Operating systems design

philosophy ESMAIL ASYABI- FEBRUARY 2015

What is Operating systemsExtended virtual machine

Resource manager

OS as a extended virtual machineAccessing the hardware directly could be very complex

◦ kernels implement a set of hardware abstractions. ◦ They hide the complexity, ◦ They provide a clean and uniform interface to the underlying hardware.

Kernel Types

Kernels can be classified into three broad categories

◦ Monolithic kernels◦ Micro kernel◦ Hybrid Kernel (This is bit controversial. We will see why it is )

Monolithic kernelsExample : MS-DOS, BSD, HP-UX, AIX, Windows 98

Microkernel

Microkernel It moves parts of the kernel away from the dangerous kernel space into userspace

limit the amount of damage a bug can cause.

the parts run in isolated processes (so-called ‘servers’) as a consequence, they do not influence each other’s functioning.

If a subsystem will crash (e.g. networking server) , all other subsystems will continue to function.

microkernel operating systems have a system in place which will automatically reload crashed servers.

Mach, Minix, QNX, L4

Hybrid Kernel

 Microsoft Windows NT, 2000, XP, Vista, and 7. Apple Mac OS X uses a hybrid kernel 

Less Is More (XEN) Xen attempts to do less than the previous version. The reason for this is that Xen runs at a

very high level of privilege above even the operating system. A bug in a program

may compromise the data that that program can access, a bug in a kernel might

compromise an entire system, but a bug in Xen can compromise every virtual

machine running on a machine. For this reason, it is important that the Xen code

be as secure and bug-free as possible.

OS as a resource managerFCFS

RR

SJF

Fairness

Fairness (equality) Equality simply means that two things are judged to have the same value.

Judging two people as having the same value before the law (i.E. Equal protection) is an objectively good thing.

Judging that any two people have the exact same needs is objectively wrong though. Equality might be good in some things, but its opposite- individuality also has its own uses and

benefits.

Fairness vs sameness

Sameness In the OSFor example in process scheduling

P3

P2P2 P2

P1

P1

P2

P2 P3

Fairness (equity)the quality of being fair

P3P1 P2

P1 P2 P3

Fairness (Sameness VS Equity)

QoS Aware Operating Systems (clouds)