スライド 1

Parallel Programming in MPI
part 2
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Answer of the previous exercise
• 作成したプログラムについて、以下を説明してください。
Explain your program with the following points of view.
• どのように解決したか。
How did you solve the problem.
• 何が難しかったか。
Where was the difficulty.
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Today's Topic
• ノンブロッキング通信
Non-Blocking Communication
• 通信の完了を待つ間に他の処理を行う
Execute other instructions while waiting for the completion of a communication.
• 集団通信関数の実装
Implementation of collective communications
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• デッドロック Deadlock
3
Today's Topic
• ノンブロッキング通信
Non-Blocking Communication
• 通信の完了を待つ間に他の処理を行う
Execute other instructions while waiting for the completion of a communication.
• 集団通信関数の実装
Implementation of collective communications
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• デッドロック Deadlock
4
ノンブロッキング通信関数
Non-blocking communication functions
• ノンブロッキング = ある命令の完了を待たずに次の命令に移る
Non-blocking = Do not wait for the completion of an instruction and
proceed to the next instruction
• Example) MPI_Irecv & MPI_Wait
Blocking
Non-Blocking
MPI_Recv
Wait for the
arrival of data
data
Proceed to the next
instruction without
waiting for the data
MPI_Irecv
next instructions
MPI_Wait
data
next instructions
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MPI_Irecv
Usage:
int MPI_Irecv(void *b, int c, MPI_Datatype d, int src,
int t, MPI_Comm comm, MPI_Request *r);
• Non-Blocking Receive
• Parameters:
start address for storing received data,
number of elements, data type,
rank of the source, tag (= 0, in most cases),
communicator (= MPI_COMM_WORLD, in most cases),
request
• request: 通信要求 Communication Request
• この通信の完了を待つ際に用いる
Used for Waiting completion of this communication
• Example)
MPI_Request req;
...
MPI_Irecv(a, 100, MPI_INT, 0, 0, MPI_COMM_WORLD, &req);
...
MPI_Wait(&req, &status);
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6
MPI_Isend
• Non-Blocking Send
Usage:
int MPI_Isend(void *b, int c, MPI_Datatype d,
int dest, int t, MPI_Comm comm,
MPI_Request *r);
• Parameters:
start address for sending data,
number of elements, data type,
rank of the destination, tag (= 0, in most cases),
communicator (= MPI_COMM_WORLD, in most cases),
request
• Example)
MPI_Request req;
...
MPI_Isend(a, 100, MPI_INT, 1, 0, MPI_COMM_WORLD, &req);
...
MPI_Wait(&req, &status);
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Non-Blocking Send?
• Blocking send (MPI_Send):
送信データが別の場所にコピーされるのを待つ
Wait for the data to be copied to somewhere else.
• ネットワークにデータを送出し終わるか、一時的にデータのコピーを作成するまで。
Until completion of the data to be transferred to the network
or, until completion of the data to be copied to a temporal memory.
• Non-Blocking send (MPI_Recv):
待たない
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Notice: ノンブロッキング通信中はデータが不定
Data is not sure in non-blocking communications
• MPI_Irecv:
• 受信データの格納場所と指定した変数の値は MPI_Waitまで不定
Value of the variable specified for receiving data is not fixed before
MPI_Wait
Value of A at here
can be 10 or 50
MPI_Irecv to A
A
10
arrived
data
...
~=A
...
A
50
50
MPI_Wait
Value of A is 50
~=A
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Notice: ノンブロッキング通信中はデータが不定
Data is not sure in non-blocking communications
• MPI_Isend:
• 送信データを格納した変数を MPI_Waitより前に書き換えると、実際に送信さ
れる値は不定
If the variable that stored the data to be sent is modified before MPI_Wait,
the value to be actually sent is unpredictable.
Modifying value of A here
causes incorrect
communication
MPI_Isend A
...
A = 50
...
A
10
A
50
data sent
10 or 50
MPI_Wait
You can modify value of A at
here without any problem
A = 100
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MPI_Wait
Usage:
int MPI_Wait(MPI_Request *req, MPI_Status *stat);
• ノンブロッキング通信（MPI_Isend、 MPI_Irecv）の完了を待つ。
Wait for the completion of MPI_Isend or MPI_Irecv
• 送信データの書き換えや受信データの参照が行える
Make sure that sending data can be modified,
or receiving data can be referred.
• Parameters:
request, status
• status:
MPI_Irecv 完了時に受信データの statusを格納
The status of the received data is stored at the completion of
MPI_Irecv
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MPI_Waitall
Usage:
int MPI_Waitall(int c,
MPI_Request *requests, MPI_Status *statuses);
• 指定した数のノンブロッキング通信の完了を待つ
Wait for the completion of specified number of non-blocking
communications
• Parameters:
count, requests, statuses
• count:
ノンブロッキング通信の数
The number of non-blocking communications
• requests, statuses:
少なくとも count個の要素を持つ MPI_Request と MPI_Statusの配列
Arrays of MPI_Request or MPI_Status that consists at least 'count'
number of elements.
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Today's Topic
• ノンブロッキング通信
Non-Blocking Communication
• 通信の完了を待つ間に他の処理を行う
Execute other instructions while waiting for the completion of a communication.
• 集団通信関数の実装
Implementation of collective communications
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• デッドロック Deadlock
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集団通信関数の中身
Inside of the functions of collective communications
• 通常，集団通信関数は，
MPI_Send, MPI_Recv, MPI_Isend, MPI_Irecv
等の一対一通信で実装される
Usually, functions of collective communications are
implemented by using message passing functions.
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Inside of MPI_Bcast
• One of the most simple implementations
int MPI_Bcast(char *a, int c, MPI_Datatype d, int root,
MPI_Comm comm)
{
int i, myid, procs;
MPI_Status st;
MPI_Comm_rank(comm, &myid);
MPI_Comm_size(comm, &procs);
if (myid == root){
for (i = 0; i < procs)
if (i != root)
MPI_Send(a, c, d, i, 0, comm);
} else{
MPI_Recv(a, c, d, root, 0, comm, &st);
}
return 0;
}
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Another implementation: With MPI_Isend
int MPI_Bcast(char *a, int c, MPI_Datatype d, int root,
MPI_Comm comm)
{
int i, myid, procs, cntr;
MPI_Status st, *stats;
MPI_Request *reqs;
MPI_Comm_rank(comm, &myid);
MPI_Comm_rank(comm, &procs);
if (myid == root){
stats = (MPI_Status *)malloc(sizeof(MPI_Status)*procs);
reqs = (MPI_Request *)malloc(sizeof(MPI_Request)*procs);
cntr = 0;
for (i = 0; i < procs)
if (i != root)
MPI_Isend(a, c, d, i, 0, comm, &(reqs[cntr++]));
MPI_Waitall(procs-1, reqs, stats);
free(stats);
free(reqs);
} else{
MPI_Recv(a, c, d, root, 0, comm, &st);
}
return 0;
}
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Flow of the Simple Implementation
Rank 0
Rank 1
Rank 2
Rank 3
Rank 4
Isend to 1
Irecv
from 0
Irecv
from 0
Irecv
from 0
Irecv
from 0
wait
wait
wait
Isend to 2
Rank 5
Rank 6
Rank 7
Irecv
from 0
Irecv
from 0
Irecv
from 0
wait
wait
Isend to 3
Isend to 4
wait
wait
Isend to 5
Isend to 6
Isend to 7
waitall
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Time for Simple Implementation
• 1 link can transfer 1 message at a time
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
Total Time = T * (P-1)
T: Time for transferring 1 message
P: Number of processes
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Another implementation: Binomial Tree
int MPI_Bcast(char *a, int c, MPI_Datatype d,
int root, MPI_Comm comm)
{
int i, myid, procs;
MPI_Status st;
int mask, relative_rank, src, dst;
int tag = 1, success = 0;
MPI_Comm_rank(comm, &myid);
MPI_Comm_rank(comm, &procs);
relative_rank = myid - root;
if (relative_rank < 0)
relative_rank += procs;
mask = 1;
while (mask < num_procs){
if (relative_rank & mask){
src = myid - mask;
if (src < 0) src += procs;
MPI_Recv(a, c, d, src, 0, comm, &st);
break;
}
mask <<= 1;
}
mask >>= 1;
while (mask > 0){
if (relative_rank + mask < procs){
dst = myid + mask;
if (dst >= procs) dst -= procs;
MPI_Send (a, c, d, dst, 0, comm);
}
mask >>= 1;
}
return 0;
}
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Flow of Binomial Tree
• Use 'mask' to determine when and how to Send/Recv
Rank 0
Rank 1
Rank 2
Rank 3
Rank 4
Rank 5
Rank 6
Rank 7
mask = 1 mask = 1 mask = 1 mask = 1 mask = 1 mask = 1 mask = 1 mask = 1
mask = 2 Recv
mask = 2 Recv
mask = 2 Recv
mask = 2 Recv
mask = 4
mask = 4
from 6
from 4 Recv
from 0 Recv
from 2 Recv
from 4
mask = 4
from 0
from
0
Send to 4
mask = 2
Send to 2
mask = 1
Send to 3
mask = 2
Send to 6
mask = 1
Send to 5
mask = 1
Send to 7
mask = 1
Send to 1
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Time for Binomial Tree
• Use multiple links at a time
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
Total Time = T * log2P
T: Time for transferring 1 message
P: Number of processes
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Today's Topic
• ノンブロッキング通信
Non-Blocking Communication
• 通信の完了を待つ間に他の処理を行う
Execute other instructions while waiting for the completion of a communication.
• 集団通信関数の実装
Implementation of collective communications
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• デッドロック Deadlock
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MPIプログラムの時間計測
Measure the time of MPI programs
• MPI_Wtime
• 現在時間（秒）を実数で返す関数 Returns the current time in seconds.
• Example)
Measure
time here
...
double t1, t2;
...
t1 = MPI_Wtime();
処理
t2 = MPI_Wtime();
printf("Elapsed time: %e sec.\n", t2 – t1);
並列プログラムにおける時間計測の問題
Problem on measuring time in parallel programs
• プロセス毎に違う時間を測定：
どの時間が本当の所要時間か?
Each process measures different time.
Which time is the time we want?
Rank 0
t1 = MPI_Wtime();
Rank 1
Read
Measure
time here
t1 = MPI_Wtime();
Read
Send
Read
Rank 2
t1 = MPI_Wtime();
Receive
Receive
t1 = MPI_Wtime();
Send
t1 = MPI_Wtime();
t1 = MPI_Wtime();
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集団通信 MPI_Barrierを使った解決策
Use MPI_Barrier
• 時間計測前にMPI_Barrierで同期
Synchronize processes before each measurement
• For measuring total execution time.
Rank 0
MPI_Barrier
t1 = MPI_Wtime();
Measure
time here
Rank 1
MPI_Barrier
Receive
Rank 2
MPI_Barrier
Receive
Read
Read
Send
Read
MPI_Barrier
Send
MPI_Barrier
MPI_Barrier
t1 = MPI_Wtime();
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より細かい解析
Detailed analysis
• Average
• MPI_Reduce can be used to achieve the average:
double t1, t2, t, total;
t1 = MPI_Wtime();
...
t2 = MPI_Wtime();
t = t2 – t1;
MPI_Reduce(&t, &total, 1, MPI_DOUBLE, MPI_SUM,
0, MPI_COMM_WORLD);
if (myrank == 0)
printf("Ave. elapsed: %e sec.\n", total/procs);
• MAX and MIN
• Use MPI_Gather to gather all of the results to Rank 0.
• Let Rank 0 to find MAX and MIN
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最大(Max)、平均(Ave)、最小(Min)の関係
Relationships among Max, Ave and Min
• プロセス毎の負荷（仕事量）のばらつき検証に利用
Can be used for checking the load-balance.
Max – Ave
is large
Max – Ave
is small
Ave – Min
is large
NG
Mostly OK
Ave – Min
is small
NG
OK
Time includes Computation Time and Communication Time
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通信時間の計測
Measuring time for communications
double t1, t2, t3, t4 comm=0;
t3 = MPI_Wtime();
for (i = 0; i < N; i++){
computation
t1 = MPI_Wtime();
communication
t2 = MPI_Wtime(); comm += t2 – t1;
computation
t1 = MPI_Wtime();
communication
t2 = MPI_Wtime(); comm += t2 – t1;
}
t4 = MPI_Wtime();
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Analyze computation time
• Computation time = Total time - Communication time
• Or, just measure the computation time
• 計算時間のばらつき＝負荷の不均衡の度合い
Balance of computation time shows
balance of the amount of computation
• 注意: 通信時間には、負荷の不均衡によって生じた待ち時間が含まれ
るので、単純な評価は難しい
Communication time is difficult to analyze since
it consists waiting time caused by load-imbalance.
==> Balance computation first.
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Today's Topic
• ノンブロッキング通信
Non-Blocking Communication
• 通信の完了を待つ間に他の処理を行う
Execute other instructions while waiting for the completion of a communication.
• 集団通信関数の実装
Implementation of collective communications
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• デッドロック Deadlock
30
Deadlock
• 何らかの理由で、プログラムを進行させることができなくなった状態
A status of a program in which it cannot proceed by some reasons.
• MPIプログラムでデッドロックが発生しやすい場所：
Places you need to be careful for deadlocks:
1. MPI_Recv, MPI_Wait, MPI_Waitall
Wrong case:
if (myid == 0){
MPI_Recv from rank 1
MPI_Send to rank 1
}
if (myid == 1){
MPI_Recv from rank 0
MPI_Send to rank 0
}
One solution:
use MPI_Irecv
if (myid == 0){
MPI_Irecv from rank 1
MPI_Send to rank 1
MPI_Wait
}
if (myid == 1){
MPI_Irecv from rank 0
MPI_Send to rank 0
MPI_Wait
}
2. Collective communications
全部のプロセスが同じ集団通信関数を実行するまで先に進めない
A program cannot proceed until all processes call
the same collective communication function
Summary
• ノンブロッキング通信の効果
Effect of non-blocking communication
• 通信開始と通信完了待ちを分離
Split the start and the completion of a communication
• 通信と計算のオーバラップを可能にする
Enable overlapping of communication and computation .
• 集団通信の実装
Implementation of collective communication.
• 内部で送信と受信を組み合わせて実装
Construct algorithms with sends and receives.
• 所要時間はアルゴリズムに依存
Time depends on the algorithm.
• MPIプログラムの時間計測
Measuring execution time of MPI programs
• 並列プログラムではデッドロックに注意
Be careful about deadlocks.
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Report) Make Reduce function by yourself
• 次のページのプログラムの my_reduce関数の中身を追加してプロ
グラムを完成させる
Fill the inside of 'my_reduce' function in the program
shown in the next slide
• my_reduce: MPI_Reduceの簡略版
Simplified version of MPI_Reduce
• 整数の総和のみ. ルートランクは 0限定．
コミュニケータは MPI_COMM_WORLD
Calculates total sum of integer numbers. The root rank is always 0.
The communicator is always MPI_COMM_WORLD.
• アルゴリズムは好きなものを考えてよい
Any algorithm is OK.
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#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
#define N 20
int my_reduce(int *a, int *b, int c)
{
complete here by yourself
return 0;
}
int main(int argc, char *argv[])
{
int i, myid, procs;
int a[N], b[N];
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &myid);
MPI_Comm_size(MPI_COMM_WORLD, &procs);
for (i = 0; i < N; i++){
a[i] = i;
b[i] = 0;
}
my_reduce(a, b, N);
if (myid == 0)
for (i = 0; i < N; i++)
printf("b[%d] = %d , correct answer = %d\n", i, b[i], i*procs);
MPI_Finalize();
return 0;
}
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