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A `TOP-C' application is compiled and run using `topcc' (similarly to `gcc') or `topc++' (similarly to `g++'). For example, assuming a `procgroup' file in the current directory to specify the remote hosts for the slave processes, one executes:
topcc --mpi parfactor.c # or else: topc++ --mpi parfactor.cc ./a.out
For purposes of documentation, we will standardize on an explanation of
topcc
. Wherever topcc
is mentioned, the description is
equally valid for topc++
.
A typical TOP-C application has the following structure:
#include <topc.h> ... define four callback functions for TOPC_master_slave() ... int main( int argc, char **argv ) { TOPC_init( &argc, &argv ); ... TOPC_master_slave( generate_task_input, do_task, check_task_result, update_shared_data ) ... TOPC_finalize(); }
For an alternative interface often useful for parallelizing existing
sequential code, see section `TOP-C' Raw Interface: TOPC_raw_XXX
. However, for new
applications, the standard interface will usually be cleaner.
In a `TOP-C' application, the programmer defines four callback functions and passes control to the `TOP-C' library through the following command.
TOPC_master_slave(GenerateTaskInput, DoTask, CheckTaskResult, UpdateSharedData);
Pictorially, TOP-C arranges for the flow of control among the four callback functions as follows:
(ON MASTER) task input GenerateTaskInput() ----------> task input (ON A SLAVE) task output -----------> DoTask(input) -----------> task output (ON MASTER) action -----------> CheckTaskResult(input, output) -----------> if (action == UPDATE): task input, task output (ON ALL PROCESSES) -----------------------> UpdateSharedData(input, output)
When there is only one slave, The `TOP-C' algorithm can be summarized by the following C code.
{ void *input, *output; TOPC_ACTION action; while ( (input = GenerateTaskInput()) != NOTASK ) { do { output = DoTask(input); action = CheckTaskResult(input, output); } while (action == REDO); /* REDO not useful for only one slave */ if (action == UPDATE) then UpdateSharedData(input, output); } }
On a first reading, it is recommended to read the next few
sections through Section section Actions Returned by CheckTaskResult()
, and then return to this
section.
For a better understanding of the case of multiple slaves, this simplified excerpt from the `TOP-C' source code describes the `TOP-C' algorithm.
TOPC_BUF input, output; int number_free_slaves = num_slaves; TOPC_ACTION action; do { wait_for_free_slave(); input = COMM_generate_task_input(); if (input.data != NOTASK.data) { SUBMIT_TO_SLAVE: output = DoTask(input.data); num_free_slaves--; } else if (num_free_slaves < num_slaves) receive_task_output(); /* needed to insure progress condition */ } while (input.data != NOTASK.data || num_free_slaves < num_slaves);
The code for wait_for_free_slave()
can be expanded as follows.
void wait_for_free_slave() { do { while ( result_is_available(&input, &output) ) { action = CheckTaskResult(input.data, output.data); if (action == UPDATE) UpdateSharedData(input.data, output.data); if (action == REDO) /* Use updated shared data, when redoing */ SUBMIT_TO_SLAVE: output = DoTask(input.data); num_free_slaves++; } while (num_free_slaves == 0); }
Note that the term result refers to an `(input,output)' pair.
The routine CheckTaskResult()
returns an action, which
determines the
control structure for a parallel algorithm. A common definition is:
TOPC_ACTION CheckTaskResult( void *input, void *output ) { if (output == NULL) return NO_ACTION; else if ( ! TOPC_is_up_to_date() ) return REDO; else return return UPDATE; }
TOPC_is_up_to_date()
returns true if and only if during the interval
between when the task input was originally generated and when the task
output was returned by the most recent slave, no other slave process had
returned a task output during the interim that had caused the shared data
to be modified through an UPDATE
action. An UPDATE
action causes
UpdateSharedData()
to be invoked on each process. Further
discussion can be found in the section on section TOP-C Utilities.
`TOP-C' also supports operation on SMP (shared memory) hardware.
`TOP-C' is designed so that the same application source code may
operate efficiently both under distributed and under shared memory. In
SMP, all data outside of the four callback functions is shared,
by default. Hence, an UPDATE
action under shared memory causes
only the master process to invoke UpdateSharedData()
. To avoid
inconsistencies in the data, by default `TOP-C' arranges that no
slave process may run DoTask()
while UpdateSharedData()
is
running. `TOP-C' also provides support for finer levels of
granularity through application-defined private variables and critical
sections. Further discussion can be found in the section on section Optimizing TOP-C Code for the Shared Memory Model.
The `TOP-C' programmer's model is based on three key concepts:
Task descriptions (task inputs) are generated on the master, and assigned to a slave. The slave executes the task and returns the result to the master. The master may update its own private data based on the result, or it may update data on all processes. Such global updates take place on each slave after the slave completes its current task. Updates are lazy in that they occur only after a task completes, although it is possible to issue a non-binding request to `TOP-C' to abort the current tasks (section Aborting Tasks). A SPMD (Single Program Multiple Data) style of programming is encouraged.
In both shared and distributed memory architectures, one must worry about the order of reads and writes as multiple slaves autonomously update data. The utilities below are meant to ease that chore, by supporting the ease of the SPMD programming style, while still maintaining good efficiency and generality for a broad range of applications. The software can easily be ported to a variety of architectures.
A task input or task output is an arbitrary buffer of bytes.
Internally, it is an arbitrary user-defined data structure, which is
opaque to TOP-C. A task input or task output must be encapsulated by a
function: TOPC_MSG( void *buf, int buf_size )
, which is then
returned by the callback functions GenerateTaskInput()
and DoTask()
.
TOPC_MSG()
copies the
application data to its own buffers in `TOP-C' space
using a shallow copy.
Hence, it is the application's responsibility to free or re-use
the original space pointed to by buf.
See section section The Main TOP-C Library Calls, for more
details on TOPC_MSG()
.
When a `TOP-C' callback function is passed a
task input or task output, that buffer is a pointer
into `TOP-C' space. The callback function may freely
use or even modify the data in the buffer, but the buffer
pointer will no longer valid when the callback function returns.
If a later invocation of a callback function will wish to use the data
in the buffer, the `TOP-C' application must cache a private
copy and point to it with a global variable.
If a heterogeneous
architecture is used, there is an issue of converting data formats.
This is the application's responsibility. UNIX utilities are available
such as htonl()
, ntohl()
(for integer byte ordering), and
the `XDR' library
(RFC 1832, eXternal Data Representation, general standard, also
including struct's, etc.). `Corba''s `IDL' presents another
possible architecture-independent solution.
Note that systems following
the IEEE floating point standard should already have compatible floating
point.
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