A very old talk comparing Linux and NetBSD. Talk delivered at FOSS.in/2005

1. NetBSD and Linux for Embedded Systems
Mahendra M
Mahendra_M@infosys.com
http://www.infosys.com
This work is licensed under a Creative Commons License
http://creativecommons.org/licenses/by-sa/2.5/

2. About the talk
 This talk is NOT about to compare NetBSD and Linux and
find out which is better.
– We shall compare – but not criticize !!
 We shall try to understand both systems in their own merit
 It is about sharing our experiences with both the systems –
more from a embedded & telecom OS perspective.
 We hope to help you choose.

3. Agenda
 A short introduction to NetBSD and Linux based systems
 Suitability for embedded systems
 Introduction to the features available in NB and Linux
– Specifically for Embedded Systems
 Feature wise comparison of the two kernels
– Core architecture, Threading model, Real time capabilities,
modularity, performance, ease of deployment, portability
 Linux capabilities.
 NetBSD Capabilities
– Kqueue : Event notification framework
 Availability of Code for future needs
 Feasibility and time to market along with License issues.

4. About the Systems
 Linux
– Refers only to the kernel
– Distributions are built around it. ( distros available for
embedded systems )
– Has a history of around 15 years
– Is under the GPL license ( v2.0 ) and supports binary modules.
 NetBSD
– A BSD “ distribution” targeted at portability.
– Includes a kernel, libraries, config tools, scripts and build
systems.
– Targeted at portability
– The BSD systems have had a history of around 25 years.
– Is under a BSD license.

5. Common stuff ...
 Highly portable – ports exist for a large number of
architectures and reference boards.
 Both are POSIX compliant
 Good VM, Networking, Threading subsystems
 Both have been time tested in the market
 Active development happening
– Directly on Linux kernel – on multiple trees maintained by
different people.
– NetBSD tend to let features stabilize on FreeBSD/OpenBSD
and then pick it up :-)
 Let us get on with the differences....

6. Memory foot print
 Linux is slowly getting bloated.
– There are projects that are working on reducing the size of
Linux kernel
– “ Linux Tiny” project provides a set of patches that can be
applied to the Linux kernel to reduce the kernel size (built
image and run time memory requirements)
 NetBSD projects boast of reduced kernel size.
– TINY configuration file is distributed along with the source.
 Both kernels can be easily reduced to a size of less than
1MB ( uncompressed : ~900k, compressed : ~400k )
 With sufficient hacking around, it can be reduced even
further.

7. Process Scheduling
 Both OS follow a similar model in running user space
processes.
– Time slice based – and O(1) in nature.
– They have priority based levels and processes are placed on
these levels based on their priority
– Both support dynamic priority modification based on
interactiveness of a process
 Not needed for embedded systems – and can be turned off at
compile time.
– Both support the same user space priorities (40)
 Linux has two run-queues : An active and an expired run-
queue.
 Linux also has a run-queue per CPU – for SMP boxes.

8. Process Run-queue
RunQueue
Doubly linked lists of tasks
Active
Task 1 Task 2 ... Task N [Priority: 1]
...
Task 1 Task 2 Task M [Priority:140]
Expired
...
...
Migration Thread
From kernel/sched.c

9. Other stuff ...
 Kernel pre-emption.
– Linux kernel supports kernel pre-emption, but only at pre-
emptible points inside in the kernel.
– There are still areas within the kernel which cannot be pre-
empted.
– NetBSD does not support kernel pre-emption.
 Hence real-time support is pretty poor on NetBSD systems
 SMP Support ( not really critical in embedded systems )
– Very good in Linux.
– Worked upon in NetBSD.

10. Real-time performance
 Hard real-time performance
– Not available in both kernels by default.
– FSM labs provides patches to the Linux kernel ( under GPL )
and commercial patches to NetBSD.
 Works by running Linux/NetBSD kernel as low priority tasks
under a real time kernel.
 Soft real-time performance.
– Linux is pretty good in providing soft real time facilities.
– In addition to the 40 user land priorities, Linux provides an
additional 100 real-time priorities
– Linux supports two soft real time scheduling options
 SCHED_FIFO – First in first out
 SCHED_RR – Round robin
– Real time tasks cannot be pre-empted.

11. Threading models
 Both kernels are thread aware and are POSIX compliant
 They support different threading models
– This is where the difference starts and things get interesting..
 Linux
– Supports 1:1 model of threading ( NPTL )
– The kernel does not distinguish between threads and
processes
 A process is a group of thread ids – thats it.
– All threads in a process are visible as tasks to the kernel and
active threads are allocated time-slices for scheduling.
 All active threads will get their time-slices
– Can set real time priorities to tasks.
– APIs are available for real time threads handling

12. Threading models ( contd .. )
 NetBSD
– Treats threads (lwp) and processes differently
– Supports Scheduler Activations.
 m:n model of threading
– Not all threads are visible to the kernel scheduler. User space
code takes part in telling the kernel which thread to schedule.
– The threads in a process have co-operative scheduling.
 A thread can keep running for most of the time – careful
programing required.
 Both groups had debated the models before taking up their
approaches. Both methods are “ claimed” to be better than
the other.
 Today, Linux seems to have better thread/process creation,
spawning and context switching times.

13. Debugging support
 NetBSD has much better debugging support.
– DDB – an in-kernel debugger
– Supports kernel crash dumps
– Supports KGDB (source level debug) – remote debugging.
 Linux
– Patches available for crash dumps and KDB & KGDB (not
available for all architectures)
– Linux Trace Toolkit, UML etc. are also available methods.
– Has “ Kernel Hacking” Option in kernel configuration
 Allows debugging using “ Magic SysRQs” , Debugging of actions
like memory allocations, file systems, highmem, stack overflows,
page allocations etc.
 Also allows stats collection for some modules.
– Also, early printk() support is pretty recent.

14. Flash Devices and File Systems
 Support is poor in NetBSD
 Linux supports
– Advanced MTD support ( device concatenation, FTL, NFTL )
– JFFS2 ( JFFS3 is being worked on )
– RAM file systems and ramdisk support
– Supports initrd
 In NetBSD
– OpenSource implementations of the above are not available.
( Commercial products are available )
– Results in considerable lead time in development.
– Supports MFS ( not memory efficient ) : tmpfs is being worked
upon
– For boot time, it allows embedding a file-system into the kernel

15. KQueue : Event notification mechanism
 NetBSD supports a generic event notification framework –
kqueues.
 Excellent replacement for select() / poll() APIs.
– No need to pass entire descriptor set to each call.
– On return, applications need not check all file descriptors for
updates.
– Reduces data copying ( fd list from kernel to user space and
vice-versa )
 Can handle multiple types of events.
– Signals, Vnode/Process monitoring, Timer events etc.
 Can club multiple occurrences of an event into a single event
 New event types can be easily added.
 All with just two system calls !!

16. Build systems and configuration
 NetBSD is better
– The entire system : kernel, compilers and tools, libraries and
applications can be compiled ( native or cross platform ) using
a single script – build.sh
– The same script can build distributions, tarballs, archives and
can also update existing systems and install fresh systems.
– Adding new components to the build framework is extremely
easy.
 Linux – Such systems exist but it is not as flexible as BSD
 Both NetBSD and Linux provide different configuration
mechanisms.
– NetBSD builds a device tree format.
– Linux provides a GUI interface for configuring the kernel...

17. Other differences.
 Linux kernel code throws out lot of warnings during
compilation. NetBSD is very clean in this aspect.
 Linux supports MMU Less operations.
 Loadable kernel modules
– Better supported in Linux.
 Better device support in Linux
 With 2.6 kernel, the device driver model has vastly improved
( attend other talks at FOSS.in for more details )
 Portability has been a design goal for NetBSD.
 Development models are vastly different :-)
– NetBSD is more “ cathedral” like !!
 In Linux, you get answers easily :-)

18. Business related
 License (violation) is a serious cause of concern
– BSD License is very liberal
– One of the main reasons why telecom companies go for BSD
– eg: Juniper ( JUNOS )
 Protocol stacks and third party code
– Are available usually for most BSDs and Linux.
– To be more portable, they tend to ignore benefits of one OS
 New device support
– Vendors of new devices like Network Processors, etc. release
code only/mainly for Linux ( kernel modules etc. ).
– Extra effort is required in such cases to port things to NetBSD.
 In case of Linux, extra time is required in selecting and
integrating components of the system.

19. Links
 http://linux-bangalore.org/blug/meetings/200401/scheduler-2.6.pdf
 http://www.netbsd.org/ - One site for all NetBSD stuff
 http://people.freebsd.org/~jlemon/papers/kqueue.pdf - Kqueue design
 http://camars.kaist.ac.kr/courses/530/97/Readings/scheduler-act.pdf - SA design
 http://josh.trancesoftware.com/linux/ - Details of Linux 2.6 scheduler
 http://www.wasabisystems.com/gpl/linux.htm - A comparison
 http://bulk.fefe.de/scalability/ - Started a Battle and cleanup !!
 http://lkcd.sourceforge.net/
 http://kgdb.sf.net/
 http://oss.sgi.com/projects/kdb/
 http://www.linuxjournal.com/article/3829 - Using Linux Trace Toolkit
 http://www.opersys.com/LTT
 http://netbsd-soc.sourceforge.net/projects/tmpfs/ - TMPFS project page
 http://www.selenic.com/linux-tiny - Linux Tiny home page

20. Finally ...
 Questions ??
 Thanks to
– Organizers for giving me a chance to speak at FOSS.in
– NetBSD and Linux developers who helped me during my work
– Linux kernel team for doing a wonderful job !!
 Special thanks to YOU for listening...
You can contact me at :
Mahendra_M@infosys.com