1. Concurrency

2. Modern OS central themes
Multiprogramming
"the management of multiple processes within a single computer system"
Distributed processing
"the management of multiple processes executing on multiple, distributed computer systems"
Fundamental to both themes is concurrency

3. The concept of concurrency
Processes are concurrent if they exist at the same time
Processes that are either interleaved or overlapped in time to yield the appearance of simultaneous execution P1 P2 P3
time

4. A simple example
A program to provide character echo: input character from keyboard and output character on display
repeat
parbegin
out:= in;
output (out, display);
input (in, keyboard)
parend
This program will
work or not depending
on the sequence the
concurrent operations
take place
(copy is wrong)

5. How to implement concurrency
Programming language structures and mechanisms
most programs can be decomposed in at least three processes: input, process, output
Operating systems structures and mechanisms
process interaction
mutual exclusion

6. Process Interaction Degree of Awareness Relationship
Influence one process has on another
Control problems
unaware
competition
results idependent
timing may be affected
mutual exclusion
deadlock (renewable)
starvation
indirectly aware
cooperation- sharing
results may depend
timing may be affected
mutual exclusion
deadlock (renewable)
starvation coherence
directly aware
cooperation-
communication
results may depend
timing may be affected
deadlock (consumable)
starvation

7. Competition for resources
Mutual exclusion
critical resources (nonsharable)
a process manages each critical resource
Deadlock
a process is holding a critical resource (c1) and waiting for another critical resource (c2)
another process is holding (c2) and waiting for (c1)
Starvation
one process is never granted access to a critical resource for other processes always get higher priority (and access) to the critical resource

8. Mutual exclusion requirements
only one process at a time is allowed access to a critical resource
a process that halts in a noncritical section should not affect others
a process should not be allowed to deadlock or starve
no delays should occur when a process requires a critical resource no others are using
a process remains in its critical section for a finite time only
Technical: Dekker and Peterson algorithms,
and Dijkstra semaphores

9. Monitors
" a concurrency construct that contains both the data and procedures to perform allocation of a particular serially reusable shared resource or group of resources"
only one process is allowed to enter the monitor
if a process calls the monitor and a resource is already allocated the monitor calls wait (outside of monitor)
when a process releases a resource the monitor calls signal (and grants the resource to a process waiting for it)
it gives priority to waiting versus new

10. Deadlock
"the permanent blocking of a set of processes that either compete for systems resources or communicate with each other"
resources
reusable (not depleted with use)
consumable (produced and consumed)
conditions for deadlock
mutual exclusion
hold-and-wait
no preemption
circular wait (deadly embrace)

11. Deadlock management Principle Resource allocation policy schemas
advantages
disadvantages
Prevention
conservative
request all resources at once
preemption
resource
ordering
processes with single burst of activity
resources whose state can be saved and restored easily
problem solved by design
inefficient, delays process initiation
preempts more than necessary,
needs to be restarted
preempts without need
Detection
very liberal
invoked periodically to test deadlock
does not delay process initiation
preemption losses
Avoidance
midway between conservative and liberal
search to find at least one safe path
no preemption needed
resources must be known, long blockings