Table of Contents

FreeBSD 8.0 Kernel Size Reduction

This document aims to customize the kernel for reducing its size and use additional files to cause the reduction.

Tools

du -cHA, top, perl files to compress the size of /bin and /sbin

The following table shows the effect of each kernel size changes

Different Changes
Change Kernel Size before Changing Kernel Size After Changing Difference Useful Links Notes
Machine Type Determination 145670 KB 143339 KB 2331 KB Show Kernels Sizes See Section1
MSDOSFS, UFS_DIRHASH, UFS_GJOURNAL 143339 KB 142848 KB 491 KB Show Kernels Sizes –
INET6, SYSVSHM, SYSVMSG, SYSVSEM, KTRACE 142848 KB 141177 KB 1671 KB Show Kernels Sizes –
Using Dynamic Executables and Remove Static Ones /bin =⇒ 971 KB & /sbin =⇒ 3607 KB /bin =⇒ 971 KB & /sbin =⇒ 2978 629 KB 1.Size of /bin directory before applying Changes
2.Size of /sbin directory before applying changes
3.Show The directories sizes After Applying changes 		See Section2
Sysctl maxusers=4 1./var =⇒ 70582 KB
2.Free Available Memory = 445 		1./var =⇒ 70556 KB
2. Free Available Memory = 446 		1. /var =⇒ 26 KB
2. For Available Memory = 1MB 		Show /var Before Changing maxusers Sizes
Show /var Before Changing maxusers Sizes
Show /var After Changing maxusers Sizes
Show Available Memory at maxusers=243
Show /var Available Memory at maxusers=4 		See Section3
Using Perl Files /bin =⇒ 728 KB /bin =⇒ 1367 KB 639 KB Show /bin before using the perl file
Show /bin after using the perl file
Download perl files with snapshots 		See Section4
Sysctl net.inet.tcp.msl=“5000” Does not affect kernel size – – – See Section5

Section1

There were three different machines loaded in the kernel configuration file and minimized to only the exact machine type.Be Careful, if you don't know your machine type, you may get a kernel crash.

Section2

  1. Steps : # cd /usr/src
    # make buildworld
    # make buildkernel
    # make installkernel
    # shutdown -r now
    1. Notice that if you get any problem with your current binaries, you won't be able to retrieve the ones.

Section3

  1. You will face a problem in Kernel Building or compilation if you set maxusers to 2 & you were running X Windows System
  2. To use this at each boot process =⇒ I. recompile it inside the configuration file. or II. put it inside /etc/sysctl.conf

Section4

  1. Steps : # use names.files to include all the binaries you need
    # run loadbin.pl
    # you can use this command at startup by running it from load.conf file
    # don't forget to use the termination.pl to compress all the files again and you can run it combined with the shutdown process
    # see snapshots for more details about how to run the document.
    # This process reduces the directory size for half the size approximately.
  2. The same process can be done for /sbin directory but don't forget to include the the sbins needed in the names.files file

Section5

Lessen max segment life to conserve resources =⇒ ACK waiting time in milliseconds to 5000

Important Notes

  1. You can use extra kernel size reduction by using the options MUTEX_NOINLINE, RWLOCK_NOINLINE & SX_NOINLINE which reduce the kernel text segment.
  2. Don't use any debug options when you recompile the kernel.
  3. Don't try to remove any configurations used to mount root at kernel compilation process or you will face a problem during your next boot Some of these processes are FFS, GEOM_GPT, ..etc
  4. You can perform only one executable for all bins using crunchgen().
  5. Using -O3 debug option for CFLAGS or COPTFLAGS doesn't guarantee producing unbroken kernel so its better to avoid it.

Resources

FreeBSD Handbook

Comments and Suggestions

You can send your comments and suggestions to mfarrag@FreeBSD.org