Kernel in Linux, kernel services & configurations

Kernel :- kernel is the heart of the operating system. It manages communication with hardware, decides which processes to run, and provides each process with an isolated, virtual address space in which to run. The kernel is what the GRUB boot loader loads into memory. The kernel loads device driver modules. It also allocates hardware resources such as IRQ ports, I/O addresses, and DMA channels. A recompiled kernel can lead to:

• Greatly improved speed at which kernel services operate.
• Direct support for commonly used drivers.
• Dynamic loading of appropriate drivers as modules.
• Lower memory consumption by removing unneeded components.
• Support for high-end hardware, such as memory above 4GB,hardware array controllers, symmetric multiprocessing (multiple CPU) support, and more.

Kernels can be organized as one big unit or as a lot of interconnected pieces. Kernels are called up by boot loaders when you start your system.

Monolithic Versus Modular

A monolithic kernel is a kernel in which all the device modules are built directly into the kernel.

Modular kernels have many of their devices built as separate loadable modules. Monolithic kernels can communicate with devices faster, since the kernels can talk to the hardware only indirectly through a module table. Unfortunately, the typical monolithic kernel is huge, which reduces available RAM. In addition, some systems just can't boot a kernel that's too large.

Linux once had problems loading modular kernels for some hardware. With a monolithic kernel, the drivers are already there and are often more appropriate for certain components such as embedded hardware.

A modular kernel has greater flexibility. We can compile almost all drivers as modules, and then each module can be inserted into the kernel whenever you need it. Modules keep the initial kernel size low, which decreases the boot time and improves overall performance. If Linux has trouble loading a kernel module, We can use the modprobe or insmod command to load modules as needed, and add those options to the /etc/modprobe.conf file.(in Centos5/Rhel5).

Upgrading  Kernel :-

Updating the kernel is not as difficult as it looks. We should never overwrite or upgrade an existing kernel, as mistakes happen. New kernels are handled by installing the newly built kernel in /boot and then adding another boot option to your boot loader configuration file (/boot/grub/grub.conf) for the new kernel. GRUB treats the new kernel as if it were an entirely new operating system. If we install the new kernel directly from a Red Hat configured RPM, it updates the boot loader automatically.

If you do make a drastic mistake and the kernel doesn't boot, you can simply reboot the system and select the old kernel from the GRUB menu.

There are different versions of kernel-devel, kernel-PAE, kernel-xen, and kernel-headers packages for each supported architecture.

Available  Linux Kernels (and Related Packages)

Kernel RPM	Description / Architecture
kernel-versionnum.i686	Designed for PCs with a single Intel/AMD CPU; also works with dual-core systems
kernel-versionnum.ia64	Designed for Itanium2 systems
kernel-devel-versionnum	Installs drivers and other information to help compile third-party drivers
kernel-PAE-versionnum	If you have more than 4GB of RAM, install the PAE kernel associated with your CPU architecture
kernel-PAE-devel-versionnum	If you have more than 4GB of RAM, install the PAE kernel associated with your CPU architecture
kernel-headers-versionnum	Includes kernel headers; often sufficient for drivers
kernel-versionnum.src.rpm	Includes the source code for the RHEL kernel