NetBSD Documentation: Kernel

Frequently Asked Kernel Questions

• Where to download the kernel source
• How to build a kernel
• What exactly is a GENERIC kernel?
• What does mclpool limit reached: increase NMBCLUSTERS mean?
• What does WARNING: SPL NOT LOWERED ON SYSCALL EXIT mean?
• What does Stray interrupt on IRQ 7 mean?
• Why are kernels compiled with -msoft-float
• Kernel compiles on low memory machine seem very slow
• Problems compiling a -current kernel
• Debugging a kernel crash dump
• Getting backtraces when debugging a kernel crash dump
• What is DDB and what can I do in it?
• Generating a kernel crash dump
• Adding a kernel to a boot floppy
• I'm having trouble mounting the SCSI device for a new partition I've just created. What's up with this device-numbering scheme?

Frequently Asked Hardware Questions

• What does device not configured mean?
• Debugging ATAPI or ATA (IDE) devices
• Debugging USB devices
• Recognising a new PnP device
• Recognising a new PCMCIA device
• Is PLIP (Parallel Line IP) supported
• What is UBC?

Further reading

• NetBSD specific documentation
• Other online documentation

Frequently Asked Kernel Questions

Where to download the kernel source (top)

How to build a kernel (top)

What exactly is a GENERIC kernel? (top)

The term GENERIC refers to a kernel that is configured to run on just about any machine supported by the machine architecture. The term originated from a line in the kernel configuration file which specified that the root device was "generic" as well as a configuration option. This option and that format of the configuration line is no longer used, but the name will probably stick for a while.

Since these kernels tend to include support for all the available device drivers and many models of machines that you are not using, you are encouraged to compile your own custom kernel.

What does mclpool limit reached: increase NMBCLUSTERS mean? (top)

This means the kernel has run out of space to map mbuf clusters. mbuf clusters are used by the network code to store packets and other network related data.

The default setting for NMBCLUSTERS is 1024 (256 in NetBSD 1.5 and earlier), so if you have this problem, try doubling the value until the error message disappears. To display the current value of NMBCLUSTERS you can use sysctl(8) as follows:

	# sysctl kern.mbuf.nmbclusters

Alternatively, try

        # echo 'print nmbclusters' | gdb -q /netbsd

See also options(4) for more details on kernel configuration options. To change the value, add

	options NMBCLUSTERS=2048

to your kernel configuration, or patch the binary:

        # gdb --write /netbsd
        (gdb) set nmbclusters=2048
        (gdb) quit

Note that if you patch the binary only, you will need to reboot for the change to take effect. If you're on a platform which supports it, you can set the value with:

	# sysctl -w kern.mbuf.nmbclusters=2048

This will work, but will be lost on the next reboot. Combining this, and patching the binary, would mean no need to build a new kernel or reboot.

What does WARNING: SPL NOT LOWERED ON SYSCALL EXIT mean? (top)

This kernel message means that there is a bug in the kernel where a syscall did "int x = splfoo();" and did not call "splx(x);" before it returned. The splx(x); function in this example would restore the system priority level to the one encoded in x, which was a value previously returned by one of the other spl functions (in this case, the made up example of splfoo();).

If you get this kernel message you should be dropped into ddb(4), the in-kernel debugger. A stack trace in ddb, accomplished by pushing 't', might show you the offending syscall(). It is probably a good idea to send-pr(1) the output of the trace command (as well as any other relevant information), since you should under no circumstances be getting this kernel message.

See also spl(9) for more information on spl functions.

What does Stray interrupt on IRQ 7 mean? (top)

The "Stray interrupt on IRQ 7" kernel message means that the interrupt controller reported an unmasked interrupt on IRQ 7, but no driver attached to that IRQ 'claimed' it.

There are two reasons this can happen:

You should only get this kernel message running a kernel with the DEBUG option defined.

Why are kernels compiled with -msoft-float (top)

When a process makes a system call the kernel needs to save the processor state, so that it can later switch back to the process. Floating point registers tend to be large and relatively plentiful, making saving and restoring them an expensive operation. If the FPU is in the middle of an operation the CPU will additionally be forced to sit and wait for it to finish before it can then copy the registers.

Avoiding floating point registers in the kernel gives a significant performance win for system calls. Some processors, such as sparc, can also use lazy FP context switching to sometimes avoid having to save and restore FP registers even when switching between processes.

On some architectures the compiler can use floating point registers to speed up certain operations (such as block memory copies), breaking the above, so '-msoft-float' is required.

Kernel compiles on low memory machine seem very slow (top)

By default NetBSD installs a GENERIC kernel which includes drivers for almost every supported item of hardware, network protocol, and filesystem. While this allows one kernel to run on virtually every machine for a given port, it does result in it using more space than is really needed, particularly on a small memory machine. This is compounded by kernel compiles using the -O2 optimisation level which tells the compiler to use extra memory and time to make the kernel as fast as possible.

One option when building your own kernel is to use "make COPTS=-O" which instructs the compiler to perform only the most effective optimisations. This will result in a fractionally slower kernel, but it will take less time to compile.

If you are intending to take several 'compile and reboot into new kernel' passes while customising a kernel on a low memory machine it may make sense to make the first few passes using "make COPTS=-O", and then switch to "make" for the final pass.

Of course, generally the fastest way to compile a kernel on a low memory machine is to use another machine, or temporarily add some more memory!

Problems compiling a -current kernel (top)

The first point to note is you should subscribe to the current-users mail list. Tracking -current without reading current-users is akin to driving in the dark without any lights. You have been warned :).

It is always worth downloading the latest config.tar.gz, compiling, installing and rerunning on your config file - config changes reasonably frequently between releases.

Sometimes, binaries and/or libraries need to be updated before you will be able to build -current on a release. In these situations, it may be simpler to install from a binary snapshot, and then build -current. Snapshots of -current for i386 (for example) can be found in /pub/NetBSD/arch/i386/snapshot/. The src/UPDATING file contains information about these important changes which you should be aware of when attempting to build -current, or a -current kernel.

Debugging a kernel crash dump (top)

You can use the usual gdb(1) commands, such as 'bt' to get a backtrace.

Getting backtraces when debugging a kernel crash dump (top)

You can get backtraces of an arbitrary process from gdb when debugging a kernel crash dump with two easy steps:

What is DDB and what can I do in it? (top)

DDB is the optional in-kernel debugger. It is usually entered via one of three methods:

Some of the more useful commands are:

Generating a kernel crash dump (top)

Usually the kernel will automatically generate a crashdump on panic, which is then picked up by savecore(8) on reboot. However you can force a crashdump in ddb(4) by using sync (or reboot 0x100). If the kernel panics or hangs while attempting to sync the buffer cache you can use reboot 0x104 which will bypass the sync.

Adding a kernel to a boot floppy (top)

Some ports are already setup to build a boot floppy by "cd /usr/src/distrib/<ARCH>/floppies ; make ". (You may need to build the INSTALL kernel manually before running this. If you have an existing boot.fs image you can replace the kernel with:

This assumes you have "pseudo-device vnd" in your kernel config file, and a ready to use kernel.

I'm having trouble mounting the SCSI device for a new partition I've just created. What's up with this device-numbering scheme? (top)

By default, SCSI devices under NetBSD are numbered starting from 0 in the order of SCSI ID number. In other words, you lowest-numbered SCSI device will be /dev/sd0, the next device will be /dev/sd1, etc. Notice that this is the assignment that they are given during the boot process.

If you compile your own kernels, you can set the SCSI devices to point to any SCSI ID number you want with a kernel configuration line like:

sd0             at scsibus0 target 4 lun 0
sd*             at scsibus? target ? lun ?

The above lines will make device sd0 point to the disk at SCSI ID#4 and the rest of the devices will be assigned as described above. This is often referred to as "hardwiring" your SCSI devices, and is recommended if you are using RAIDframe or ccd so as to avoid the device IDs being changed out from under the configuration if one device is powered off or broken.

Frequently Asked Hardware Questions

What does device not configured mean? (top)

Debugging ATAPI or ATA (IDE) devices (top)

If a kernel is compiled with WDCDEBUG defined, then gdb can be used to patch wdcdebug_atapi_mask and wdcdebug_mask. Setting the appropriate bits in these variables will cause the kernel to output verbose information about ATAPI and ATA operations. (Currently NetBSD defaults to WDCDEBUG enabled.)

For the maximum level of output, use:

	# gdb --write /netbsd
	(gdb) set wdcdebug_atapi_mask=0xff
	(gdb) set wdcdebug_mask=0xff
	(gdb) quit

Note: This will produce an extremely large quantity of output. To select individual options, look for the list of bitflags directly above:

Debugging USB devices (top)

If you have problems with USB devices, you can enable verbose messages in the usb driver:

Recognising a new PnP device (top)

This assumes the device is of a generic type which is already supported, but the device ID is not recognised. Adding a device that performs differently includes writing code.

Recognising a new PCMCIA device (top)

This assumes the device is of a generic type which is already supported, but the device ID is not recognised. Adding a device that performs differently includes writing code.

Is PLIP (Parallel Line IP) supported (top)

There are patches from Martin Husemann which add PLIP support to NetBSD/i386, submitted as PR 1278. The patches at the end of the PR should apply to the NetBSD 1.3.3 source tree.

What is UBC? (top)

UBC stands for the Unified Buffer Cache project. It was written by Chuck Silvers, and has been integrated into NetBSD since 1.5L (Nov 2000). When upgrading from a non-UBC setup, you'll need to rerun config(8) again, but before you do, you'll want to remove any settings for "BUFCACHE", "NBUF" or "BUFPAGES", and let the size of the buffer cache go back to the default. Under UBC, the traditional buffer cache is no longer used for storing regular file data, only metadata, so you want to allow the VM system to manage most of your physical memory. The default buffer cache size will be fine for most people, regardless of the amount of memory in the machine. The amount of memory in the boot messages about "using X buffers containing Y memory" no longer indicates the amount of memory available for caching file data, so don't worry if those numbers don't change.

The important difference is that more memory will be available for caching regular file data, so filesystem i/o will be faster since there will be more times when the data you're accessing is already in memory. How much faster depends on what you're doing, but you'll probably notice the difference.

See also: UBC: An Efficient Unified I/O and Memory Caching Subsystem for NetBSD by Chuck Silvers.

Further reading

NetBSD specific documentation (top)

Other online documentation (top)

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