我正在尝试使用MPI和C ++开发并行随机游走模拟。
在我的模拟中，每个过程都可以被认为是一个可以包含粒子的细胞（随机游走者）。单元在一个维度上与周期性边界条件（即环形拓扑）对齐。
在每个时间步骤中，粒子可以保留在其细胞中或以一定概率进入左或右相邻细胞。为了使它更容易一些，只有每个单元格列表中的最后一个粒子可以行走。如果粒子行走，则必须将其发送到具有相应等级的进程（MPI_Isend + MPI_Probe + MPI_Recv + MPI_Waitall）。
然而，在第一步之后，我的粒子开始消失，即消息以某种方式“丢失”。
下面是一个最小的例子（抱歉，如果它仍然很长）。为了更好地跟踪粒子运动，每个粒子都有一个ID，它对应于它开始的过程的等级。在每个步骤之后，每个单元格打印存储在其中的粒子的ID。
#include <mpi.h>
#include <vector>
#include <iostream>
#include <random>
#include <string>
#include <sstream>
#include <chrono>
#include <algorithm>
using namespace std;
class Particle
{
public:
int ID; // this is the rank of the process which initialized the particle
Particle () : ID(0) {};
Particle (int ID) : ID(ID) {};
};
stringstream msg;
string msgString;
int main(int argc, char** argv)
{
// Initialize the MPI environment
MPI_Init(NULL, NULL);
// Get the number of processes
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
// Get the rank of the process
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
// communication declarations
MPI_Status status;
// get the ranks of neighbors (periodic boundary conditions)
int neighbors[2];
neighbors[0] = (world_size + world_rank - 1) % world_size; // left neighbor
neighbors[1] = (world_size + world_rank + 1) % world_size; // right neighbor
// declare particle type
MPI_Datatype type_particle;
MPI_Type_contiguous (1, MPI_INT, &type_particle);
MPI_Type_commit (&type_particle);
// every process inits 1 particle with ID = world_rank
vector<Particle> particles;
particles.push_back (Particle(world_rank));
// obtain a seed from the timer
typedef std::chrono::high_resolution_clock myclock;
myclock::time_point beginning = myclock::now();
myclock::duration d = myclock::now() - beginning;
unsigned seed2 = d.count();
default_random_engine generator (seed2);
uniform_real_distribution<double> distribution (0, 1);
// ------------------------------------------------------------------
// begin time loop
//-------------------------------------------------------------------
for (int t=0; t<10; t++)
{
// ------------------------------------------------------------------
// 1) write a message string containing the current list of particles
//-------------------------------------------------------------------
// write the rank and the particle IDs into the msgString
msg << "rank " << world_rank << ": ";
for (auto& i : particles)
{
msg << i.ID << " ";
}
msg << "\n";
msgString = msg.str();
msg.str (string()); msg.clear ();
// to print the messages in order, the messages are gathered by root (rank 0) and then printed
// first, gather nums to root
int num = msgString.size();
int rcounts[world_size];
MPI_Gather( &num, 1, MPI_INT, rcounts, 1, MPI_INT, 0, MPI_COMM_WORLD);
// root now has correct rcounts, using these we set displs[] so
// that data is placed contiguously (or concatenated) at receive end
int displs[world_size];
displs[0] = 0;
for (int i=1; i<world_size; ++i)
{
displs[i] = displs[i-1]+rcounts[i-1]*sizeof(char);
}
// create receive buffer
int rbuf_size = displs[world_size-1]+rcounts[world_size-1];
char *rbuf = new char[rbuf_size];
// gather the messages
MPI_Gatherv( &msgString[0], num, MPI_CHAR, rbuf, rcounts, displs, MPI_CHAR,
0, MPI_COMM_WORLD);
// root prints the messages
if (world_rank == 0)
{
cout << endl << "step " << t << endl;
for (int i=0; i<rbuf_size; i++)
cout << rbuf[i];
}
// ------------------------------------------------------------------
// 2) send particles randomly to neighbors
//-------------------------------------------------------------------
Particle snd_buf;
int sndDest = -1;
// 2a) if there are particles left, prepare a message. otherwise, proceed to step 2b)
if (!particles.empty ())
{
// write the last particle in the list to a buffer
snd_buf = particles.back ();
// flip a coin. with a probability of 50 %, the last particle in the list gets sent to a random neighbor
double rnd = distribution (generator);
if (rnd <= .5)
{
particles.pop_back ();
// pick random neighbor
if (rnd < .25)
{
sndDest = neighbors[0]; // send to the left
}
else
{
sndDest = neighbors[1]; // send to the right
}
}
}
// 2b) always send a message to each neighbor (even if it's empty)
MPI_Request requests[2];
for (int i=0; i<2; i++)
{
int dest = neighbors[i];
MPI_Isend (
&snd_buf, // void* data
sndDest==dest ? 1 : 0, // int count <---------------- send 0 particles to every neighbor except the one specified by sndDest
type_particle, // MPI_Datatype
dest, // int destination
0, // int tag
MPI_COMM_WORLD, // MPI_Comm
&requests[i]
);
}
// ------------------------------------------------------------------
// 3) probe and receive messages from each neighbor
//-------------------------------------------------------------------
for (int i=0; i<2; i++)
{
int src = neighbors[i];
// probe to determine if the message is empty or not
MPI_Probe (
src, // int source,
0, // int tag,
MPI_COMM_WORLD, // MPI_Comm comm,
&status // MPI_Status* status
);
int nRcvdParticles = 0;
MPI_Get_count (&status, type_particle, &nRcvdParticles);
// if the message if non-empty, receive it
if (nRcvdParticles > 0) // this proc can receive max. 1 particle from each neighbor
{
Particle rcv_buf;
MPI_Recv (
&rcv_buf, // void* data
1, // int count
type_particle, // MPI_Datatype
src, // int source
0, // int tag
MPI_COMM_WORLD, // MPI_Comm comm
MPI_STATUS_IGNORE // MPI_Status* status
);
// add received particle to the list
particles.push_back (rcv_buf);
}
}
MPI_Waitall (2, requests, MPI_STATUSES_IGNORE);
}
// ------------------------------------------------------------------
// end time loop
//-------------------------------------------------------------------
// Finalize the MPI environment.
MPI_Finalize();
if (world_rank == 0)
cout << "\nMPI_Finalize()\n";
return 0;
}
我使用8个进程运行模拟，下面是输出的示例。在步骤1中，它似乎仍能正常工作，但从第2步开始，粒子开始消失。
step 0
rank 0: 0
rank 1: 1
rank 2: 2
rank 3: 3
rank 4: 4
rank 5: 5
rank 6: 6
rank 7: 7
step 1
rank 0: 0
rank 1: 1
rank 2: 2 3
rank 3:
rank 4: 4 5
rank 5:
rank 6: 6 7
rank 7:
step 2
rank 0: 0
rank 1:
rank 2: 2
rank 3:
rank 4: 4
rank 5:
rank 6: 6 7
rank 7:
step 3
rank 0: 0
rank 1:
rank 2: 2
rank 3:
rank 4:
rank 5:
rank 6: 6
rank 7:
step 4
rank 0: 0
rank 1:
rank 2: 2
rank 3:
rank 4:
rank 5:
rank 6: 6
rank 7:
我不知道代码有什么问题...不知何故，组合MPI_Isend + MPI_Probe + MPI_Recv + MPI_Waitall似乎无法正常工作......真的很感激任何帮助！
答案 0 :(得分：5)
您的代码中存在错误。以下逻辑（省略了不相关的代码和参数）是错误的：
MPI_Probe(..., &status);
MPI_Get_count (&status, type_particle, &nRcvdParticles);
// if the message if non-empty, receive it
if (nRcvdParticles > 0)
{
MPI_Recv();
}
MPI_Probe 不从邮件队列中删除零大小的邮件。这样做的唯一MPI调用是MPI_Recv以及MPI_Irecv + MPI_Test / MPI_Wait的组合。您必须接收所有消息，包括零大小的消息，否则它们将阻止接收具有相同（源，标记）组合的其他消息。虽然接收零大小的消息不会在接收缓冲区中写入任何内容，但它会从队列中删除消息信封，并且可以接收下一个匹配的消息。
解决方案：在条件运算符之前将调用移至MPI_Recv。