Remember
Just to remember to the reader: the actual code is working with a client-server communication. The server part - the MPI profiler - sends information to the client - the interface - through a MPI interconnect done with MPI_Open_port.
In order to do so a protocol has to be defined for the messages.
Client-Server organisation
Connection
The Profiler-Interface organisation (4 MPI processes)
The connection of the profiler and the interface is done via the MPI_Open_port function, that opens and gives a port address. Each MPI process from the profiler publishes its own port, and therefore the interface has to connect to every single one of them. That actually means there is n servers and 1 client. It is unusual of a client-server model, as usually only 1 server delivers information to several clients. Nonetheless the profiler processes are the servers, as they are the ones that publish an accessible address.
With OpenMPI
OpenMPI provides an address that looks like:
117112832.0;tcp://192.168.1.71:36441+117112833.0;tcp://192.168.1.71:42986:300
An early attempt to guess the port was unsuccessful. Some part may change from a process to another without other logic than available resource (the port for example).
But a problem came when connecting in the interface. As the previous diagram shows it, several profilers connect to a single interface process. What is not shown is that the interface has in fact 1 thread per profiler, to deal with the communication. The data is then centralised in a single GUI. This approach is a typical Hybrid MPI programming approach. Therefore the interface has to initialise the MPI environment with MPI_Init_thread (rather than MPI_Init) and ask for a MPI_THREAD_MULTIPLE initialisation. By default OpenMPI doesn't provide such support.
The solution is rather simple: recompile OpenMPI with the threading support:
./configure --enable-mpi-threads
With MPICH-2
The Ness machine provided a MPICH-2 implementation already installed. For some reasons it didn't support dynamic linking, but static one is fine. For this implementation the port string looks like:
tag=0 port=52970 description=ness.epcc.ed.ac.uk ifname=129.215.175.1
That is radically different of the OpenMPI one, showing once more that guessing the port isn't a interesting idea.
As MPICH-2 is natively installed on Ness with the multi-threading, the configuration option isn't yet known.
Retrieving the port
The profiler opens and publish a port. As a matter of fact, the user has to read the port and give them in input to the interface. In order to give as much freedom as possible to the user several ways of doing it are available:
- Printing the port to the standard output stream
- Printing the port to the standard error output stream
- Writing the ports into a defined file
$> ./ring --help Profiler of an MPI program\nUse a MPI visualisation GUI to see information Possible options: --port-in-stdout [default] write the port into the standard output --port-in-stderr write the port into the standard error output --port-in-file file write the port into the file using MPI-I/O Note that only the last given option is used --help display that help
To use the file writing functionality simply start you program like:
$> mpiexec -n 4 ring --port-in-file port.txt
Note: so far adding the option manually as 2D array of char doesn't work, and no further looking as been made to make it work.
Writing each process' port in a single file
In order to write each process' port in a single file the MPI I/O functions are used. The standard defines several ways of doing so. In that case a simple subarray is defined with the size of the port as a base length. MPI I/O writes data as a whole line into a file, as this stores characters a new line is created for each port. The interface can therefore read the file line by line to find every port and know the number of started processes.
Extract of child_comm.c
if ( port == INFILE )
{
MPI_Datatype subarray;
MPI_File file_ptr;
int smallarray, bigarray, stride;
smallarray = (strlen(port_name)+1);
bigarray = world_size*smallarray;
stride = world_rank*smallarray;
fprintf(stderr, "!profiler(%d)! will write his port in '%s'\n", world_rank, file);
MPI_Type_create_subarray(1, &bigarray, &smallarray, &stride, MPI_ORDER_C, INTRA_MESSAGE_MPITYPE, &subarray);
MPI_Type_commit(&subarray);
if ( MPI_File_open(MPI_COMM_WORLD, file, MPI_MODE_WRONLY|MPI_MODE_CREATE, MPI_INFO_NULL, &file_ptr) != MPI_SUCCESS )
{
fprintf(stderr, "!profiler(%d)! failed to open file '%s'. ABORTING\n", world_rank, file);
MPI_Abort(MPI_COMM_WORLD, -1);
}
if ( MPI_File_set_view(file_ptr, 0, INTRA_MESSAGE_MPITYPE, subarray, "native", MPI_INFO_NULL) != MPI_SUCCESS )
{
fprintf(stderr, "!profiler(%d)! failed to set the file view! ABORTING\n", world_rank);
MPI_Abort(MPI_COMM_WORLD, -1);
}
if ( MPI_File_write_all(file_ptr, strcat(port_name, "\n"), smallarray, INTRA_MESSAGE_MPITYPE, MPI_STATUS_IGNORE) != MPI_SUCCESS )
{
fprintf(stderr, "!profiler(%d)! failed to write '%s'. ABORTING\n", world_rank, file);
MPI_Abort(MPI_COMM_WORLD, -1);
}
MPI_File_close(&file_ptr);
}
Communication
The profiler side
As far as the profiler is concerned, the communication with the interface could be either synchronous or asynchronous. The current implementation uses MPI_Ssend as simple choice, but later version could use asynchronous call and waiting before the next one is done. Or even deal with a request list to wait for.
The profiler uses internal functions defined into child_comm.h to communicate with the interface.
child_comm.h
#ifndef CHILDCOMM
#define CHILDCOMM
#include "intra_comm.h"
extern int world_rank;
extern double global_time;
typedef enum PortType { STDOUT, STDERR, INFILE } PortType;
int start_child(int world_size, PortType port_type, char* file);
int alive_child();
int sendto_child(Intra_message* message);
int wait_child(double time_in);
#endif // CHILDCOMM
intra_comm.h
#ifndef INTRA_COMM
#define INTRA_COMM
#define INTRA_MESSAGE_SIZE 64
typedef char Intra_message;
#define INTERCOMM_TAG 0
#define PROFNAME "!profiler!"
#ifdef __cplusplus
#define INTRA_MESSAGE_MPITYPE MPI::CHAR
#else
#define INTRA_MESSAGE_MPITYPE MPI_CHAR
#endif
/*
* ACTIONS
*/
typedef enum Message { MESSAGE_INIT,
MESSAGE_Ssend,
MESSAGE_Bsend,
MESSAGE_Issend,
MESSAGE_Recv,
MESSAGE_Irecv,
MESSAGE_Wait,
MESSAGE_QUIT } Message;
#endif // INTRA_COMM
The functions' name are explicit, and the intra_comm.h header defines the actual protocol information: it is therefore used by both profiler and interface. The actual sending is done by character stings, renamed as Intra_message. As the interface is coded in C++ the INTRA_MESSAGE_MPITYPE is defined using both C and C++ MPI standard definitions.
The message is composed of several fields, all separated by a space, which always includes main fields:
- action::enum Message the occurring action
- time in::double the Unix time when entering the MPI function
- time out::double the Unix time when returning the MPI function
- communicator::unsigned int the communicator unique number - not implemented yet
- destination::int the destination process
The information are written using standard C I/O calls:
sprintf(message, "%d %lf %lf %d\0", MESSAGE_Ssend, time_in, time_out, dest);
The interface side
Starting message box of the interface (GNU/Linux Gnome 3)
On the interface side the profilers' port could be defined either manually or by reading the file written as explained before. When this is done, one thread per process is started and their duty is to communicate with the profiler (the object is therefore called MPIWatch). The MPIWatch object is only responsible for receiving (and sending) information to the profiler, therefore each of them is attached to a Monitor object, that is responsible of the analyse of messages. In order to communicate the MPIWatch pushes arriving message onto a stack and signal to the Monitor that new messages are available. The Monitor then analyse the message and display information in the according places.
The couple MPIWatch - Monitor was done for logical purposes:
- Only the MPIWatch is actually aware of the MPI functions needed to sends and receive information to the profiler. If in the future another system is used, only this class has to be changed.
- Only the MPIWatch needs a separated thread, dealing with the messages contents is done on the main thread.
- Only the Monitor has the knowledge of what a message contains. New protocol functionnalities does not affect the way to transfer data between profiler and interface
- Only the Monitor knows about the GUI, that are shared "windows" among the several monitors.
As the interface is implemented in C++, the standard stream library is used to decapsulate the messages. The main fields are extract for each messages, and the according to the message action each additional information.
Extract of monitor.cpp
QString m = watcher->pop_pool();
std::istringstream stream(m.toStdString());
int message;
double time_in, time_out;
stream >> message >> time_in >> time_out;
switch(message)
{
/* ... */
case MESSAGE_Bsend:
// adds to call counts
statWidget->addTo(proc, N_Bsend);
// add time info
statWidget->addTo(proc, T_Bsend, time_out-time_in);
break;
/* ... */
}
Conclusion
The Profiler-Interface communication is done on two levels. The first one is the actual communication, done through MPI. This requires a port opening and publish mechanism, that the user has to give as an input to the profiler.
But the communication is also what information is sent. This is generated by each overloaded MPI function, and is analysed in the interface side by a Monitor object.
Decoupling the communication on these two levels allows an abstraction of actually sending and analysing the information.
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