Chapter 8 Kernel Configuration

8.1. I would like to customize my kernel. Is it difficult?
8.2. My kernel compiles fail because _hw_float is missing. How do I solve this problem?
8.3. Why is my kernel so big (over 10MB)?
8.4. Why do I get interrupt conflicts with multi-port serial code?
8.5. Why does every kernel I try to build fail to compile, even GENERIC?
8.6. How can I verify which scheduler is in use on a running system?
8.7. What is kern.quantum?
8.8. What is kern.sched.quantum?

8.1. I would like to customize my kernel. Is it difficult?

Not at all! Check out the kernel config section of the Handbook.

Note: We recommend that you make a dated snapshot of your new /kernel called /kernel.YYMMDD after you get it working properly. Also back up your new /modules directory to /modules.YYMMDD. That way, if you make a mistake the next time you play with your configuration you can boot the backup kernel instead of having to fall back to kernel.GENERIC. This is particularly important if you are now booting from a controller that GENERIC does not support.

8.2. My kernel compiles fail because _hw_float is missing. How do I solve this problem?

You probably removed npx0 (see npx(4)) from your kernel configuration file because you do not have a math co-processor. The npx0 device is MANDATORY. Somewhere inside your hardware lies a device that provides hardware floating-point support, even if it is no longer a separate device as used in the good old 386 days. You must include the npx0 device. Even if you manage to build a kernel without npx0 support, it will not boot anyway.

8.3. Why is my kernel so big (over 10MB)?

Chances are, you compiled your kernel in debug mode. Kernels built in debug mode contain many symbols that are used for debugging, thus greatly increasing the size of the kernel. Note that there will be little or no performance decrease from running a debug kernel, and it is useful to keep one around in case of a system panic.

However, if you are running low on disk space, or you simply do not want to run a debug kernel, make sure that both of the following are true:

You do not have a line in your kernel configuration file that reads:
```
makeoptions DEBUG=-g
```
You are not running config(8) with the -g option.

Either of the above settings will cause your kernel to be built in debug mode. As long as you make sure you follow the steps above, you can build your kernel normally, and you should notice a fairly large size decrease; most kernels tend to be around 1.5MB to 2MB.

8.4. Why do I get interrupt conflicts with multi-port serial code?

When I compile a kernel with multi-port serial code, it tells me that only the first port is probed and the rest skipped due to interrupt conflicts. How do I fix this?

The problem here is that FreeBSD has code built-in to keep the kernel from getting trashed due to hardware or software conflicts. The way to fix this is to leave out the IRQ settings on all but one port. Here is an example:

#
# Multiport high-speed serial line - 16550 UARTS
#
device sio2 at isa? port 0x2a0 tty irq 5 flags 0x501 vector siointr
device sio3 at isa? port 0x2a8 tty flags 0x501 vector siointr
device sio4 at isa? port 0x2b0 tty flags 0x501 vector siointr
device sio5 at isa? port 0x2b8 tty flags 0x501 vector siointr

8.5. Why does every kernel I try to build fail to compile, even GENERIC?

There are a number of possible causes for this problem. They are, in no particular order:

You are not using the new make buildkernel and make installkernel targets, and your source tree is different from the one used to build the currently running system (e.g., you are compiling 4.3-RELEASE on a 4.0-RELEASE system). If you are attempting an upgrade, please read the /usr/src/UPDATING file, paying particular attention to the “COMMON ITEMS” section at the end.
You are using the new make buildkernel and make installkernel targets, but you failed to assert the completion of the make buildworld target. The make buildkernel target relies on files generated by the make buildworld target to complete its job correctly.
Even if you are trying to build FreeBSD-STABLE, it is possible that you fetched the source tree at a time when it was either being modified, or broken for other reasons; only releases are absolutely guaranteed to be buildable, although FreeBSD-STABLE builds fine the majority of the time. If you have not already done so, try re-fetching the source tree and see if the problem goes away. Try using a different server in case the one you are using is having problems.

8.6. How can I verify which scheduler is in use on a running system?

If you are running FreeBSD version 5.2.1 or earlier, check for the existence of the kern.quantum sysctl. If you have it, you should see something like this:

% sysctl kern.quantum
kern.sched.quantum: 99960

If the kern.quantum sysctl exists, you are using the 4BSD scheduler. If not, you will get an error printed by sysctl(8) (which you can safely ignore):

% sysctl kern.sched.quantum
sysctl: unknown oid 'kern.sched.quantum'

In FreeBSD version 5.3-RELEASE and later, the name of the scheduler currently being used is directly available as the value of the kern.sched.name sysctl:

% sysctl kern.sched.name
kern.sched.name: 4BSD

8.7. What is kern.quantum?

kern.quantum is the maximum number of ticks a process can run without being preempted. It is specific to the 4BSD scheduler, so you can use its presence or absence to determine which scheduler is in use. In FreeBSD 5.X or later kern.quantum has been renamed to kern.sched.quantum.

8.8. What is kern.sched.quantum?

See Q: 8.7.