Concurrent Processes
Lecture 5
Introduction
• Modern operating systems can handle more
than one process at a time
• System scheduler manages processes and their
competition for the CPU
• Memory manager role is to manage sharing of
main memory between active processes
• Look at how processes coexist and
communicate with each other on modern
computer
Concurrency pros and cons
• Concurrency is good for users
– One of the reasons for multiprogramming
• Working on the same problem, simultaneous
execution of programs, background execution
• Concurrency is a “pain in the neck” for the
system
– Access to shared data structures
– Deadlock due to resource contention
– Enabling process interaction
Concepts:
• A process is a program in execution.
• A process has state, including resources like
memory and open files, as well as the contents of
its program counter.
• This is the process’s execution context.
• A concurrent program has more than one
execution context.
• This type of program is also referred to as a
multi-threaded program.
• A parallel program is a concurrent program with
more than one thread executing simultaneously.
Relationships between processes
• Fully indenendant – separate applications
running on the one system
• Indenpendant but related – example users
running their own copy of a data entry
program but accessing and updating the
one database
• Concurrent processes – set of cooperating
processes, example C program
Resources
• Processes compete for CPU, memory, I/O
devices, secondary storage
• Other resources may include data items in main
memory such as message queues, shared data
structures.
• Resources can be reusable or consumable,
reusable examples are CPU, main memory
(serial reusable example is printer). Consumable
resource is created by one process and
consumed by another example is message sent
between one process and another.
Mutual Exclusive
• It is necessary for some resources to remain
allocated to a process for as long as process
requires. Serial reusable resources require
mutual exclusion. Example printer is being used
by a process then it must remain allocated to the
process until printout is complete.
• Mutual exclusion gives rise to another problem
which OS must handle – deadlock.
Deadlock
• Deadlock is where a process is waiting for
an event that will never occur.
• Example Process P1 has a printer
allocated and is attempting to open a file
F. Process P2 has already obtained
exclusive access to file F and now
requests printer. Each process is waiting
for a resource held by the other, while
holding a resource required by the other.
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File F
Mutual Exclusion First Attempt
• Busy Waiting
– Process is always checking to see if it can
enter the critical section
– Process can do nothing productive until it gets
permission to enter its critical section
Co-routine
• Designed to be able to pass execution
control back and forth between
themselves
• Inadequate to support concurrent
processing
Second Attempt
• Each process can examine the other’s status but cannot
alter it
• When a process wants to enter the critical section is
checks the other processes first
• If no other process is in the critical section, it sets its
status for the critical section
• This method does not guarantee mutual exclusion
• Each process can check the flags and then proceed to
enter the critical section at the same time
Third Attempt
• Set flag to enter critical section before check
other processes
• If another process is in the critical section when
the flag is set, the process is blocked until the
other process releases the critical section
• Deadlock is possible when two process set their
flags to enter the critical section. Now each
process must wait for the other process to
release the critical section
Fourth Attempt
• A process sets its flag to indicate its desire to
enter its critical section but is prepared to reset
the flag
• Other processes are checked. If they are in the
critical region, the flag is reset and later set to
indicate desire to enter the critical region. This
is repeated until the process can enter the
critical region
Fourth Attempt (contd.)
• It is possible for each process to set their
flag, check other processes, and reset
their flags. This scenario will not last very
long so it is not deadlock. It is undesirable
Correct Solution
• Each process gets a turn at the critical
section
• If a process wants the critical section, it
sets its flag and may have to wait for its
turn
Critical region
• The area of a process which is sensitive to
inter-process complications is called the
critical region.
• To guarantee mutual exclusion only one
process is allowed to enter its critical
region at a time.
• OS need a system to ensure process
cannot enter critial region if another
process is in it’s. (semaphores)
Mutual Exclusion with Busy
Waiting
• Figure 2-9 Mutual exclusion using critical
regions.
Critical Regions (1)
Four conditions to provide mutual exclusion
1.
2.
3.
4.
No two processes simultaneously in critical region
No assumptions made about speeds or numbers of
CPUs
No process running outside its critical region may
block another process
No process must wait forever to enter its critical
region
Semaphores:
What are they?
• Semaphore is a system of communication using
flags. There are 2 stations a sender and
reciever and they must be able to clearly see the
other station.
• For long distance communication, there could be
a number of repeater stations between the
sender and reciever. Generally a station
comprises of 2 people.
Semaphores: In Software
(contd.)
• Dijkstra in 1965 proposed semaphores as a solution to
the problems of concurrent processes.
• The fundamental principle is: That two or more
processes can cooperate by means of simple signals,
such that a process can be forced to stop at a specified
place until it has received a specific signal.
• For signaling, special variables called semaphores are
used.
– Primitive signal (s) is used to transmit a signal
– Primitive wait (s) is used to receive a signal
File and record locking
• Mutual exclusion is required file
processing where several users are
accessing the same file (databases)
• Example airline booking system, one seat
left two agents access at same time before
file is updated and seat is double booked.
• Solution is file locking preventing access
to the file while it is being updated
Types of file locking
• File lock – whole file is locked, prevents
any other process accessing any part of
that file.
• Write lock – if a specific set of data (eg
order file) is write locked prevents other
process modifying or reading data
• Read lock – set of data is read locked
other processes may read data but no
process can modify it.