Setting Kernel Options

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Because all Linux software runs atop the kernel, the kernel's performance, and hence its optimizations, is particularly important. What's more, the kernel's configuration tools enable you to set its optimizations from a menu. Figure 1.1 shows this menu, obtained by typing make xconfig in the kernel source directory, for a 2.5-series kernel.

Figure 1.1: The Linux kernel provides many options related to the CPU.

The Processor Type and Features option area contains kernel settings related to the CPU type and related options. Other options include:

SMP Support Symmetric Multi-Processing (SMP) refers to a computer with multiple CPUs. The kernel requires special support to take advantage of more than one CPU per computer.

APIC Support The Advanced Programmable Interrupt Controller (APIC) is an interrupt controller built into some CPUs, as opposed to built into the motherboard chipset. Select this option if your CPU supports this feature.

Machine Check Exception Motherboards and CPUs increasingly support hardware problem-detection circuitry. This option adds support for some such tools, such as temperature sensors that detect when the computer is overheating.

CPU Frequency Scaling Some CPUs support changing their clock speeds "on the fly," which can be useful for conserving battery life on laptops. Select this option (in the Power Management Options area) to add support for this feature. Check http://www.brodo.de/cpufreq/ for more information.

Laptop Support Some laptops provide software "hooks" to enable an OS to check on the CPU's status and power management. You can compile support for these hooks into the kernel.

Device Interfaces Some device files in the /dev/cpu directory provide access to the CPU, but only if you enable the appropriate support in the kernel.

High Memory Support There are three settings for high-memory support: Off supports machines with less than one gigabyte of RAM, 4GB supports one to four gigabytes, and 64GB supports more than four gigabytes.

Math Emulation The ยก386, some i486, and some early Pentium clone CPUs lacked a floating-point unit (FPU), aka a math coprocessor. Some Linux software relies upon the presence of an FPU, so the kernel includes an option to emulate one when it's not present. You only need to include this support if your CPU is a very old one that lacks an integrated FPU and if you haven't added a separate FPU chip to the motherboard.

MTRR Support Intel's Pentium Pro and later CPUs offer a feature called Memory Type Range Register (MTRR), which improves support for access to memory that's shared between the CPU and some other device. This configuration is most common on laptops and other computers with video hardware integrated into the motherboard. To enable this support, you must compile support for it into the kernel.

Note If you're using a 2.4.x or earlier kernel, the configuration tool looks different from the one shown in Figure 1.1, but it provides many of the same options.

Kernel configuration is a very important topic in and of itself. Chapter 15, "Creating a Custom Kernel," describes the process in more detail.

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