Major Linux Vs UNIX Kernel Differences

UNIX has been regarded as the mother of most of the operating systems. Some of the popular members of this family Include :

• System V Release 4(SVR4) developed by AT&T.
• 4.4 BSD From university of California
• AIX from IBM.
• HP-UX from Hewlett-Packard.
• Solaris from Sun Microsystems.

Linux is relatively a new member of this family. Linux was initially written by Linus Torvalds in 1991 for IBM compatible personal computers. As an OS, GNU Linux has experienced huge success and popularity in last 20 years with most of the commercial servers now using GNU Linux. To add to the popularity, end users have also started using Linux these days with most of the popular Laptop and PC manufactures giving GNU Linux as a per-installed OS.
For those who are still confused between Linux being an OS or kernel, Linux in true sense as written by Linus was a kernel that was written by referring to book on Unix internals (Though the Linux kernel has adopted good features from many other Unix like kernels too) while the commercially available distributions that contain utilities like graphical desktop, text editors, compilers etc on top of the Linux kernel are complete operating systems.
Though Linux Kernel borrows most of its features from Unix/Unix-Like kernels but still there are many points where the two type of kernels differ significantly. In this article, the main focus will be on these differences. The list is not exhaustive but contains the main differences.

1. Monolithic Vs Micro-kernel Approach

Monolithic kernels are those where all the kernel code runs as a single process while Micro-kernel kernels are those where the core of a kernel (that controls the different pieces of OS) runs in one process while other services like device drivers etc run as different processes. Linux follows monolithic approach while there are a couple of exceptions in Unix-Like kernels that follow Micro-kernel approach.

2. Adding/Removing features to kernel

While traditional Unix/Unix-like systems require static linking of new modules being added, Linux supports a powerful feature where-in kernel components like device drivers etc can be loaded and unloaded dynamically. This feature is known as Loadable kernel modules (LKM). Any new component can be added/removed as an LKM to the kernel. This means there is no need to compile the whole kernel again. Also, if a component is not needed, it can easily be unloaded. This feature makes Linux kernel very flexible.

3. Kernel Threading

Many Unix-Like kernels are organized as a set of kernel threads. A kernel thread can be thought of as an independent execution flow. A kernel thread can run user process or some kernel code. The basic Idea is to do context switches between the kernel threads which is less expensive than context switches between processes as threads operate in same address space. While many Unix-Like OS use kernel threads for process context switching, Linux uses kernel threads only for executing some kernel code periodically.

4. Multi-threaded application support

Almost all modern OS, be it Unix-Like or Linux distributions, support multi-threading. A multi-threaded application is one which creates more than one execution flows. These independent execution flows are known as threads. Threads are light weight processes. In most of the Unix-Like systems, light weight processes are based on kernel threads while in Linux these LWP are created by a call to function clone() which lets the application to create a separate process like fork() does but the difference being that with clone() the newly generated process can share its physical memory, opened files, address space etc. As these newly created process works in a shared environment, so they are given a different name ‘threads’. So we see that Linux and Unix/Unix-Like differ in the way multi-threaded environment is handled internally.

5. STREAMS

Streams I/O subsystem is included in most of the Unix kernels and has become a preferred interface for writing device drivers, terminal drivers etc. While on the other hand there is nothing like Streams in Linux.

6. Preemptive Vs Non-Preemptive Kernels

Preemptive kernels are the kernels which can preempt the currently executing process. It means that a process which is currently executing can be forcibly interrupted if a process with higher priority is ready for execution. On the other hand, Non preemptive kernels are those where a running process cannot be forcibly interrupted even if a higher priority process is ready for execution. Normally, Linux OS are Non preemptive while some of the Unix systems like Solaris 2.x etc are fully preemptive. Usually Real time OS have fully preemptive kernels. These days we have Linux Real time OS which have fully preemptive kernels.
So we see that though Linux was born out of the basic idea from Unix but still it differs from Unix/Unix-Like kernels in many ways. Despite of these differences Linux still inherits a lot from Unix and is still considered as a member of Unix family of kernels.