Work Sharing and Thread Data Management

• OpenMP utilizes work sharing constructs to facilitate dividing parallelizable work among a number of threads.
• The work sharing constructs are:
  • for: divide loop iterations among threads.
  • sections: divide sections of codes among themselves.
  • single: the section is executed by a single thread.
  • task: for offloading

• Used when parallelize predetermined number of independent work units.
• Within a primary sections construct, there can be multiple section construct.

• A section can be executed by any available thread in the current team, including having multiple sections done by the same thread.

• Inside csc466, create a file named hello_sections.c with the following contents:

Given the following functions: $y=x^{4} + 15x^{3} + 10x^{2} + 2x$ develop an OpenMP program called poly_openmp.c with sections/section directives. Each section should handle the calculations for one term of the polynomial.

• Limits the execution of a block to a single thread.
• All other threads will skip the execution of this block but wait until the block is finished before moving on.
• To enable proceed without waiting, a nowait clause can be added.
• Inside csc466, create the following files:

hello_sections_nosingle.c: ```c linenums=”1” –8<– “docs/csc466/lectures/data/openmp/hello_sections_nosingle.c”

1
2
3
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5
 
`hello_sections_single.c`: 

```c linenums="1"
--8<-- "docs/csc466/lectures/data/openmp/hello_sections_single.c"
 

hello_sections_single_nowait.c:

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/hello_sections_nowait.c"

• Compile and run the above files:

• Data declared outside of a parallel region will be shared among all threads.
• Data declared inside of a parallel region will be private to individual thread.
• Inside csc466, create a file named counter_openmp.c with the following contents:

c linenums="1" --8<-- "docs/csc466/lectures/data/openmp/counter_openmp.c"

• Shared variables: accessible and modifiable by all threads
• Private variables: each thread gets its own copy (uninitialized by default)
• Firstprivate: each thread getss an initialized copy of a variable.
• Lastprivate save the last computed value of a variable after parallel execution

```c linenums=”1” –8<– “docs/csc466/lectures/data/openmp/shared_private.c”