unix derivatives: mac osx and ubuntu

8/10/2019 UNIX Derivatives: Mac OSX and Ubuntu

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UNIVERSITY OF THE SOUTHERN CARIBBEAN

MARACAS ROYAL ROAD, MARACAS, ST. JOSEPH

UNIX Derivatives: Mac OS X and Ubuntu

A Kernel Approach

An Assignment

Presented in Partial Fulfilment

Of the Requirements for the Course

CPTR461: OPERATING SYSTEMS 1

Instructor: Connell Byron Hunte

By:

Kyle Roach

25th

January 2014

Approval.


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Windows is the best operating system. This is a statement often made by ignorant

Windows users who have never used another operating system. In the first quarter of 2014,

88.99% of the worldwide desktop users had a Windows system as their operating system of

choice (Stat Counter Global Stats 2014). Unfortunately this number includes users who have

never used or been exposed to other operating systems. To them, Windows is the best and only

choice for an operating system. However to smaller population, we know that other operatingsystems offer features and tools which far outweigh those of Windows. Outside of Windows

systems, the other operating systems are all UNIX-based in some way. UNIX, the computer

operating system from Bell Labs, is still regarded as one of the most powerful, versatile, and

flexible operating systems in the computer world. Its popularity is due to many factors, including

its ability to run a wide variety of machines, from micros to supercomputers, and its portability --

all of which led to its adoption by many manufacturers (Bell Labs 2002). After modifications

made my developers over the years we ended up with two prominent kernels XNU and Linux.

In my paper I will seek to point out the differences and similarities between these two kernels

and how they affect the operating systems that they run.

On many online forums about operating systems, you would often see the comment being

made that OS X is a Linux-based operating system. However OS X is actually built on the XNUkernel which like Linux, got its origins from UNIX. The operating systems we will use if needed

are Apples OS X with its XNU kernel and Ubuntu with its Linux kernel. The criteria we will

use to compare these are process management, storage management, and network management.

The first and foremost biggest difference between the two operating systems lies in the

architecture of the kernel. The kernel is the indispensable and therefore most important of an

operating system. The kernel is the true brains of the OS and controls virtually all aspects of

interacting with the underlying hardware (Taggart 2008). Basically without this essential

component of the operating system your computer would just be an expensive paperweight with

spinning fans. Roughly the operating system itself consists of two parts: the kernel space

(privileged mode) and user space (unprivileged mode). It runs every basic system service like

process and memory management, interrupt handling and I/O communication, file system, etc. in

kernel space. There are two different concepts of kernels: monolithic kernel and -kernel

(microkernel) (Roch 2004). A monolithic kernel is one in which the entire kernel exists as one

large file. The second approach is known as the microkernel. An OS built on the microkernel

approach will have many smaller kernels that all communicate with each other (Taggart 2008).

Linux uses the monolithic approach in which its kernel is one large file that is made up

of over 5.9 million lines of code. In fact, almost all of the hardware drivers are contained in the

kernel itself. This makes installing "drivers" in Linux a non-issue. Most hardware simply works

out of the box.

On the other hand XNU is hybrid kernel, containing features of both monolithic kernels

and microkernels, attempting to make the best use of both technologies, such as the messagepassing capability of microkernels enabling greater modularity and larger portions of the OS to

benefit from protected memory, as well as retaining the speed of monolithic kernels for certain

critical tasks (Taggart 2008). The basis for the XNU kernel however is the Mach microkernel.

The Kernel and Device Drivers layer of OS X consists of the Mach kernel environment, device

drivers, BSD library functions (libSystem), and other low-level components. The layer includes

support for file systems, networking, security, interprocess communication, programming

languages, device drivers, and extensions to the kernel. These combined form what we know as


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Darwin.

Process Management

Now that we have identified the difference in kernel architecture, we can continue to how

they each manage processes. The process is one of the fundamental abstractions in UNIX

operating systems. A process is a program in execution. A program itself is not a process a

process is an active program and related resources. Indeed, two or more processes can exist that

are executing the same program. In fact, two or more processes can exist that share various

resources, such as open files or an address space (Love 2005). In OS X, processes do not

normally share memory. Instead, the kernel assigns each process its own address space,

controlling access to these address spaces. This control ensures that no application can

inadvertently access or modify another applications memory (protection) (Apple 2004).

Threads of execution, often shortened to threads, are the objects of activity within the

process. To Linux, a thread is just a special kind of process. It does not differentiate between

threads and processes (Love 2005). In OS X each process is made up of one or more threads,each of which represents a single path of execution through the application's code. Every

application starts with a single thread, which runs the application's main function. Processes do

not directly interact with the processor, this is handled by threads. Applications can spawn

additional threads, each of which executes the code of a specific function. When an application

spawns a new thread, that thread becomes an independent entity inside of the application's

process space (Apple 2013).

Multitasking operating systems come in two flavours: cooperative

multitaskingandpreemptive multitasking. Both OS X and Ubuntu use pre-emptive multitasking.

In preemptive multitasking, the scheduler decides when a process is to cease running and a new

process is to resume running. Both OS X and Ubuntu employ two general schedule techniques:

time-sharing and fixed priority. A time-sharing threads priority is raised and lowered to balanceits resource consumption against other time-sharing threads. Fixed-priority threads execute for a

certain quantum of time, and then are put at the end of the queue of threads of equal priority

(Apple 2004), (Love 2005).

Uniprogramming environments ran a task till it was finished and then started another

task. With the introduction of multi programming and scheduling processes, came the problem of

deadlocks. In OS X there are two deadlock implementations, a deadlock and a livelock. In a

livelock situation, a thread gives up its first lock in an attempt to acquire its second lock. Once it

acquires the second lock, it goes back and tries to acquire the first lock again and this process

continues. Both OS X and Ubuntu use the same methods for detecting, preventing, and resolving

deadlocks.

Storage Management

All computer applications need to store and retrieve information. While a process is

running, it can store a limited amount of information within its own address space. However to

store information persistently, a file system is needed. UNIX has provided four basic

filesystem-related abstractions: files, directory entries, inodes, and mount points. The file-system

component of Darwin is based on extensions to BSD and an enhanced Virtual File System (VFS)

design (Apple 2004). The Virtual File System is the subsystem of the kernel that implements the


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filesystem-related interfaces provided to user-space programs. This enables programs to use

standard UNIX system calls to read and write to different filesystems on different media (Love

2005). Both OS X and Ubuntu use a virtual filesystem as an abstraction layer between the

user-space and the filesystem. There are three file system designs. (a) Single directory shared by

all users. (b) One directory per user. (c) Arbitrary tree per user. In both OSs one directory is used

per user (Tanenbaum, Woodhull 2006). Both OS X and Ubuntu have the same file system layout,however in OS X many of these directories are hidden by default from the user, and permissions

to limit a users access to system files and other user directories are set automatically (Cone

2011). A Linux system, just like UNIX, makes no difference between a file and a directory, since

a directory is just a file containing names of other files. Programs, services, texts, images, and so

forth, are all files. Input and output devices, and generally all devices, are considered to be files,

according to the system. In order to manage all those files in an orderly fashion, man likes to

think of them in an ordered tree-like structure on the hard disk, as we know from MS-DOS (Disk

Operating System) for instance. The large branches contain more branches, and the branches at

the end contain the tree's leaves or normal files (Garrels 2004). The default file system in OS X

is HFS+ while Ubuntu uses Ext2, Ext3, Ext4. The kind of problem resulting in an error message

saying that permission is denied somewhere is usually a problem with access rights in mostcases. Also, comments like, "It worked yesterday," and "When I run this as root it works," are

most likely caused by the wrong file permissions. Both Ubuntu and OS X provide the chmod

system call to modify user permissions.

Networking Management

Networking allows us to communicate with people all around the world. Networking also

occurs at a device level, where devices communicate and transfer data across networks. Both OS

X and Ubuntu both provide support for various communication protocols out of the box which

provide for easy network management and usage. In network communication is described in

what we call a protocol stack. Each layer of the protocol stack is connected or accessed by a

group of protocols Integrated into OS Xs kernel is a customized version of the BerkeleySoftware Distribution (BSD) operating system. BSD serves as the basis for the file systems and

networking facilities of OS X (Apple 2004). This provides strong TCP/IP networking with

support for industry standards such as SLIP, PPP, and NFS. OS X can interoperate easily with

other systems as well as act as an enterprise server, providing vital functions such as NFS

(remote file access) and email services, or Internet services such as HTTP, FTP, routing, and

firewall (security) services (Apple 2004).

The network function is a very important part of the Linux kernel. More than 30% of the

Web server on the Internet are constructed on Linux. The protocol stack of Linux is a part of the

kernel and is embedded in the kernel code. The structure of the network protocol makes it

possible to support lots of protocols at the same time. Linux provides a system call named

sys_socketcall to provide an interface to application programs. It provides standard BSD Socket

interface to realize the separation of the user program and the network functions of the Linux

kernel. The data in the user space is copied into the kernel space. The kernel and the user

programs are different running units, and have their own behavior. In order to protect the unity of

the data, the user program and the kernel have a copy of the data of their own independently

(Shaoren, & Chuanxiong 2000). TCP/IP networking has been present in Linux since its

beginnings. It has been implemented from scratch. It is one of the most robust, fast and reliable


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implementations and is one of the key factors of the success of Linux. Linux and networking are

made for each other, in so much that not connecting your Linux system to the network may result

in slow startup and other troubles. Even if you don't use any network connections to other

computers, networking protocols are used for internal system and application communications.

Linux expects to be networked (Garrels 2004).

In conclusion the UNIX operating system and standards gave rise to spark of kernel

modifications, which integrally led to the development of UNIX- based operating systems, such

as OS X and Ubuntu. We saw that even though both operating systems were UNIX-based, they

still had differences, which were primarily dependent on the structure and architecture of their

kernels. Both operating systems offer advanced tools which allow them to be competent in the

business world, providing solid alternatives to the Windows systems meta. In my opinion, OS X

and its XNU kernel provides more power and features compared to Ubuntu and its Linux kernel,

primarily because the XNU kernel is hybrid which allows features from both kernel

architectures. However as the saying goes, A man will always use the right tool for the job. In

saying this, there is no best operating system, for the needs of consumer will always determine

what features of an os he needs. Windows isnt the only operating system on the market.


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References

Stat Counter Global Stats. (2014). Top 7 Desktop OSs from January to March 2014.Retrieved

from http://gs.statcounter.com/#desktop-os-ww-monthly-201401-201403-bar

Bell Labs. (2002). The Creation of the UNIX Operating System.Retrieved from

http://www.bell-labs.com/history/unix/

Roch, B. (2004).Monolithic Kernel vs Microkernel.Retrieved from

http://www.davepowell.org/media/teaching/operating-systems/handouts/COMP354-Lec0

2-KernelComparisons.pdf

Tanenbaum, A. S., & Woodhull, A. S. (2006) Operating Systems Design and Implementation,

(3rd ed.). New Jersey: Prentice Hall.

Taggart, Michael. (2008).Linux vs. Windows vs. OS X - The Kernel Debate. Retrieved from

http://roala.blogspot.com/2008/01/linux-vs-windows-vs-os-x-kernel-debate.html

Love, R. (2005).Linux Kernel Development (2nd ed.). Indianapolis, IN: Sams Publishing.

Apple Inc. (2004). Kernel Programming Guide.Retrieved from

https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/KernelProgra

mming/Architecture/Architecture.html

Apple Inc. (2013). Thread Programming Guide. Retrieved from

https://developer.apple.com/library/mac/documentation/cocoa/conceptual/multithreading/


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CreatingThreads/CreatingThreads.html#//apple_ref/doc/uid/10000057i-CH15-SW2

Apple Inc. (2004).Mac Technology Overview.Retrieved from

https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/OSX_Tech

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Rusling, D. A. (2000). The Linux Kernel.New Jersey: Pearson Education

Shaoren, Z., & Chuanxiong, G. (2000). Analysis and Evaluation of the TCP/IP Protocol Stack of

LINUX*. 2000 International Conference on Communication Technology Proceedings

(Vol. 1). doi: 10.1109/ICCT.2000.889245

Garrels, M. (2004).Introduction to Linux.California. Fultus Corporation

Cone, M. (2011).How to Set File Permissions in Mac OS X. Retrieved from

http://www.macinstruct.com/node/415
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