4.3 MPI and Threading

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Del curso dictado por Rice University

Distributed Programming in Java

117 calificaciones

Rice University

Distributed Programming in Java

117 calificaciones

Curso 3 de 3 en SpecializationParallel, Concurrent, and Distributed Programming in Java

This course teaches learners (industry professionals and students) the fundamental concepts of Distributed Programming in the context of Java 8. Distributed programming enables developers to use multiple nodes in a data center to increase throughput and/or reduce latency of selected applications. By the end of this course, you will learn how to use popular distributed programming frameworks for Java programs, including Hadoop, Spark, Sockets, Remote Method Invocation (RMI), Multicast Sockets, Kafka, Message Passing Interface (MPI), as well as different approaches to combine distribution with multithreading. Why take this course? • All data center servers are organized as collections of distributed servers, and it is important for you to also learn how to use multiple servers for increased bandwidth and reduced latency. • In addition to learning specific frameworks for distributed programming, this course will teach you how to integrate multicore and distributed parallelism in a unified approach. • Each of the four modules in the course includes an assigned mini-project that will provide you with the necessary hands-on experience to use the concepts learned in the course on your own, after the course ends. • During the course, you will have online access to the instructor and the mentors to get individualized answers to your questions posted on forums. The desired learning outcomes of this course are as follows: • Distributed map-reduce programming in Java using the Hadoop and Spark frameworks • Client-server programming using Java's Socket and Remote Method Invocation (RMI) interfaces • Message-passing programming in Java using the Message Passing Interface (MPI) • Approaches to combine distribution with multithreading, including processes and threads, distributed actors, and reactive programming Mastery of these concepts will enable you to immediately apply them in the context of distributed Java programs, and will also provide the foundation for mastering other distributed programming frameworks that you may encounter in the future (e.g., in Scala or C++).

De la lección

COMBINING DISTRIBUTION AND MULTITHREADING

In this module, we will study the roles of processes and threads as basic building blocks of parallel, concurrent, and distributed Java programs. With this background, we will then learn how to implement multithreaded servers for increased responsiveness in distributed applications written using sockets, and apply this knowledge in the mini-project on implementing a parallel file server using both multithreading and sockets. An analogous approach can also be used to combine MPI and multithreading, so as to improve the performance of distributed MPI applications. Distributed actors serve as yet another example of combining distribution and multithreading. A notable property of the actor model is that the same high-level constructs can be used to communicate among actors running in the same process and among actors in different processes; the difference between the two cases depends on the application configuration, rather the application code. Finally, we will learn about the reactive programming model,and its suitability for implementing distributed service oriented architectures using asynchronous events.

4.1 Processes and Threads9:48

4.2 Multithreaded Servers6:59

4.3 MPI and Threading7:09

4.4 Distributed Actors8:49

4.5 Distributed Reactive Programming7:01

Conoce a los instructores

Vivek Sarkar
Professor
Department of Computer Science

Let us now extend what we learned about the Message Passing Interface

by understanding how we can add threading to it.

So we know in our distributed system,

we have these compute nodes that have a network interface.

They have a local memory, and

they are connected to an interconnect network.

And we've learned the Message Passing Interface where

effectively calls like MPI SEND

and MPI RECV go through the network interface,

into the interconnect, so as to achieve the communication that you want.

Now, if you look at real hardware today in data centers,

it turns out that the processing part actually has multiple processors,

which are also referred to as processor cores, executing on the node.

So when we run an MPI process, we are actually just running a single thread and

only using one of these processors.

And we should, of course, be using all of them to their fullest.

One way to do this is to create separate MPI ranks,

one per processor, but another way is to add threading.

So, what we want to do is effectively create threads that run on the processors T0,

T1, T2, so that we can get the benefits of both message passing and

multi-threading to use the full power of the computer.

Also, it's worth noting that these processors can

communicate through the same local memory.

So these threads can exchange data structures in a local memory

without using calls to MPI.

It's only when you need to exchange data on other nodes that you need to use MPI.

So in this approach to combining MPI with threading, what we'll do is,

we'll have one rank per node.

And we can refer to as T0 as the master thread.

This is the one that starts up MPI just like you've seen before.

So it will call MPI INIT, and then at the end call MPI FINALIZE.

But what it can do is create a number of other threads,

so it can create worker threads.

And these threads can execute in parallel using techniques that

you would learn in a course on parallelism.

So the idea is that when the master thread

has to perform some operations like MPI SEND,

MPI RECV, or MPI REDUCE,

it may be blocked waiting on communications from other nodes.

But these other threads can still continue useful work on the node.

You'll basically have one rank per node, so this node could be running MPI rank 0.

Another node could be running rank 1, but

you have parallelism within the rank.

Now, it's interesting that MPI offers a few different modes.

So in MPI, you have different

threading modes.

One is referred to as funneled,

And the idea here is that all MPI calls are performed by one thread.

So from MPI's perspective, all the communications are occurring

with a single thread like what we called the master thread over here.

And it is completely unaware of all the other worker threads.

Another one is called serialized.

And here, you can at most have one MPI call at a time.

Here's where you can leverage the concepts that you'd learn from

a concurrency class to know that while there are multiple threads,

only one thread can be performing an MPI call at any one time.

And this helps the implementation, when in this mode, for

MPI to know that there's not going to be any contention on MPI resources.

But it could be that T0 makes an MPI call in one phase,

T1 makes an MPI call in another phase, and so on.

And finally, there is the most general mode which is called multiple.

And here you can have multiple calls at the same time.

So this gives more flexibility, but it puts

more burden on the MPI implementation to take care of any contention.

There are some caveats over here.

So, for example, in the multiple mode,

you cannot have two threads waiting on the same event.

So if T1 wanted to do an MPI wait on a request R1,

T2 is not allowed to also wait on the same

MPI request, so this is not allowed.

It will have to wait on some other request instead.

So now, you have it.

If you've learned the basic concepts of parallelism and concurrency, and

you know how to do message passing in its most fundamental way.

You can combine the two, you have MPI and threading.

And you can pick the mode that you want to use depending on how you want to leverage

the parallelism.

And with that,

you can exploit the computers available to you in any data center to the fullest.

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