1. Wombat: A Portable User-Mode Linux for Embedded Systems
Jiyong Park
Real-Time Operating Systems Laboratory,
Seoul National University, Korea

2. Reference Paper
Ben Leslie, Carl van Schaik, and Gernot Heiser, “Wombat: A Portable User-Mode Linux for Embedded Systems,” Proceedings of the 6th Linux Conference, April 2005

3. Motivation Provide Linux APIs to embedded programmers
Linux is not suitable for embedded systems
It does not support hard real-time.
It is not highly robust.
It has big trusted computing base (TCB).
Proprietary firmware are subject to GPL.
But, Linux supports many useful legacy applications.
Solution
Wombat: User-mode Linux on top of micro kernel

4. Wombat: Australian marsupial
What is Wombat?
Linux kernel modified to run on top of L4/Iguana
Very similar to L4Linux
Key mechanisms are exactly the same.
I will not explain them again.
Key differences
Wombat: run on top of Iguana
L4Linux: run on top of L4 directly
Wombat: developed by NICTA
L4Linux: developed by DUT
Wombat: Australian marsupial

5. Relationship Between Wombat and Iguana
Real-time applications are managed by Iguana
Non real-time Linux tasks are managed by Wombat
Wombat itself is managed by Iguana
Untrusted Legacy Applications
Wombat Linux
Server
Real-time Applications
Similar to RTLinux
managed by
Iguana L4
Hardware

6. Benefits of Using Iguana
Real-time
Real-time tasks and non real-time Linux tasks can run on the same machine.
Robust
Real-time tasks are isolated with the fault or crash at Linux world.
Linux server can be upgraded on-the-fly.
TCB
TCB is small (L4 + Iguana).
GPL
Firmware are free from GPL.

7. Structure of Wombat Wombat server Linux processes Compatibility mode
Native mode
Address space
Protection domain
L4 thread
Compatibility mode Linux process
Real-time
tasks
Iguana Servers
Real-time
tasks
Interrupt threads
Bottom half thread
Main thread
Wombat server
Native mode Linux process
Iguana Server
Real-time
tasks

8. Native and Compatibility Modes
Native mode Linux processes
Run inside the shared address space (similar to uClinux)
Can use Iguana services
Can communicate with real-time tasks
Context switching and communication are fast
Address space model is different with conventional Linux processes
Compatibility mode Linux processes
Full binary Linux compatibility
Can not use Iguana services
Can not communicate with real-time tasks
Context switching and communication is slow

9. Mechanisms of Wombat
System call handling, exception handling, scheduling, and etc. are the same as L4Linux.

10. Performance – System Call
Null system call
Linux: us
Wombat: 1.38 us
UML: 8.99 us
Pipe system call
Linux: 5.05 us
Wombat: 8.83 us
UML: 66.8 us
UML processes
UML
Linux processes
Linux
UML: User-Mode Linux

11. Performance – Context Switch
Context switching time of Wombat is very close to that of Linux.
Number of active processes
Context switching time(us)
UML with 16k cache
UML with 0k cache
Linux, Wombat with 16k cache
Linux, Wombat with 0k cache
2.8GHz

12. Performance – Use of FASS
L4-embedded supports FASS on ARM processor.
Linux currently does not support FASS on ARM.
So, Wombat achieves better performance than Linux.
Latencies
Linux
Wombat
Rel.
lat_ctx -s 0 2
190.8
6.48
29.44
lat_ctx -s 0 3
197.1
18.82
10.47
lat_ctx -s 0 4
199.5
19.78
10.09
lat_ctx -s 0 10
215.7
44.07
4.89
lat_ctx -s 4 2
257.7
7.15
36.04
lat_ctx -s 4 3
259.3
23.26
11.15
lat_ctx -s 4 4
293.4
40.28
7.28
lat_ctx -s 4 10
285.1
141.96
2.01
lat_fifo
377
80.07
4.71
lat_pipe
378.4
81.56
4.64
lat_unix
764.5
107.48
7.11
lat_syscall
0.82
4.73
0.17
lat_proc procedure
0.21 1
lat_proc fork
4334
5706
0.76
lat_proc exec
4600
6400
0.72
XScale 200MHz (time unit: us)

13. Conclusion Wombat goes one step further than L4Linux.
Real-time tasks can coexist with Linux tasks
Native mode Linux process reduces overhead of context switchings
However, Linux itself is becoming suitable for real-time systems.

14. Thank You!!!