difference between omp critical and omp single

Question

I am trying to understand the exact difference between #pragma omp critical and #pragma omp single in OpenMP:

Microsoft definitions for these a

Answer 1

single and critical belong to two completely different classes of OpenMP constructs. single is a worksharing construct, alongside for and sections. Worksharing constructs are used to distribute a certain amount of work among the threads. Such constructs are "collective" in the sense that in correct OpenMP programs all threads must encounter them while executing and moreover in the same sequential order, also including the barrier constructs. The three worksharing constructs cover three different general cases:

• for (a.k.a. loop construct) distributes automatically the iterations of a loop among the threads - in most cases all threads get work to do;
• sections distributes a sequence of independent blocks of code among the threads - some threads get work to do. This is a generalisation of the for construct as a loop with 100 iterations could be expressed as e.g. 10 sections of loops with 10 iterations each.
• single singles out a block of code for execution by one thread only, often the first one to encounter it (an implementation detail) - only one thread gets work. single is to a great extent equivalent to sections with a single section only.

A common trait of all worksharing constructs is the presence of an implicit barrier at their end, which barrier might be turned off by adding the nowait clause to the corresponding OpenMP construct, but the standard does not require such behaviour and with some OpenMP runtimes the barrier might continue to be there despite the presence of nowait. Incorrectly ordered (i.e. out of sequence in some of the threads) worksharing constructs might therefore lead to deadlocks. A correct OpenMP program will never deadlock when the barriers are present.

critical is a synchronisation construct, alongside master, atomic, and others. Synchronisation constructs are used to prevent race conditions and to bring order in the execution of things.

• critical prevents race conditions by preventing the simultaneous execution of code among the threads in the so-called contention group. This means all threads from all parallel regions encountering similarly named critical constructs get serialised;
• atomic turns certain simple memory operations into atomic ones, usually by utilising special assembly instructions. Atomics complete at once as a single non-breakable unit. For example, an atomic read from some location by one thread, which happens concurrently with an atomic write to the same location by another thread, will either return the old value or the updated value, but never some kind of an intermediate mash-up of bits from both the old and the new values;
• master singles out a block of code for execution by the master thread (thread with ID of 0) only. Unlike single, there is no implicit barrier at the end of the construct and also there is no requirement that all threads must encounter the master construct. Also, the lack of implicit barrier means that master does not flush the shared memory view of the threads (this is an important but very poorly understood part of OpenMP). master is basically a shorthand for if (omp_get_thread_num() == 0) { ... }.

critical is a very versatile construct as it is able to serialise different pieces of code in very different parts of the program code, even in different parallel regions (significant in the case of nested parallelism only). Each critical construct has an optional name provided in parenthesis immediately after. Anonymous critical constructs share the same implementation-specific name. Once a thread enters such a construct, any other thread encountering another construct of the same name is put on hold until the original thread exits its construct. Then the serialisation process continues with the rest of the threads.

An illustration of the concepts above follows. The following code:

#pragma omp parallel num_threads(3)
{
   foo();
   bar();
   ...
}

results in something like:

thread 0: -----< foo() >< bar() >-------------->
thread 1: ---< foo() >< bar() >---------------->
thread 2: -------------< foo() >< bar() >------>

(thread 2 is purposely a latecomer)

Having the foo(); call within a single construct:

#pragma omp parallel num_threads(3)
{
   #pragma omp single
   foo();
   bar();
   ...
}

results in something like:

thread 0: ------[-------|]< bar() >----->
thread 1: ---[< foo() >-|]< bar() >----->
thread 2: -------------[|]< bar() >----->

Here [ ... ] denotes the scope of the single construct and | is the implicit barrier at its end. Note how the latecomer thread 2 makes all other threads wait. Thread 1 executes the foo() call as the example OpenMP runtime chooses to assign the job to the first thread to encounter the construct.

Adding a nowait clause might remove the implicit barrier, resulting in something like:

thread 0: ------[]< bar() >----------->
thread 1: ---[< foo() >]< bar() >----->
thread 2: -------------[]< bar() >---->

Having the foo(); call within an anonymous critical construct:

#pragma omp parallel num_threads(3)
{
   #pragma omp critical
   foo();
   bar();
   ...
}

results in something like:

thread 0: ------xxxxxxxx[< foo() >]< bar() >-------------->
thread 1: ---[< foo() >]< bar() >------------------------->
thread 2: -------------xxxxxxxxxxxx[< foo() >]< bar() >--->

With xxxxx... is shown the time a thread spends waiting for other threads executing a critical construct of the same name before it could enter its own construct.

Critical constructs of different names do not synchronise with each other. E.g.:

#pragma omp parallel num_threads(3)
{
   if (omp_get_thread_num() > 1) {
     #pragma omp critical(foo2)
     foo();
   }
   else {
     #pragma omp critical(foo01)
     foo();
   }
   bar();
   ...
}

results in something like:

thread 0: ------xxxxxxxx[< foo() >]< bar() >---->
thread 1: ---[< foo() >]< bar() >--------------->
thread 2: -------------[< foo() >]< bar() >----->

Now thread 2 does not synchronise with the other threads because its critical construct is named differently and therefore makes a potentially dangerous simultaneous call into foo().

On the other hand, anonymous critical constructs (and in general constructs with the same name) synchronise with one another no matter where in the code they are:

#pragma omp parallel num_threads(3)
{
   #pragma omp critical
   foo();
   ...
   #pragma omp critical
   bar();
   ...
}

and the resulting execution timeline:

thread 0: ------xxxxxxxx[< foo() >]< ... >xxxxxxxxxxxxxxx[< bar() >]------------>
thread 1: ---[< foo() >]< ... >xxxxxxxxxxxxxxx[< bar() >]----------------------->
thread 2: -------------xxxxxxxxxxxx[< foo() >]< ... >xxxxxxxxxxxxxxx[< bar() >]->

Answer 2

single and critical are two very different things. As you mentioned:

• single specifies that a section of code should be executed by single thread (not necessarily the master thread)
• critical specifies that code is executed by one thread at a time

So the former will be executed only once while the later will be executed as many times as there are of threads.

For example the following code

int a=0, b=0;
#pragma omp parallel num_threads(4)
{
    #pragma omp single
    a++;
    #pragma omp critical
    b++;
}
printf("single: %d -- critical: %d\n", a, b);

will print

single: 1 -- critical: 4

I hope you see the difference better now.

For the sake of completeness, I can add that:

• master is very similar to single with two differences:
  1. master will be executed by the master only while single can be executed by whichever thread reaching the region first; and
  2. single has an implicit barrier upon completion of the region, where all threads wait for synchronization, while master doesn't have any.
• atomic is very similar to critical, but is restricted for a selection of simple operations.

I added these precisions since these two pairs of instructions are often the ones people tend to mix-up...