程序代写代做代考 compiler Java Fortran c/c++ Message-Passing Programming with MPI

Message-Passing Programming with MPI
Message-Passing Concepts

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3

Overview
• This lecture will cover – message passing model – SPMD
– communication modes
– collective communications
4

Programming Models
Serial Programming
Message-Passing Parallel Programming Concepts
Concepts
Arrays
Control flow Variables
Subroutines Human-readable OO
Processes
SPMD Groups
Send/Receive
Collectives
Python
Java
struct
C/C++
Fortran
if/then/else
Languages
Libraries
MPI
MPI_Init()
Implementations
Intel MPI OpenMPI
MPICH2
Cray MPI
IBM MPI
gcc -O3
crayftn
pgcc -fast
icc
javac
Implementations
craycc
5

Message Passing Model
• The message passing model is based on the notion of
过程的概念,
– can think of a process as an instance of a running program,
together with the program􏰇s data
• In the message passing model, parallelism is achieved by having many processes co-operate on the same task
• Each process has access only to its own data – ie all variables are private
• Processes communicate with each other by sending and receiving messages
– typically library calls from a conventional sequential language 6
processes

Sequential Paradigm
Memory
M
连续的范式
Process
Processor
7
P

Parallel Paradigm
Processes 0123
Message Passing Interface
isdated.jdohotnedtoknowtpngsicds.de
Communication Network
8

Distributed-Memory Architectures
Interconnect
P
M
P
M
P
M
P
M
P
M
P
M
P
M
P
M
9

Process Communication
Program
Process 2
Recv(1,b)
a=b+1
Process 1
a=23
Send(2,a)
a.
ababare.pro
23
24
Data
23 23
。在
10

SPMD
• Most message passing programs use the Single- Program-Multiple-Data (SPMD) model
• All processes run (their own copy of) the same program
• Each process has a separate copy of the data
• To make this useful, each process has a unique identifier
• Processes can follow different control paths through the program, depending on their process ID
• Usually run one process per processor / core
11

Emulating General Message Passing (C)
main (int argc, char **argv) {
if (controller_process)
{
} }
asmlopnoces.mg
Controller( /* Arguments */ ); }
else
{
Worker
( /* Arguments */ );
12

Emulating General Message Passing (F)
PROGRAM SPMD
IF (controller_process) THEN
ELSE
CALL WORKER
END PROGRAM SPMD
( ! Arguments ! )
ENDIF
CALL CONTROLLER ( ! Arguments ! )
13

Messages
• A message transfers a number of data items of a certain type from the memory of one process to the memory of another process
• A message typically contains – the ID of the sending processor – the ID of the receiving processor – the type of the data items
– the number of data items – the data itself
– a message type identifier
14

Communication modes
• Sending a message can either be synchronous or asynchronous
• A synchronous send is not completed until the message has started to be received
• An asynchronous send completes as soon as the makephoneausendemail.tn
message has gone
• Receives are usually synchronous – the receiving process must wait until the message arrives
15

Synchronous send
类似
• Analogy with faxing a letter.
• Know when letter has started to be received.
16

Asynchronous send
• Analogy with posting a letter.
• Only know when letter has been posted, not when it has been received.
17

Point-to-Point Communications
• We have considered two processes – one sender
– one receiver
• This is called point-to-point communication – simplest form of message passing
– relies on matching send and receive
• Close analogy to sending personal emails
18

Collective Communications
• A simple message communicates between two processes • There are many instances where communication between
groups of processes is required
• Can be built from simple messages, but often implemented separately, for efficiency
19

Barrier
• Global synchronisation 全球眇
Barrier
Barrier
Barrier
20

Broadcast
• One to all communication
21

Broadcast
• From one process to all others 8
88 8
8
8
22

Scatter 䩢
• Information scattered to many processes
1
2 012345
3
0
4
5
23

Gather
• Information gathered onto one process 1
2 012345
3
0
4
5
24

Reduction Operations
• Combine data from several processes to form a single result
Strike?
kgpare
25

Reduction
• Form a global sum, product, max, min, etc. 1
20 15
3
5
4
26

Launching a Message-Passing Program
• Write a single piece of source code
• with calls to message-passing functions such as send / receive
• Compile with a standard compiler and link to a message- passing library provided for you
• both open-source and vendor-supplied libraries exist
• Run multiple copies of same executable on parallel machine • each copy is a separate process
• each has its own private data completely distinct from others
• each copy can be at a completely different line in the program
• Running is usually done via a launcher program
• 􏰀please run N copies of my executable called program.exe􏰁
启动程序
27

Issues
• Sends and receives must match – danger of deadlock
– program will stall (forever!)
• Possible to write very complicated programs, but 􏰈 – most scientific codes have a simple structure
– often results in simple communications patterns
• Use collective communications where possible – may be implemented in efficient ways
28

Summary (i)
• Messages are the only form of communication – all communication is therefore explicit 明星的 .
• Most systems use the SPMD model
– all processes run exactly the same code
– each has a unique ID
– processes can take different branches in the same codes
• Basic communications form is point-to-point
– collective communications implement more complicated patterns
that often occur in many codes
29

Summary (ii)
• Message-Passing is a programming model
– that is implemented by MPI
– the Message-Passing Interface is a library of function/subroutine calls
• Essential to understand the basic concepts – private variables
– explicit communications
– SPMD
• Major difficulty is understanding the Message-Passing model – a very different model to sequential programming
if (x < 0) p􏰃in􏰉(􏰀E􏰃􏰃o􏰃􏰁); exit; 30