Hello world!!: February 2012

Tuesday, February 28, 2012

Multithreading in C# - Basics

· Multithreading – is the parallel execution of code

· Use of multithreading

1. Maintaining a responsive UI

2. Allow requests to process simultaneously

· Join –wait for the thread to end/complete the execution

· Sleep – pauses the current thread for specified period

· IsAlive – property returns true if thread is alive

Note: Thread will be in blocked state while waiting on a join/sleep. And will not consume resources

· Yeild – pauses thread running on same processor.

· Preempted – a thread is said to be preempted when its execution is interrupted due to external factor

· Interrupt and abort – interrupt will wait for until thread block to complete, while abort breaks where it is executing

· Thread scheduler manages multithreading

1. It’s a function typically CLR delegates to Operating System

2. TS ensures all active threads are allocated appropriate execution time

3. And threads are waiting/blocked do not consume CPU resource

4. On a Single processor system, TS perform time-slicing

5. On a multi processor system, TS perform mixture of time slicing and concurrency

· Process Vs Thread – a thread in execution is a process. Threads run parallel within a single process. Processes are fully isolated from each other, where threads are partially isolated from each other. (threads share heap memory with other threads)

· Foreground and background threads – by default foreground threads are created. (Thread.IsBackground=true)

· Thread Priority – property determines how much execution time it gets (Lowest, BelowNormal, Normal, AboveNormal, Highest)

· Thread pooling –allow execution of the threads simultaneously. Once the limit is reached it queues up and continues execution when other finishes (ThreadPool.QueueUserWorkItem)

· Asynchronous delegate – If a thread wants to notify (return value back) after it has finished execution, then this can be achieved using asynchronous delegate.

1. Create a delegate for the method to be called (Func delegate)

2. Call BeginInvoke on delegate with IAsyncResult return value

Note: BeginInvoke return immediately to the caller.

· ThreadState: gives you threads execution status.

o Unstarted

o Running

o WaitSleepJoin

o Stopped

· Thread synchronization

o Lock (Monitor.Enter, Monitor.Exit)

o Mutex

o Semaphore

o SemaphoreSlim

o SpinLock

· Lock – ensure only one thread can enter particular section of code at a time (Exclusive lock)

object locker = new object();

System.Threading.Monitor.Enter(locker);

System.Threading.Monitor.Exit(locker);

· Mutex – is an exclusive lock. Purpose is inter-process synchronization. It can work across multiple processes. Mutex can be computer-wide or application-wide.

· Semaphore – Ensures not more than a specified number of concurrent threads can access a resource. It is cross process.

Reference: http://www.albahari.com/threading/

Multithreading (computer architecture)

Multithreading (computer architecture)

Multithreading aims to increase utilization of a single core by using thread-level as well as instruction-level parallelism

Instruction-level parallelism (ILP) is a measure of how many of the operations in a computer program can be performed simultaneously.

1. e = a + b

2. f = c + d

3. g = e * f

Operation 3 depends on the results of operations 1 and 2, so it cannot be calculated until both of them are completed. However, operations 1 and 2 do not depend on any other operation, so they can be calculated simultaneously.

Advantage of multithreading:

· concurrent execution : advantage of a multithreaded program allows it to operate faster on computer systems

· One main use of multithreading, for single-CPU systems, is the ability for an application to remain responsive to input.

Drawback of multithreading:

· Careless use of threading can cause deadlock. If two threads are accessing same resource at same time can cause deadlock.

Operating systems schedule threads in one of two ways:

1. Preemptive multithreading is generally considered the superior approach, as it allows the operating system to determine when a context switch should occur.

2. Cooperative multithreading, on the other hand, relies on the threads themselves to relinquish control once they are at a stopping point

Reference:

http://en.wikipedia.org/wiki/Multithreading_(computer_architecture)

http://en.wikipedia.org/wiki/Instruction_level_parallelism

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