OpenHMPP


OpenHMPP - programming standard for heterogeneous computing. Based on a set of compiler directives, standard is a programming model designed to handle hardware accelerators without the complexity associated with GPU programming. This approach based on directives has been implemented because they enable a loose relationship between an application code and the use of a hardware accelerator.

Introduction

The OpenHMPP directive-based programming model offers a syntax to offload computations on hardware accelerators and to optimize data movement to/from the hardware memory.
The model is based on works initialized by , a common project from INRIA, CNRS, the University of Rennes 1 and the INSA of Rennes.

OpenHMPP concept

OpenHMPP is based on the concept of codelets, functions that can be remotely executed on HWAs.

The OpenHMPP codelet concept

A codelet has the following properties:
  1. It is a pure function.
  2. * It does not contain static or volatile variable declarations nor refer to any global variables except if these have been declared by a HMPP directive “resident”
  3. * It does not contain any function calls with an invisible body. This includes the use of libraries and system functions such as malloc, printf,...
  4. * Every function call must refer to a static pure function.
  5. It does not return any value.
  6. The number of arguments should be fixed.
  7. It is not recursive.
  8. Its parameters are assumed to be non-aliased.
  9. It does not contain callsite directives or other HMPP directives.
These properties ensure that a codelet RPC can be remotely executed by a HWA. This RPC and its associated data transfers can be asynchronous.

Codelet RPCs

HMPP provides synchronous and asynchronous RPC. Implementation of asynchronous operation is hardware dependent.

HMPP Memory Model

HMPP considers two address spaces: the host processor one and the HWA memory.

Directives concept

The OpenHMPP directives may be seen as “meta-information” added in the application source code. They are safe meta-information i.e. they do not change the original code behavior. They address the remote execution of a function as well as the transfers of data to/from the HWA memory.
The table below introduces the OpenHMPP directives. OpenHMPP directives address different needs: some of them are dedicated to declarations and others are dedicated to the management of the execution.
Control flow instructionsDirectives for data management
Declarationscodelet
group		resident
map
mapbyname
Operational Directivescallsite
synchronize
region		allocate
release
advancedload
delegatedstore

Concept of set of directives

One of the fundamental points of the HMPP approach is the concept of directives and their associated labels which makes it possible to expose a coherent structure on a whole set of directives disseminated in an application.
There are two kinds of labels:
In order to simplify the notations, regular expressions will be used to describe the syntax of the HMPP directives.
The color convention below is used for the description of syntax directives:
The general syntax of OpenHMPP directives is:
#pragma hmpp <grp_label> ? directive_type *
!$hmpp <grp_label> ? directive_type *
Where:
The parameters associated to a directive may be of different types.
Below are the directive parameters defined in OpenHMPP:

Directives for declaring and executing a codelet

A codelet directive declares a computation to be remotely executed on a hardware accelerator.
For the codelet directive:
The syntax of the directive is:
#pragma hmpp <grp_label> codelet_label codelet
?]?
.io=*
.size=]*
.const=true]*

*]
More than one codelet directive can be added to a function in order to specify different uses or different execution contexts. However, there can be only one codelet directive for a given call site label.
The callsite directive specifies how the use a codelet at a given point in the program.
The syntax of the directive is:
#pragma hmpp <grp_label> codelet_label callsite
?
.size=]*
.advancedload=*
.addr="expr"]*
.noupdate=true]*
An example is shown here :

/* declaration of the codelet */
#pragma hmpp simple1 codelet, args.io=inout, target=CUDA
static void matvec

Data transfers directives to optimize communication overhead

When using a HWA, the main bottleneck is often the data transfers between the HWA and the main processor.

To limit the communication overhead, data transfers can be overlapped with successive executions of the same codelet by using the asynchronous property of the HWA.
  • allocate directive
The allocate directive locks the HWA and allocates the needed amount of memory.
#pragma hmpp <grp_label> allocate .size=]*
  • release directive
The release directive specifies when to release the HWA for a group or a stand-alone codelet.
#pragma hmpp <grp_label> release
  • advancedload directive
The advancedload directive prefetches data before the remote execution of the codelet.

#pragma hmpp <grp_label> ? advancedload
,args
.size=]*
.addr="expr"]*
.section=]*

  • delegatedstore directive
The delegatedstore directive is a synchronization barrier to wait for an asynchronous codelet execution to complete and to then download the results.

#pragma hmpp <grp_label> ? delegatedstore
,args
.addr="expr"]*
.section=]*
  • Asynchronous Computations
The synchronize directive specifies to wait until the completion of an asynchronous callsite execution.
For the synchronize directive, the codelet label is always mandatory and the group label is required if the codelet belongs to a group.
#pragma hmpp <grp_label> codelet_label synchronize
  • Example
In the following example, the device initialization, memory allocation and upload of the input data are done only once outside the loop and not in each iteration of the loop.
The synchronize directive allows to wait for the asynchronous execution of the codelet to complete before launching another iteration. Finally the delegatedstore directive outside the loop uploads the sgemm result.

int main

Sharing data between codelets

Those directives map together all the arguments sharing the given name for all the group.
The types and dimensions of all mapped arguments must be identical.
The map directive maps several arguments on the device.
#pragma hmpp <grp_label> map, args
This directive is quite similar as the map directive except that the arguments to be mapped are directly specified by their name. The mapbyname directive is equivalent to multiple map directives.
#pragma hmpp <grp_label> mapbyname +

Global variable

The resident directive declares some variables as global within a group. Those variables can then be directly accessed from any codelet belonging to the group.
This directive applies to the declaration statement just following it in the source code.
The syntax of this directive is:
#pragma hmpp <grp_label> resident
.io=*
.size=]*
.addr="expr"]*
.const=true]*
The notation ::var_name with the prefix ::, indicates an application's variable declared as resident.

Acceleration of regions

A region is a merge of the codelet/callsite directives. The goal is to avoid code restructuration to build the codelet. Therefore, all the attributes available for codelet or callsite directives can be used on regions directives.
In C language:
#pragma hmpp region
.io=*
<
.const=true]*
*]
.size=]*
.advancedload=*
.addr="expr"]*
.noupdate=true]*
?
]*

Implementations

The OpenHMPP Open Standard is based on HMPP Version 2.3.
The OpenHMPP directive-based programming model is implemented in:
  • CAPS Compilers, CAPS Entreprise compilers for hybrid computing
  • PathScale ENZO Compiler Suite
OpenHMPP is used by HPC actors in Oil & Gas, Energy, Manufacturing, Finance, Education & Research.