

The timer that is programmable an apparatus to have an illustration linked to the latencies inherent within the different OS options considered. The motorist for the timer had been furnished by GE Fanuc.