hyPACK-2013 Mode-1 : Mixed Mode of Programming : MPI-Pthreads
MPI is a standard message-passing interface for applications and libraries
running on concurrent computers with logically distributed memory.
In message-passing model, the data is moved from the address space of one process to that of another
by means of a cooperative operation such as a send/receive pair.
Message-passing programs are often written using the asynchronous or loosely
synchronous paradigms.
The message passing-programming paradigm assumes a partitioned address space
and supports
explicit parallelization.
The interactions in typical message passing programs are accomplished by sending and
receiving messages, the basic operations in the message-passing paradigm are send and receive.
On Multi-Core Processors, the reasons for supporting efficient implementation of send
and receive operations for performance reasons.
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MPI-Pthreads/Codes
The software threading model on the computing platform, which consists of multi-core processors play an important role to understand performance aspects of an application. Several factors such as proper use of threading, understanding of how threads operate,
algorithm used for your application, the threading application programming interface (API), the compiler runtime environment,
and the number of multi-cores used for application are play a key-role from performance point of
view on Multi-Core processors.
Using different APIs, and Compilation switches for the numerical and non-numerical computations can be optimised and performance tuning
can be achieved to get sustained performance on a node (Multi-Core Processors) in Message Passing Environment.
Mixed (hybrid) mode-programming models such as MPI-OpenMP, MPI-Pthreads, &
MPI-TBB are commonly used on Message Passing cluster of Multi-Core processors.
By utilizing the mixed(hybrid) mode-programming model (MPI-Pthreads) ,we should be able to take
advantage of the benefits of shared and non-shared memory models.
The majority of mixed mode applications involve a hierarchical model,
MPI parallelisation occurring at the top level, and OpenMP parallelisation occurring below.
MPI-Pthreads
Message passing codes written in MPI are portable and should transfer easily to cluster of
Multi-Core processor Systems.
Message passing is required to communicate between nodes (boxes) using different networks,and message passing
in node (SMPs or Multi-core processors) require communication within node. Performance depends upon the
efficient implementaton within a Multi-Core Processor box or a node of Message Passing Cluster.
OpenMP is an Application Program Interface (API) that may be used to explicitly direct multi-threaded, shared memory
parallelism. It is a specification for a set of compiler directives, library routines and environment variables
that can be used to specify shared memory parallelism in Fortran and C/C++ programs.
The OpenMP is a shared memory standard supported by most of the hardware and software vendors.
OpenMP is comprised of three primary API components such as Compiler Directives,
Runtime Library Routines, and Environment Variables
OpenMP is portable and the API is specified for C/C++ and Fortran.
Multiple platforms have been implemented including most Unix platforms and Windows NT. Efforts are going on
to implement on Multi-Core processors to enhance the performance. The available programming environment on most
of the Multi-Core processors will address the thread affinity to core and overheads in OpenMP programming environment.
A combination of shared memory and message
passing parallelisation paradigms within the same application (mixed mode programming) may provide a more efficient
parallelisation strategy than pure MPI.
While mixed code may involve other programming languages such as High Performance Fortran (HPF) and POSIX threads.
Mixed MPI and OpenMP codes are likely to represent the most widespread use of mixed mode programming on SMP
cluster due to their portability and the fact that they represent industry standards for distributed and shared
memory systems respectively.
While SMP clusters offer the greatest reason for developing mixed mode code, both the OpenMP and MPI paradigms
have different advantages and disadvantages and by developing such a model these characteristics might even be
exploited to give the best performance on a single SMP system.
Thread Safety in MPI-Pthreads :
Although a large number of MPI implementations are thread-safe, mixed mode programming cannot be
guaranteed. To ensure the code is portable all MPI calls should be made within thread sequential
regions of the code.
In mixed mode-programming model, the advantage of the benefits of both models can be taken in which a mixed mode
program make use of the explicit control data placement policies of MPI with the finer grain parallelism of OpenMP.
The majority of mixed mode applications involve a hierarchical model, MPI parallelisation occurring at the top level,
and OpenMP parallelisation occurring below.
MPI Pthreads
The MPI-Pthreads mixed programming paradigm performs MPI-related tasks across nodes (boxes) of
Message Passing cluster of Multi-core processors and facilitate a variety of thread-related
tasks.
In typical hybrid parallel
program, MPI process execute on each multi-processor or compute node which consists of multiple cores.
Inside each sections of code, the MPI processes for threads to occupy the multi-core CPU'S, and these
threads can interact via shared variables.
Explicit threading of POSIX (Pthreads) model, provides a richer API in the form of
condition waits, locks of different types, and increased flexibility for building different
synchronization operations. Explicit threading is more widely used than in OpenMP, the rich
set of tools on Multi-Core processors may help programmer to understand performance issues.
Hybrid or mixed mode of programs using both MPI and
Pthreads execute faster than programs using only MPI on Multi-core processors for data intensive
specific class of applications.
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