Schedulers and building kernels

ULE vs. 4BSD

The ULE scheduler was recently introduced and provides similar pre-emptiveness to that of the 2.6 Linux kernel. The ULE scheduler was initially rolled out during the 5.x series; however, due to system instability the default scheduler was reverted to the classic 4BSD scheduler. The ULE scheduler still isn’t enabled by default, as proven by the fact that its configuration line is commented out in the default kernel config file. The adventurous may choose to uncomment the ULE scheduler, and comment out the 4BSD scheduler and rebuild their kernels.

Compiling the kernel

Compiling FreeBSD’s kernel will be a little bit different for those used to compiling Linux kernels. There is no graphical, or semi-graphical text based interface to select the various parameters: as with many other things in FreeBSD a single, simple text file will do the trick. The default kernel configuration files are located in /usr/src/sys/{YOUR_ARCH} where arch will most likely be i386 or amd64. It is highly recommended to copy the GENERIC file to a safe location, like /root/config/kernel/MYKERNEL and then symlink it back to /usr/src/sys. This is to prevent the accidental deletion of your config if you decided to blow away /usr/src.

The name of the kernel config file is not enough to distinguish it from other kernel configuration files; instead the IDENT line must be changed to reflect the name you choose for your configuration, MYKERNEL in this particular case (for this article).

To build and install the kernel with your custom config, simply change directories to /usr/src and invoke make buildkernel KERNCONF=MYKERNEL. The command to install the newly built kernel is similar: make installkernel KERNCONF=MYKERNEL.

# Should be changed to the identifier of your custom config
ident    GENERIC
-- Becomes --
ident    MYKERNEL

# Scheduler Options
#options        SCHED_ULE               # ULE scheduler
options         SCHED_4BSD              # 4BSD scheduler (This is the classic scheduler)


cd /usr/src
make buildkernel KERNCONF="KERN_LABEL"
make installkernel KERNCONF="KERN_LABEL"