Thursday, November 20, 2014

Knowledge: Parallelism

In this post, we will cover some simple concepts about parallelism.

1. Process v.s Thread
1. Threads are easier to create than processes since they
don't require a separate address space.

2. Multithreading requires careful programming since threads
share data strucures that should only be modified by one thread
at a time.  Unlike threads, processes don't share the same
address space.

3.  Threads are considered lightweight because they use far
less resources than processes.

4.  Processes are independent of each other.  Threads, since they
share the same address space are interdependent, so caution
must be taken so that different threads don't step on each other.  
This is really another way of stating #2 above.

5.  A process can consist of multiple threads.




2. mutex vs. semaphores?
A mutex is like a lock. Mutexes are used in parallel programming to ensure that only one thread can access a shared resource at a time.


Semaphores is in computer science, particularly in operating systems, a semaphore is a variable or abstract data type that is used for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment.

A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record as units are required or become free, and, if necessary, wait until a unit of the resource becomes available. Semaphores are a useful tool in the prevention of race conditions; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores.

3. How to implement a mutex?

void acquire_lock( Lock lock1) {
    while (test_and_set(lock1));
}

int test_and_set(int x) {
     if (x) {
         return 1;
     } else {
         x = 1;
         return 0;
     }
}

void acquire_unlokc(Lock lock1) {
    lock1 = 0;   // we need it to be atomic instruction
}


4. What is synchronized  method in Java?
To make a method synchronized, simply add the synchronized keyword to its declaration:
public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

    public synchronized void decrement() {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}

If count is an instance of SynchronizedCounter, then making these methods synchronized has two effects:
  • First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
  • Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.
Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.


Whenever a synchronized method is called, the mutex is locked. When the method is finished, the mutex is unlocked. This ensures that only one synchronized method is called on a given object.


5. How to prevent deadlocks?
Lock Ordering
Deadlock occurs when multiple threads need the same locks but obtain them in different order.

If you make sure that all locks are always taken in the same order by any thread, deadlocks cannot occur. Look at this example:

Thread 1:
lock A
lock B

Thread 2:
wait for A lock C (when A locked)

Thread 3:
wait for A wait for B wait for C


If a thread, like Thread 3, needs several locks, it must take them in the decided order. It cannot take a lock later in the sequence until it has obtained the earlier locks.

For instance, neither Thread 2 or Thread 3 can lock C until they have locked A first. Since Thread 1 holds lock A, Thread 2 and 3 must first wait until lock A is unlocked. Then they must succeed in locking A, before they can attempt to lock B or C.

Lock ordering is a simple yet effective deadlock prevention mechanism. However, it can only be used if you know about all locks needed ahead of taking any of the locks. This is not always the case.


Lock Timeout
Another deadlock prevention mechanism is to put a timeout on lock attempts meaning a thread trying to obtain a lock will only try for so long before giving up. If a thread does not succeed in taking all necessary locks within the given timeout, it will backup, free all locks taken, wait for a random amount of time and then retry. The random amount of time waited serves to give other threads trying to take the same locks a chance to take all locks, and thus let the application continue running without locking.

Here is an example of two threads trying to take the same two locks in different order, where the threads back up and retry:

Thread 1 locks AThread 2 locks B
Thread 1 attempts to lock B but is blockedThread 2 attempts to lock A but is blocked
Thread 1's lock attempt on B times outThread 1 backs up and releases A as wellThread 1 waits randomly (e.g. 257 millis) before retrying.
Thread 2's lock attempt on A times outThread 2 backs up and releases B as wellThread 2 waits randomly (e.g. 43 millis) before retrying.


In the above example Thread 2 will retry taking the locks about 200 millis before Thread 1 and will therefore likely succeed at taking both locks. Thread 1 will then wait already trying to take lock A. When Thread 2 finishes, Thread 1 will be able to take both locks too (unless Thread 2 or another thread takes the locks in between).

An issue to keep in mind is, that just because a lock times out it does not necessarily mean that the threads had deadlocked. It could also just mean that the thread holding the lock (causing the other thread to time out) takes a long time to complete its task.

Additionally, if enough threads compete for the same resources they still risk trying to take the threads at the same time again and again, even if timing out and backing up. This may not occur with 2 threads each waiting between 0 and 500 millis before retrying, but with 10 or 20 threads the situation is different. Then the likeliness of two threads waiting the same time before retrying (or close enough to cause problems) is a lot higher.

A problem with the lock timeout mechanism is that it is not possible to set a timeout for entering a synchronized block in Java. You will have to create a custom lock class or use one of the Java 5 concurrency constructs in the java.util.concurrency package. Writing custom locks isn't difficult but it is outside the scope of this text. Later texts in the Java concurrency trails will cover custom locks.


Deadlock Detection
Deadlock detection is a heavier deadlock prevention mechanism aimed at cases in which lock ordering isn't possible, and lock timeout isn't feasible.

Every time a thread takes a lock it is noted in a data structure (map, graph etc.) of threads and locks. Additionally, whenever a thread requests a lock this is also noted in this data structure.

When a thread requests a lock but the request is denied, the thread can traverse the lock graph to check for deadlocks. For instance, if a Thread A requests lock 7, but lock 7 is held by Thread B, then Thread A can check if Thread B has requested any of the locks Thread A holds (if any). If Thread B has requested so, a deadlock has occurred (Thread A having taken lock 1, requesting lock 7, Thread B having taken lock 7, requesting lock 1).

Of course a deadlock scenario may be a lot more complicated than two threads holding each others locks. Thread A may wait for Thread B, Thread B waits for Thread C, Thread C waits for Thread D, and Thread D waits for Thread A. In order for Thread A to detect a deadlock it must transitively examine all requested locks by Thread B. From Thread B's requested locks Thread A will get to Thread C, and then to Thread D, from which it finds one of the locks Thread A itself is holding. Then it knows a deadlock has occurred.

Below is a graph of locks taken and requested by 4 threads (A, B, C and D). A data structure like this that can be used to detect deadlocks.




So what do the threads do if a deadlock is detected?

One possible action is to release all locks, backup, wait a random amount of time and then retry. This is similar to the simpler lock timeout mechanism except threads only backup when a deadlock has actually occurred. Not just because their lock requests timed out. However, if a lot of threads are competing for the same locks they may repeatedly end up in a deadlock even if they back up and wait.

A better option is to determine or assign a priority of the threads so that only one (or a few) thread backs up. The rest of the threads continue taking the locks they need as if no deadlock had occurred. If the priority assigned to the threads is fixed, the same threads will always be given higher priority. To avoid this you may assign the priority randomly whenever a deadlock is detected.

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