Lõimed ja protssesdevaheline komnikatsioon

protsessi kood kahend süsteemis.

https://www.youtube.com/watch?v=trQAn9J0Q4E


kasutaja lõimed :

POSIX Pthreads (lõimed)
Win32 threads (lõimed)
java threads (lõimed)

kerneli lõimed:

Windows XP/2000 (lõimed)
Solaris (lõimed)
Linux (lõimed)
Tru64 UniX (lõimed)
Mac OS X (lõimed)

ühe kerneli lõimega on seotud mitu kasutaja lõime .

näited:

Solaris Green threads (lõimed)
GNU Portable threads (lõimed)

iga kasutaja lõim on seotud kerneli lõimuga
(vt pilt 1.)

Näited:

Windows NT/XP2000
Linux
Solaris 9 and later


Kahe tasandiline mudel
(vt pilt 2.)


Näited:

IRIX
HP-UX
Tru64 UNIX
Solaris 8 and earlier



Lõime juhtumid:

Semantics of fork() and exec() system calls

Thread cancellation
Signal handling
Threads pools
Threads specific data
Scheduler activations


Semantics of fork() and exec()

Does fork() duplicate inly the calling thread or all threads?


Signal Handling

Thread Pools

single and multithreads protcess

Thread Specific Data

Allows each thread to have its own copy of data

Usefull when you do not have control over the thread
creation process (i.e., when using a thread pool)


Benefits

Responsiveness

Resource Sharing

Economy

Utilization of MP Architectures

Pthreads

A POSIX standard (IEEE 1003.1c) API for threads
creation and synchroniceation

API specifies behavior of the thread 

WIN XP threads ( lõimed )

Linux Threads

Linux refers to them as tasks rather than threads

Thread creation is done throught clone() system call

Clone() allows a child dask to share the address space of the parent task (process)


Java threads

java threads are managed by the JVM

Java threads may be created by:

           Extending thread class
           Implemending the Runnable interface

 






Interprocess Comunication

• Interprocess cumunication
• deadlocks
• Classic IPC Problems

Producer - Consumer Problem

Buffer is shared (ie., it is a shared variable)

Progress in time ....

Both processes are started at the sa,e time and consumer ases some old value initially

A race Condition

Because of the timing and wich process starts fisrt

Critical Sections

Critical sections

a section of code in which the process acesses and modifies shared variables


Mutal Exclusion

A method of preventing for ensuring that one of are in a critical sections 

Mutal exclusin Problem starvation

Defination

indefinitely delaying the scheuling of a process in favor of other processes

Cause

Usually a bias in a sustem scheluling policies

solution 

implement some from a again

Another Problem Deadlocks

More on semaphores

• Two types of semaphores
- binary semaphores can only be 0 or 1
- counting semaphores can be any non-negative integer

• Semaphores are an OS service implemented using one of the methods shown early
-usually by disabling interrupts for a very short time

What is deadlock?

• process deadlock
-a process is deadlocked when it is waiting on an event which will never happen

•system deadlock
- a system is deadlocked when one or more processes are deadlocked

Necessary conditions for a deadlock

• mutal exclusion
-shared resources are used in a mutually exclusive manner

• hold & wait
-processes hold onto resources they already have while waiting for the allocation of other resources

no preemption
resources can not be preemted until the process releases them

circular wait
a circular chain of processes exists in which each process holds recoursec wanted by the next process in the chain

No deadlock situations

if you can prevent at least one of the necessary deadlock conditions then you won't have a deadlock.


Denying " no problem "

implementation

• when a process is refused a resources request, i MUST release all other resourses it holds
• Reousurses can be removed from a process before it is finished with them

Advantages

• it works
• bossibly better resource utilization

Problems

• the cost of removing a process' resources
• the process is likely to lose work it has done 
• possibility of starvation