mirror of
https://github.com/OpenMP/Examples.git
synced 2025-04-07 23:22:11 +01:00
42 lines
1.5 KiB
TeX
42 lines
1.5 KiB
TeX
|
|
\pagebreak
|
|
\section{The OpenMP Memory Model}
|
|
\label{sec:mem_model}
|
|
|
|
In the following example, at Print 1, the value of \plc{x} could be either 2
|
|
or 5, depending on the timing of the threads, and the implementation of the assignment
|
|
to \plc{x}. There are two reasons that the value at Print 1 might not be 5.
|
|
First, Print 1 might be executed before the assignment to \plc{x} is executed.
|
|
Second, even if Print 1 is executed after the assignment, the value 5 is not guaranteed
|
|
to be seen by thread 1 because a flush may not have been executed by thread 0 since
|
|
the assignment.
|
|
|
|
The barrier after Print 1 contains implicit flushes on all threads, as well as
|
|
a thread synchronization, so the programmer is guaranteed that the value 5 will
|
|
be printed by both Print 2 and Print 3.
|
|
|
|
\cexample{mem_model}{1}
|
|
|
|
\ffreeexample{mem_model}{1}
|
|
|
|
\pagebreak
|
|
The following example demonstrates why synchronization is difficult to perform
|
|
correctly through variables. The value of flag is undefined in both prints on thread
|
|
1 and the value of data is only well-defined in the second print.
|
|
|
|
\cexample{mem_model}{2}
|
|
|
|
\fexample{mem_model}{2}
|
|
|
|
\pagebreak
|
|
The next example demonstrates why synchronization is difficult to perform correctly
|
|
through variables. Because the \plc{write}(1)-\plc{flush}(1)-\plc{flush}(2)-\plc{read}(2)
|
|
sequence cannot be guaranteed in the example, the statements on thread 0 and thread
|
|
1 may execute in either order.
|
|
|
|
\cexample{mem_model}{3}
|
|
|
|
\fexample{mem_model}{3}
|
|
|
|
|