Download Lecture 5

DT265-2
Object Oriented Software
Development 2
Lecture 5 : Concurrency
Lecturer Pat Browne
[email protected]
Syllabus
• Concurrency
– Threading and concurrency
– Thread synchronization
– Parallel extensions and platforms
• Programs in this section will be run from the
command line. Find where csc.exe is on your
machine, say
• C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe
• You can add this location you the PATH system
variable.
• csc.exe file.cs
Processes
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Concurrent programs
The hardware interface
Support for processes
Processes and threads
Processes in language systems
Processes in C#
Sequential & Concurrent programs
• A sequential program has a single thread of
control. Its execution is called a process.
• A concurrent program has multiple threads of
control. These may be executed as parallel
processes.
• Non-determinism: concurrent programs are
nondeterministic in their execution order, and
may also have nondeterministic results.
• Run-time overhead: thread construction, context
switching and synchronization take time
UML state charts & interaction
diagrams
What is a concurrent system
• A concurrent system consists of a set of activities
in the process of being executed. That is, at a
given time activities are at some stage between
their start and finish.
• The activities may be competing or cooperating.
• Activities cooperate to achieve a common goal
(e.g. they may be part of a larger activity).
Concurrent activities may need to cooperate by
synchronizing their activities or exchanging data.
What is a concurrent system
• The activities may be competing or
cooperating.
– Activities cooperate to achieve a common goal
(e.g. they may be part of a larger activity).
Concurrent activities may need to cooperate by
synchronizing their activities or exchanging data.
– Activities will compete when they share resources
but the resources can only be used by one activity
at a time, (e.g. printer, updating a database)
Devices, Buffers, Interrupts, Poling
• External devices are much slower than processors.
• Processors must proceed at their fastest rate and not be held up
by slow devices. Devices and processors operate concurrently
(in parallel), and exchange data through synchronization.
• Buffers, interrupts and polling help achieve synchronization.
• A buffer storage area accessed by both a device and the
processor.
• If an activity (process) is interrupted, it stops execution an
interrupt service routine (ISR) is called to carry out the
necessary tasks, the state of the process that has been
interrupted must be saved in order to be able to resume
execution of it later. Upon completion of execution ISR, the
previous process or other process will be restarted. Context
switching takes significant time.
• Poling is alternative to interrupts.
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Process states
Process asked
to wait
Blocked
waiting for an event
Running
on a processor
Scheduled
An event occurred
The process runs
out of time
or preempted
Each process as a
process descriptor:
Runnable
Waiting for a processor
Process id
Process priority
Process state
(blocked, running)
Resources allocated
Lists the events for
which the process
may be waiting,
Start address
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Desirable properties of concurrent
systems
• Safety: ensuring consistency ,concurrent
processes may corrupt shared data
• Liveness: processes may “starve” if not properly
coordinated.
• Fairness: A fairness guarantee states that the next
operation in a runnable thread eventually will
execute. An implementation technique that is
often used to provide fairness guarantee is preemption. Pre-emption means running threads
are periodically stopped by software in order to
let other threads proceed.
Desirable properties of concurrent
systems
• Safety = ensuring consistency
• A safety property says “nothing bad happens”.
Mutual exclusion: shared resources must be
updated atomically
• Condition synchronization: operations may be
delayed if shared resources are in the wrong
state (e.g., read from empty buffer)
Desirable properties of concurrent
systems
• Liveness = ensuring progress
• A liveness property says “something good
happens”
• No Deadlock: some process can always access
a shared resource
• No Starvation: all processes can eventually
access shared resources
Desirable properties of concurrent
systems
• Fairness and Liveness are related notions of
describing which of a collection of concurrent
processes make progress in their execution. A
fairness constraint is imposed on (the
scheduler of) the system that it fairly select
the process to be executed next. A fairness
constraint is a condition on executions (paths)
of the system model.
Liveness
• Liveness describe the requirement that a
process make progress toward a specific goal,
the achievement of which depends on the
fairness of the system. If a process never gets
to execute, it usually cannot reach its goal.
Therefore, liveness is often evaluated only
along fair paths, i.e. paths that satisfy one of
the above three fairness path requirements.
Synchronous and asynchronous input and output
• synchronous I/O policy : a process is blocked
while it waits for I/O to be completed
• asynchronous I/O policy: a process is allowed
to continue its execution and pick up the
requested input or acknowledgement of
output later
• Some systems offer these two policies as
options at the application level.
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Multi-threaded process implementation
• A single program can be subdivided into subtasks that are executed by
separate processes.
– One (main) process will be responsible for executing the program, in the
course of its execution, other processes will be created to execute subtasks
concurrently.
• Each process must be associated with memory locations that contain its
executable code, its data, its references to open files and so on (the
process’s resources).
• Some OS allow subtask processes to share the resources of the main
process. (lightweight processes) (or threads)
– the full overhead associated with a context switch is avoided.
• main processes are referred to as heavyweight processes (or simply
processes)
– a context switch between such processes will incur the full overhead of
changing the resource information.
• A multi-threaded process : A (heavyweight) process that is split into many
lightweight processes
• A multi-threaded operating system :an operating system that supports
lightweight processes
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Concurrent programming language with OS support
for use thread as kernel thread
One program,
many user threads
One program,
many user threads
Language runtime
system
Language runtime
system
Operating system kernel
• Operating system knows about and manages the threads created by each
process.
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Threads
• Multi-tasking: The ability of a computer to carry out more than
one task or program concurrently. (tasks is heavyweight processes.
• Multi-threading: an individual program which can be divided up
into a number of concurrent tasks. (task is lightweight processes
(or thread)).
• For user friendly programs
– While program is running, it interacts with user.
– To do this
• The execution of the program is divided into a separate thread, leaving the
main program thread free to listen for user activity.
• In C#, threads are created using the Thread Class. Design choice :
– Define class as subclass of Thread, and override the method run, which is
the method that is executed to perform the task assigned to a thread.
– Or
– Declare a thread as implementing the Runnable interface
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C# Thread Pool
• A thread pool is a collection of threads that can
be used to perform a number of tasks in the
background. This leaves the primary thread free
to perform other tasks asynchronously.
• Thread pools are often employed in server
applications. Each incoming request is assigned
to a thread from the thread pool, so the request
can be processed asynchronously, without tying
up the primary thread or delaying the processing
of subsequent requests.
Sockets
• A Socket is a bidirectional end-point for a
communication link between two programs.
We use
• SynchronousSocketClient.cs
• SynchronousSocketListener.cs
Sockets
• A Socket is a bidirectional end-point for a
communication link between two programs. We
use
• SynchronousSocketClient.cs
• SynchronousSocketListener.cs
• C# Server Socket Program: A C# Server Socket
Program running on a computer has a socket that
bound to a Port Number on the same computer
and listening to the client's incoming requests.
Remoting
• The .NET Remoting API is a framework that
allows objects on a client machine to
communicate with remote objects on a server.
• .NET remoting provides an abstract approach
to interprocess communication that separates
the remotable object from a specific client or
server application domain and from a specific
mechanism of communication.
Compiling from command line
• Find where csc.exe is on your machine, say
• C:\Windows\Microsoft.NET\Framework\v4.0.30319\csc.exe
• You can add this location you the PATH system
variable.
• csc.exe file.cs