RTOS Comparison Questions

1
RTOS Comparison Questions

• CS 684
• 24th March 2009

2
Outline

• Lynx OS
• VxWorks
• QNX/Neutrino
• VRTX
• RT Linux
• RTAI
• Free RTOS
• UC/OS II
• RTOS Internals
• Miscellaneous

3
Lynx OS

• How does LynxOS offer memory protection through
  hardware MMUs? / what is MMU based memory
  protection?
• What are Kernel Plug-Ins (KPIs)?
• We have KPIs in Lynx OS and similarly in VRTX
  each application can add its own system calls.
  Are both of them effectively serving the same
  way?

4
VxWorks

• VxWorks has backward compatibility to previous
  version features for exception handling and
  template support. Will that increase the
  footprint of the kernel?
• In the slides it is given that lynx OS is a self
  hosted OS and because of that there is a need for
  memory protection. Is VxWorks also a self hosted
  OS? Is that the reason for having memory
  protection in VxWorks?
• As VxWorks uses RR Scheduling, more context
  switching should happen, which in turn should
  result in more overhead. But in the slides, its
  given that VxWorks incurs less overhead. How is
  it possible?

5
VxWorks (Contd.)

• Why does RTOSes such as VxWorks and QNX encourage
  the use of message queuing or mailboxes as the
  primary IPCs?
• For VxWorks, reduced context switch time is
  achieved by saving only those register windows
  which are actually in use (on a Sparc). What do
  we mean by Sparc?
• Secondly, to increase the response time, it
  saves the register windows in a register cache
  useful for recurring tasks. What are the
  replacement algorithms used for updating the
  contents of the cache and avoiding access to
  stale data?

6
VxWorks (Contd.)

• What mechanism for IPC is used by VxWorks and how
  efficient is it as compared to other RTOSes?
• VxWorks uses preemptive round robin scheduling.
  Round robin means each task is allocated a time
  slot to run on the CPU. If it is a high priority
  task then also it will use only the allocated
  time slice then other low priority task takes
  place. Is the priority rotating or not?In most
  of the cases, there is deficiency in RR as
  compared to EDF RMS scheduling algorithm. Also,
  as RR takes more context switches which in turn
  means more latency. In spite of this, VxWorks is
  the most popular RTOS. How is it possible?

7
QNX/Neutrino

• The slides say that in QNX/Neutrino, there is
  message based priority tracking feature. What is
  priority tracking? Why is it necessary? How is it
  done based on messages?
• How does QNX/Neutrino maximize application
  portability ?

8
VRTX

• VRTX has been certified by the US FAA for use in
  mission and life critical applications such as
  avionics. When there are so many RTOSes, how do
  we verify them and select the best one for a
  particular application?

9
RT Linux

• How does the use of FIFO in RT Linux ensure that
  the real time task is never blocked?

10
RTAI

• How does LXRT in RTAI manages user space
  application in kernel space ?
• Jitter and latency are low for RTAI than RTLinux.
  How can this be justified?
• In RTAI, how significant is the overhead of using
  the LXRT API?

11
Free RTOS

• Free RTOS has the following feature It offers a
  smaller and easier real-time processing
  alternative for applications where eCos and
  embedded Linux (or RT Linux) won't fit, are not
  appropriate, or are not available. What are such
  applications, where RT Linux is not appropriate?
  What parameters make them inappropriate?

12
UC/OS - II

• UC/OS II features Memory management is performed
  using fixed size partitions. What is the typical
  size of each partition. Will there be any memory
  wastage in the partitions?
• Why UC/OS-II is suitable for use in safety
  critical embedded system? What are the features
  to be considered while selecting RTOS?

13
RTOS Internals

• How does memory management in a RTOS differ from
  that in a conventional OS, how is it implemented
  across the various RTOS's mentioned.
• An operating system can be monolithic or micro
  kernel based. Which one is the best for real time
  application design ? What is the limitation of
  each for real time application and which are
  monolithic or micro kernel based from the list
  given in the slide ?
• Elaborate feedback driven priority assignment.

14
Miscellaneous

• Selection guide (not on cost) and testing
  (simulation) tools for COTS RTOS solutions
  requested. UC/Probe from Micrium, how it has been
  used for Firebird-IV hard RT applications?
• Suppose I want to use RTOS in an application like
  Nuclear Power Plant. Which should be the RTOS I
  should select out of the existing ones?