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OpenMP (Shared-Memory Compiler Directives)

Please note: The FAQ pages at the HPCVL website are continuously being revised. Some pages might pertain to an older configuration of the system. Please let us know if you encounter problems or inaccuracies, and we will correct the entries.

This is a short introduction to the OpenMP industry standard for shared-memory parallel programming. It outlines basic features of the system and explains its usage on the HPCVL Sunfire SMP machines. This is not an introduction to OpenMP programming. References and links for further details are given.

What is OpenMP?

OpenMP is a system of so-called "compiler directives" that are used to express parallelism on a shared-memory machine. OpenMP has become an industry standard for such directives, and at this point, most parallel enabled compilers that are used on SMP machines are capable of processing OpenMP directives. The OpenMP standard has had a rather short and steep career: it was introduced in 1997 and has since sidelined all other similar systems.

OpenMP is exclusively designed for shared-memory machines, and is based on "multi-threading", i.e. the dynamic spawning of so-called "light-weight" sub-processes, commonly within loops. In favorable cases it is quite possible to create a well-scaling parallel program from a serial code by inserting a few lines of OpenMP directives into the serial precursor and recompiling. The simplicity and ease of use of OpenMP directives have made it a popular alternative to the more involved (and arguably more powerful) communication system MPI, which was designed for distributed-memory systems.

What kind of system uses OpenMP?

OpenMP was designed from the outset for shared-memory machines, commonly called SMP (Symmetric Multi-Processor) machines. These types of parallel computers have the advantage of not requiring communication between processors for parallel processing, and therefore bypassing the associated overhead. In addition, they allow multi-threading, which is a dynamic form of parallelism in which sub-processes are created and destroyed during program execution. In some cases this can be done automatically at compile time. In other cases, the compiler needs to be instructed about details of the "parallel region" of code where multi-threading is to take place. OpenMP was designed to perform this task.

OpenMP therefore needs both a shared-memory (SMP) computer and a compiler that understands OpenMP directives. The Sunfire machines at HPCVL fulfill both of these requirements.

OpenMP will not work on distributed-memory clusters, such as a Beowulf. However, it may sometimes be used with combination with distributed memory parallel systems such as MPI. However, this holds only if each of the nodes in a cluster has in itself at least 2 CPUs available.

How is OpenMP used?

OpenMP is usually used in the stepwise parallelization of pre-existing serial programs. Shared-memory parallelism is often called "loop parallelism" because of the typical situation that make OpenMP compiler directives an option.

The OpenMP compiler directives are inserted into the serial code by the user. They instruct the compiler to distribute the tasks performed in a certain region of the code (usually a loop) over several sub-processes, which in turn may be executing on different CPUs.

For instance, the following Fortran loop looks as if the repeated calls to the functionpoint() could be done in seperate processes, or better on seperate CPUs:

 do imesh=inz,nnn,nstep 
svec(1)=xmesh(imesh)
svec(2)=ymesh(imesh)
svec(3)=zmesh(imesh)
integral=integral+wints(imesh)*point(svec)
end do

If we are using a compiler that is able to automatically parallelize code, and try to use that feature, we will find that things are not that simple. The function call topoint may hide a "loop dependency", i.e. a situation where data computed in one loop iteration depend on data calculated in another. The compiler will therefore commonly reject parallelizing such a loop as "unsafe".

The use of OpenMP directives can solve this problem:

 !$omp parallel do private (imesh,svec) & 
!$omp shared (inz,nnn,nstep,xmesh,ymesh,zmesh,wints) &
!$omp reduction(+:integral)
do imesh=inz,nnn,nstep
svec(1)=xmesh(imesh)
svec(2)=ymesh(imesh)
svec(3)=zmesh(imesh)
integral=integral+wints(imesh)*point(svec)
end do
!$omp end parallel do

The three lines of directives have the effect of forcing the compiler to distribute the tasks performed in each of the loop iterations over seperate, dynamically created processes. Furthermore, they inform the compiler which variables can be used by all sub-processes (ie, shared), and which have different values for each process (ie, private). Finally, they direct the compiler to collect values of integral sperately in each process and then "reduce" them to a common value by summing them up.

OpenMP programs need to be compiled with special compiler options and will then yield parallel code. It must be pointed out that since the compiler is forced to multi-thread specific regions of the code, it is the responsibility of the programmer to ensure that such multi-threading is safe, i.e. no dependeny between iterations in the parallelized loop exist. In the above example that means that the tasks performed inside the point call are indeed independent.

Can you give me an example?

The working principle of OpenMP is perhaps best illustrated on the grounds of a programming example. The following program written in Fortran 90 computes the sum of all square-roots of integers from 0 up to a specific limit m:

 

 	 program example02 
call demo02
stop
end 
	 subroutine demo02 
integer:: m, i
real*8 :: mys
write(*,*)'how many terms?'
read(*,*) m
mys=0.d0 
!$omp parallel do private (i) &
!$omp shared (m) &
!$omp reduction (+:mys)
do i=0,m
mys=mys+dsqrt(dfloat(i))
end do
write(*,*) 'mys=',mys, ' m:',m
return
end
!$omp end parallel do

It is instructive to compare this example with the one in our MPI FAQ which performs exactly the same task. It is obvious that the OpenMP version is a good deal shorter. In fact, apart from the OpenMP directives (starting with !$omp), this is just a simple serial program.

 

In Fortran 90, anything after a ! sign is commonly interpreted as a comment, so that the above example when compiled without special options will just yield the serial version of the program. If the -openmp option is specified at compile time, the compiler will use the OpenMP directives to create a multi-threaded executable.

The instruction parallel do will cause the next do-loop to be taken as parallel region, in other words before executing that loop, multiple sub-processes will be created and the loop iterations will be distributed to those processes. The order in which this happens should not matter, since we have to be sure that the iterations are independent.

The instruction private(i) ensures that a separate value of the loop index is used for each process or thread. By default, scalar values such as i are considered private, i.e. thread-specific. We are specifying this only for demonstration purposes. It is in any case a good idea not to rely on default settings.

The instruction shared(m) makes sure that all threads are using the same maximum value. This declaration is necessary, since only arrays are considered "shared" by default. Since we are not doing anything with m, it is safe to assume a common value.

Finally, we instruct the compiler to treat the value of mys specially. Thereduce(+:mys) instruction causes a private value for mys to be initiatialized with the current mys value before thread creation. After all loop iterations have been completed, the different private values are reduced to a single on by a sum (+ sign in the directive).

After compilation, we can convince ourselves easily that we have in fact created a parallel program. Here is the execution with a maximum of m=100,000,000 and only one thread:

bash-2.05$ timex a.out in 
how many terms?
mys= 6.66666671666567E+11 m: 100000000 

real 3.90
user 3.88
sys 0.01

And here's the same run with two threads:

bash-2.05$ timex a.out in 
how many terms?
mys= 6.66666671666484E+11 m: 100000000 
real 2.00 
user 3.92
sys 0.02

We note that the result is the same to 12 significant digits, but the time of the second run is only slightly more than 1/2 of the first. The difference of about 0.1 seconds may be attributed to a constant overhead for reading in m and writing out the results.

How is OpenMP implemented on HPCVL SUN computers ?

The Sun Studio compilers on the HPCVL machines are capable of processing OpenMP directives. No special settings need to be specified in setup files to use this capability. The location of the compiler suite is included the PATH variable. 

How do I compile OpenMP code on the Sun?

To enable the interpretation of OpenMP compiler directives, the -xopenmp compiler option has to be specified both at the compile and at the link level. This holds for the Fortran, C, and C++ compilers equally. At the compile level, it is often useful to also use the -xloopinfo option which creates a list of loops and information on whether they have been parallelized or not. For instance, in the case of a Fortran program, the compiler calls will be, for compiling:

f90 -c -xopenmp -xloopinfo test.f90

and for linking

f90 -o test.exe -xopenmp test.o

Both can of course be combined:

f90 -o test.exe -xopenmp -xloopinfo test.f90

Note that the -xopenmp option is a macro which includes several sub options. Also, if no optimization is specified (as in the above lines), the optimization level will automatically be increased to -xO3 to support multi-threading. This can not be disabled.

How do I run OpenMP programs on an HPCVL machine?

Unlike MPI programs, shared-memory parallel OpenMP programs do not need a special runtime environment to run in parallel. They only need to be instructed about the number of threads (or processes) that should be used. This is usually done by setting an environment variable. The default variable used on any system that is OpenMP enabled is OMP_NUM_THREADS. For instance, in a bash, the following sequence will cause the OpenMP program test_omp.exe to be executed with 16 threads:

export OMP_NUM_THREADS=16; test_omp.exe

Incidentally, the number of threads may also be set from inside the program by means of a function call. The line

call omp_set_num_threads(16)

inside the program test_omp.f90 will have the same effect as setting the environment variable. This will take precedent over external settings. 

Where can I learn details about OpenMP?

As already pointed out, this FAQ is not an introduction to OpenMP programming. In fact, we barely scratch the surface of what can be done. OpenMP includes many directives and functions that warrant study before they can be used properly. It is necessary to point out that shared-memory programming has its pitfalls: hidden dependencies may lead to so-called "race conditions", and cache-effects such as "false sharing" can seriously degrade performance. Often, a detailed analysis of a parallel region or a loop is necessary to determine if and how it may be parallelized using OpenMP.

A good text book on OpenMP is:

Rohit Chandra, Leonardo Dagum, Dave Kohr, Dror Maydan, Jeff McDonald, and Ramesh Menon: Parallel Programming in OpenMP, Academic Press, San Diego, California, 2001; ISBN 1-55860-671-8

This text includes examples that are worked out in detail and explains concepts of shared-memory programming that might be unfamiliar to may users.

A good online tutorial for OpenMP shared-memory programming can be found at Lawrence Livermore National Laboratory.

There is a website devoted specifically to all things OpenMP, which is a good starting point for learning about it.

For Sun and Solaris specific questions, including CRE and Sun MPI, visit the Sun Documentation Site and use their Search Engine to look for "OpenMP".

HPCVL also organizes Workshops on a regular basis, and one of them is devoted to OpenMP programming. They are announce on our web site. We might see you there sometime soon.

Are there any tools to help me with OpenMP programming?

The standard debugging and profiling tool on the Sunfire machines at HPCVL is Sun Studio, which provides a GUI and is well documented internally. It is able to handle multi-threaded code, i.e. programs that were written using OpenMP. If the home directory of the compiler suite (/opt/ss10/SUNWspro/bin) is in your PATH, the tool may be invoked by simply typing sunstudio at the command prompt. Help is easily invoked, for example by clicking on "Help", selecting "Contents" and then going to "Managing Threads" in the menu that appears.
Sun Studio supplies both a means to debug your program, and to perform timing experiments on them (profiling). The tool might be used to track down bottle necks to the level of single lines of code, or even assembly language instructions.

Quite often, the best way to check the performance of a multi-threaded program is timing it by insertion of suitable routines. This can be done by calling the subroutines ETIME and DTIME, which can give you information about actual CPU time used. However, it is advisable to carefully read the documentation before using them with OpenMP programs. In this case, go to docs.sun.com and search for"Sun Studio 10: Fortran Library Reference".

HPCVL also provides a package called the HPCVL Working Template (HWT), which was created by Gang Liu and has now reached version 5.1. The HWT provides 3 main functionalities:

  1. Maintenance of multiple versions of the same code from a single source file. This is very useful, if your OpenMP code is based on a serial code that you want to convert, which usually is the case.
  2. Automatic Relative Debugging which allows you to use pre-existing code (for example the serial version of your program) as a reference to check the correctness of your OpenMP code.
  3. Simple Timing which is needed to determine bottlenecks for parallelization, to optimize code, and to check its scaling properties.

The HWT is based on libraries and script files. It is easy to use and portable (written largely in Fortran). Fortran, C, C++, and any mixture thereof are supported, as well as OpenMP and MPI for parallelism. Documentation of the HWT is available. The package is installed on the Sunfire cluster in /usr/local/hwt.

It doesn't work. Where can I get help?

Most of the Sun specific issues addressed in this FAQ are documented at http://docs.sun.com. The search engine provides a reliable means to find specific documents.
You can also call or send email to one of our support staff, who include several scientific programmers. Keep in mind that we support many people at any given time, so we cannot do the coding for you. But we can do my best to help you get your code ready for multi-processor machines.