I would like to learn about Java's multi-core processing. From what I understand, is that Threading is one type of multi-core, which I feel I have a decent grasp of. I know there are other ways to do multi-core processing, but I don't know what they are. Does anyone know of any good simple tutorials/examples, or have their own that I could look at to learn more about multi-core processing in Java?
All the tutorials that I have found get too in depth with charts, graphs, background information, etc. and that really isn't my learning style with programming. I would preferably like something quick and simple.
The primary way you use multiple cores is to use multiple threads. The simplest way to use these if via the High Level Concurrency Objects which you should be familar with. This uses threads but you don't have to deal with them directly.
Another way is to use multiple processes, but this is an indirect way of using multiple threads.
You might find this library interesting. Java Thread Affinity It allows you to assign thread to sockets, cores or cpus.
This is Oracle's tutorial about Java 7 fork/join framework
http://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html
Beyond the mentioned High Level Concurrency Objects (fork/join should be added there) which are part of the Java implementation, there are many libraries and frameworks. Google for "actor framework", "dataflow framework", mapreduce, "scientific dataflow". The dataflow model is the mainstream, all other are it's variations (e.g. actor - dataflow node with single input port, mapreduce - persistent distributed actors created by demand, etc). The minimal dataflow framework (no persistence or distribution over a machine cluster) is mine df4j library.
Related
I would like to build a distributed NoSQL database or key-value store using golang, to learn golang and practice distribute system knowledge I've learnt from school. The target use case I can think of is running MapReduce on top of it, and implement a HDFS-compatible "filesystem" to expose the data to Hadoop, similar to running Hadoop on Ceph and Amazon S3.
My question is, what difficulties should I expect to integrate such an NoSQl database with Hadoop? Or integrate with other languages (e.g., providing Ruby/Python/Node.js/C++ APIs?) if I use golang to build the system.
Ok, I'm not much of a Hadoop user so I'll give you some more general lessons learned about the issues you'll face:
Protocol. If you're going with REST Go will be fine, but expect to find some gotchas in the default HTTP library's defaults (not expiring idle keepalive connections, not necessarily knowing when a reader has closed a stream). But if you want something more compact, know that: a. the Thrift implementation for Go, last I checked, was lacking and relatively slow. b. Go has great support for RPC but it might not play well with other languages. So you might want to check out protobuf, or work on top the redis protocol or something like that.
GC. Go's GC is very simplistic (STW, not generational, etc). If you plan on heavy memory caching in the orders of multiple Gs, expect GC pauses all over the place. There are techniques to reduce GC pressure but the straight forward Go idioms aren't usually optimized for that.
mmap'ing in Go is not straightforward, so it will be a bit of a struggle if you want to leverage that.
Besides slices, lists and maps, you won't have a lot of built in data structures to work with, like a Set type. There are tons of good implementations of them out there, but you'll have to do some digging up.
Take the time to learn concurrency patterns and interface patterns in Go. It's a bit different than other languages, and as a rule of thumb, if you find yourself struggling with a pattern from other languages, you're probably doing it wrong. A good talk about Go concurrency is this one IMHO http://www.youtube.com/watch?v=QDDwwePbDtw
A few projects you might want to have a look at:
Groupcache - a distributed key/value cache written in Go by Brad Fitzpatrick for Google's own use. It's a great implementation of a simple yet super robust distributed system in Go. https://github.com/golang/groupcache and check out Brad's presentation about it: http://talks.golang.org/2013/oscon-dl.slide
InfluxDB which includes a Go based version of the great Raft algorithm: https://github.com/influxdb/influxdb
My own humble (pretty dead) project, a redis compliant database that's based on a plugin architecture. My Go has improved since, but it has some nice parts, and it includes a pretty fast server for the redis protocol. https://bitbucket.org/dvirsky/boilerdb
I am trying to pick a right web technology both for I/O heavy and CPU heavy tasks. NodeJs is perfect for handling large load and it also can be scaled out. However, I am stuck with the cpu heavy part. Is it possible to integrate another technology (e.g. Java) into node, so that I will have it running my algorithms in other threads and then use the results again in node. Is there any existing solution? Any other suggestions will be very good.
You can intergrate NodeJS with Java using node-java.
As mentioned in a previous answer, you can use node-java which is an npm module that talks to Java. You can also use J2V8 which wraps Node.js as a Java library and provides a Node.js API in Java.
The answer is lambda architecture.
NodeJs is nice by itself - handling fast queries in a lightweight manner, not doing any extra computations on data.
The CPU heavy tasks can be easily delegated to specialized components based on JVM (well, the most famous ones are on JVM). This is nicely implemented by using message brokers and microservices.
An event-based architecture, where nodejs can be hooked up to databases like Cassandra or Mongodb and cluster computing frameworks like Apache Spark (not necessarily, though, it depends on the problem) to handle the cpu-heavy parts of the system. And lightweight containers add an icing to the cake by providing nice isolated runtime environments for each of the components to live in.
That's my conclusion so far regarding this question.
I think the suggestions above sort of eliminate the need to wrap node under java or other JVM based solution for cpu-heavy tasks.
NodeJS is based on the v8 javascript engine which is written in c++.
It is therefore possible to write fully native addons in c++ for NodeJS. Check out some of these resources:
https://github.com/nodejs/node-addon-api
https://github.com/nodejs/node-addon-examples
I have a small cluster of Linux machines and an account on all of them.
I have ssh access to all of them with no-password login.
How can I use actors or other Scala's concurrency abstraction to achieve distribution ?
What is the simplest path?
Does some library can distribute the processes for me?
The computers are unreliable, they can go on and off wheter they (students) feel like it.
Does some library can distribute the processes for me and watch for ready computers?
Can I avoid bash scripts?
I'd use Akka in your place. It is a distributed computing platform for Scala and Java, based on Erlang's Actor model. In particular, the let-it-fail philosophy it inherits from Erlang is particularly well suited to an environment where the nodes might go off at any time.
You could use Scala's build in support for remote actors. Browse the web or see Scala remote actors for additional information.
You could also take a look at GridGain a very easy to use grid computing framework for Java and Scala.
What you are looking for is a grid.
For a free one take a look at http://www.jppf.org/
JavaSpaces allows you to create distributed Data Structures that facilitate Distributed Computing. Not cheap, but look at GigaSpaces for a robust implementation.
This book gave me an eye opening experience of the possibilities.
I was reading how Clojure is 'cool' because of its syntax + it runs on the JVM so it is multithreaded etc. etc.
Are languages like ruby and python single threaded in nature then? (when running as a web app).
What are the underlying differences between python/ruby and say java running on tomcat?
Doesn't the web server have a pool of threads to work with in all cases?
Both Python and Ruby have full support for multi-threading. There are some implementations (e.g. CPython, MRI, YARV) which cannot actually run threads in parallel, but that's a limitation of those specific implementations, not the language. This is similar to Java, where there are also some implementations which cannot run threads in parallel, but that doesn't mean that Java is single-threaded.
Note that in both cases there are lots of implementations which can run threads in parallel: PyPy, IronPython, Jython, IronRuby and JRuby are only few of the examples.
The main difference between Clojure on the one side and Python, Ruby, Java, C#, C++, C, PHP and pretty much every other mainstream and not-so-mainstream language on the other side is that Clojure has a sane concurrency model. All the other languages use threads, which we have known to be a bad concurrency model for at least 40 years. Clojure OTOH has a sane update model which allows it to not only present one but actually multiple sane concurrency models to the programmer: atomic updates, software transactional memory, asynchronous agents, concurrency-aware thread-local global variables, futures, promises, dataflow concurrency and in the future possibly even more.
A confused question with a lot of confused answers...
First, threading and concurrent execution are different things. Python supports threads just fine; it doesn't support concurrent execution in any real-world implementation. (In all serious implementations, only one VM thread can execute at a time; the many attempts to decouple VM threads have all failed.)
Second, this is irrelevant for web apps. You don't need Python backends to execute concurrently in the same process. You spawn separate processes for each backend, which can then each handle requests in parallel because they're not tied together at all.
Using threads for web backends is a bad idea. Why introduce the perils of threading--locking, race conditions, deadlocks--to something inherently embarrassingly parallel? It's much safer to tuck each backend away in its own isolated process, avoiding the potential for all of these problems.
(There are advantages to sharing memory space--it saves memory, by sharing static code--but that can be solved without threads.)
CPython has a Global Interpreter Lock which can reduce the performance of multi-threaded code in Python. The net effect, in some cases, is that threads can't actually run simultaneously because of locking contention. Not all Python implementations use a GIL so this may not apply to JPython, IronPython or other implementations.
The language itself does support threading and other asynchronous operations. The python libraries can also support threading internally without exposing it directly to the Python interpreter.
If you've heard anything negative about Python and threading (or that it doesn't support it), it is probably someone encountering a situation where the GIL is causing a bottleneck..
Certainly the webserver will have a pool of threads. That's only outside the control of your program. Those threads are used to handle HTTP requests. Each HTTP request is handled in a separate thread and the thread is released back to pool when the associated HTTP response is finished. If the webserver doesn't have such a pool, it would have been extremely slow in serving.
Whether a programming language is singlethreaded or multithreaded dependens on the possibility to programmatically spawn new threads using the language in question. If that isn't possible, then the language is singlethreaded, for example PHP. As far as I can see, both Ruby and Python supports multithreading.
The short answer is yes, they are single threaded.
The long answer is it depends.
JRuby is multithreaded and can be run in tomcat like other java code. MRI (default ruby) and Python both have a GIL (Global Interpreter Lock) and are thus single threaded.
The way it works for web servers is further complicated by the number of available server configurations. For most ruby applications there are (at least) two levels of servers, a proxy/static file server like nginx and then the ruby app server.
Nginx does not use threads like apache or tomcat, it uses non-blocking events (and I think forked worker processes). This allows it to deal with higher levels of concurrency than would be allowed with the overhead and scheduling inefficiencies of native threads.
The various ruby apps servers also work in different ways to get high throughput and concurrency without threads. Thin uses libev and the asynchronous evented model like Nginx. Mongrel uses a round-robin pool of worker processes. Unicorn uses native Unix IPC (select on a socket) to load balance to a pool of forked processes through one master proxy socket.
Threads are only one way to address concurrency. Multiple processes and evented models are a different approach that ties in well with the Unix base. This is fundamentally different from the way Java treats the world.
Python
Let me try to put it more simply than the more detailed answers.
The heart of the answer here doesn't really have to do with Python being single-threaded versus multi-threaded. It has a more to do with threading versus multiprocessing.
Saying Python is "single-threaded" doesn't really capture reality, because you can certainly have more than one thread running in a Python process. Just use the threading library, and create more than one thread. There, now you have just proven that Python isn't single-threaded.
But using multiple threads in Python does NOT mean you're using multiple CPU processors concurrently. In fact, the Global Interpreter Lock prevents this. So this is where questions arise.
Basically, threading in Python cannot be used for parallel CPU computation. But you CAN do parallel CPU computation with Python by using multiprocessing instead of multi-threading.
I found this article very helpful when researching this: https://timber.io/blog/multiprocessing-vs-multithreading-in-python-what-you-need-to-know/ . It includes real-world examples of when you'd want to use multiprocessing versus multi-threading.
Most languages don't define single or multithreading. Usually, that is left up to the libraries to implement.
That being said, some languages are better at it than others. CPython, for instance, has issues with interpreter locking during multithreading, Jython (python running on the JVM) does not.
Some of the real power of Clojure (IMO) is that it runs on the JVM. You get multithreading and tons of libraries for free.
A few interpreted programming
languages such as CPython and Ruby
support threading, but have a
limitation that is known as a Global
Interpreter Lock (GIL). The GIL is a
mutual exclusion lock held by the
interpreter that prevents the
interpreter from concurrently
interpreting the applications code on
two or more threads at the same time,
which effectively limits the
concurrency on multiple core systems.
from wikipedia Thread
keeping this very short..
Python supports Multi Threading.
Python does NOT support parallel execution of its Threads.
Exception:
Above statement may vary with implementations of Python not using GIL (Global Interpreter Locking).
If a particular implementation is not using GIL, then, that would be Multi Threaded as well as support Parallel Execution
Ruby
The Ruby Interpreter is single threaded, which is to say that several of its methods are not thread safe.
In the Rails world, this single-thread has mostly been pushed to the server. So, you'll see nginx running with a pool of mongrel servers, each of which has an interpreter in memory, processes 1 request at a time, and in its own thread.
Passenger, running "ruby enterprise" brings the concept of garbage collection and some thread safety into Rails, and it's nice.
Still work to be done in Rails on this area, but it's getting there slowly -- but in general, the idea is to have multiple services and servers.
How to untangle the knots in al those threads...
Clojure did not invent threading, however it has particularly strong support for it with Software Transactional Memory, Atoms, Agents, parallel map operations, ...
All other have accumulated threading support. Ruby is a special case as it has green threads in some implementations which are a kind of software emulated threads and do not use all the cores. 1.9 will put this to rest.
Regarding web servers, no they do not always work multithreaded, apache has traditionally ran as a flock of daemons which are a pool of separate single threaded processes. Now currently there are more options to run apache servers.
To summarize all modern languages support threading in one form or another.
The newer languages like scala and clojure are adding specific support to improve working with multiple threads without explicit locking as this has traditionally be the great pitfall of multithreading.
Reading these answers here... A lot of them try to sound smarter than they really are imho (im mostly talking about Ruby related stuff as thats the one i'm most familiar with).
In fact, JRuby is currently the only Ruby implementation that supports true concurrency. On JVM Ruby threads are mapped to OS native threads, without GIL interfering. So its totally correct to say that Ruby is not multithreaded.
In 1.8.x Ruby is actually run inside one OS thread, and while you do have the fake feeling of concurrency with green threads, then in reality GIL will pretty much prevent you from having true concurrency.
In Ruby 1.9 this changed a bit, as now a Ruby process can have many OS threads attached to it (plus the green threads), but again GIL will totally destroy the point and become the bottleneck.
In practice, from a regular webapp standpoint, it should not matter much if its single or multithreaded. The problem mostly arises on the server side anyhow and it mostly is a matter of scaling technique difference.
Yes Ruby and Python can handle multi-threading, but for many cases (web) is better to rely on the threads generated by the http requests from the client to the server. Even if you generate many threads on a same application to low the runtime cost or to handle many task at time, in a web application case that's usually too much time, no one will wait happily more than some fractions of a second for the response of your application in a single page, it's more wise to use AJAX (Asynchronous JavaScript And XML) techniques: make sure the design of your web shows up rapidly, and make an asynchronous insertion of those hard-coding things later.
That does not mean that multi-threading is useless for web! It's highly recommended to low the charge of your server if you want to run recursive-complicated-hardcore-applications (not for a website, I mean), but what that thing return must end in files or in databases, so then could be softly served by a http response.
I have a quadcore processor and I would really like to take advantage of all those cores when I'm running quick simulations. The problem is I'm only familiar with the small Linux cluster we have in the lab and I'm using Vista at home.
What sort of things do I want to look into for multicore programming with C or Java? What is the lingo that I want to google?
Thanks for the help.
The key word is "threading" - wouldn't work in a cluster, but it will be just fine in a single multicore machine (actually, on any kind of Windows, much better in general than spawning multiple processes -- Windows' processes are quite heavy-weight compared to Linux ones). Not quite that easy in C, very easy in Java -- for example, start here!
You want Threads and Actors
Good point ... you can't google for it unless you know some keywords.
C: google pthread, short for Posix Thread, although the win32 native interface is non-posix, see Creating Threads on MSDN.
Java: See class Thread
Finally, you should read up a bit on functional programming, actor concurrency, and immutable objects. It turns out that managing concurrency in plain old shared memory is quite difficult, but message passing and functional programming can allow you to use styles that are inherently much safer and avoid concurrency problems. Java does allow you to do everything the hard way, where data is mutable shared memory and you desperately try to manually interlock shared state structures. But you can also use an advanced style from within java. Perhaps start with this JavaWorld article: Actors on the JVM.
Check out this book: Java Concurrency in Practice
I think you should consider Clojure, too. It runs on the JVM and has good Java interoperability. As a Lisp, it's different from what you're used to with C and Java, so it might not be your cup of tea, but it's worth taking a look at the issues addressed by Clojure anyway, since the concepts are valuable regardless of what language you use. Check out this video, and then, if you're so inclined, the clojure site, which has links to some other good screencasts more specifically about Clojure in the upper right.
It depends on what your preferred language is to get the job done.
Besides the threading solutions, you may can also consider
MPI as a possibility from Java and C --- as well as from Python or R or whatever you like.
DeinoMPI appears to be popular on Windows, and OpenMPI just started with support for Windows too in the current release 1.3.3.
A lot of people have talked about threading, which is one approach, but consider another way of doing it. What if you had several JVM's started up, connected to the network, and waiting for work to come their way? How would you program an application so that it could leverage all of those JVMs without knowing whether or not they are on the same CPU?
On a quadcore machine, you should be able to run 12 or more JVMs to process work. And if you approach the problem from this angle, scaling up to multiple computers is fairly simple, although you do have to consider higher network latencies when your communication is across a real network.
Here is a good source of info on threading in C#.
You need to create multithreaded programs. Java supports multi-threading out of the box (though older JVMs ran all threads on one core). For C, you'll either need to use platform specific code to to create and manipulate threads (pthread* for Linux, CreateThread and company for Windows). Alternatively, you might want to do your threading from C++, where there are a fair number of libraries (e.g. Boost::threads) to make life a bit simpler and allow portable code.
If you want code that'll be portable across a single machine with multiple cores AND a cluster, you might look into MPI. It's really intended for the cluster situation, but has been ported to work on a single machine with multiple processors or multiple cores -- though it's not as efficient as code written specifically for a single machine.
So, that's a very broad question. You can experiment with multithreaded programming using many different programming languages including C or Java. If you wanted me to pick one for you, then I'd pick C. :)
You want to look into Windows threads, POSIX threads (or multithreading in Java, if that's language). You might want to try to find some problems to experiment with. I'd suggest trying out matrix multiplication; start with a sequential version and then try to improve the time using threads.
Also, OpenMP is available for Windows and offers a much different view of how to multithreaded programming.
Even though you asked specifically for C or Java, Erlang isn't a bad choice of language if this is just a learning exercise
It allows you to do multiprocess style programming very very easily and it has a large set of libraries that let you dive in at just about any level you like.
It was built for distributed programming in a very pragmatic way. If you are comfortable with java, the transition shouldn't be too difficult.
If you are interested, I would recommend the book, "Programming Erlang" by Joe Armstrong.
(as a note: there are other languages designed to run on in highly parallel environments like Haskell. Erlang just tends to be more pragmatic than languages like Haskell which are rooted more in theory)
If you want to do easy threading, such as parallel loops, I recommend check out .NET 4.0 Beta (C# in VS2010 Beta).
The book chapter Joe linked to is a very good one I use myself and highly recommend, but doesn't cover the new parallel extensions to the .NET framework.
yes , many threads , but if the threads are accessing the same position in the memory only one thread will execute,
we need multi memory cores
By far the easiest way to do multicore programming on Windows is to use .NET 4 and C# or F#. Here is a simple example where a parallel program (from the shootout) is 7× shorter in F# than Java and just as fast.
.NET 4 provides a lot of new infrastructure for parallel programming and it is really easy to use.
That you say "take advantage" sounds to me as something more than doing just any multi-threading. Simulations in my book are computation-intensive and in that respect the most efficient language is C. Some would say assembly but there are very few x86 assembly programmers who can beat a modern C compiler.
For the Windows NT engine (NT4, 2000, XP, Vista and 7) the mechanisms you should look into are threads, critical sections and I/O completion ports (iocp). Threads are nice but you need to be able to synchronize them among themselves and with I/O which is where cs's and iocps come in. To make sure your wringing every last bit of performance out of your code you need to profile, analyze, experiment/re-construct. Lots of fun but very time-consuming.
Multiple threads can exist in a single process. The threads that belong to the same process share the same memory area (can read from and write to the very same variables, and can interfere with one another). On the contrary, different processes live in different memory areas, and each of them has its own variables. In order to communicate, processes have to use other channels (files, pipes or sockets).
If you want to parallelize a computation, you're probably going to need multithreading, because you probably want the threads to cooperate on the same memory.
Speaking about performance, threads are faster to create and manage than processes (because the OS doesn't need to allocate a whole new virtual memory area), and inter-thread communication is usually faster than inter-process communication. But threads are harder to program. Threads can interfere with one another, and can write to each other's memory, but the way this happens is not always obvious (due to several factors, mainly instruction reordering and memory caching), and so you are going to need synchronization primitives to control access to your variables.
Taken from this answer.