Operating systems design
philosophy
ESMAIL ASYABI- FEBRUARY 2015
What is Operating systems
Extended virtual machine
Resource manager
OS as a extended virtual machine
Accessing 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 kernels
Example : 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 manager
FCFS
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 OS
For example in process scheduling
P1
P1
P2
P2
P3
P2
P2
P2
P3
Fairness (equity)
the quality of being fair
P1
P2
P1
P2
P3
P3
Fairness (Sameness VS Equity)
QoS Aware Operating Systems (clouds)