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Java Forum / General / February 2006Taking advantage of clusters or multi CPUs?
Hello, I am just wondering that how can we make sure a java program can take advantage of those cluster machines, or those with multi CPUs? ------ I mean, besides write our code in a "proper" way, do we need to "explicitly" add something into the code or tell something to the compiler or JVM? Any guide line for coding with multi CPUs or cluster machines? Thanks! > Hello, > [quoted text clipped - 7 lines] > > Thanks! I asked this question a few weeks ago. I'm currently working on a big project that has several large multi-threaded apps. We were concerned about getting enough computer to make the project lively enough. We ended up buying single board computers from Crystal Group. They have two Intel 2.8Ghz Xeon dual core processors. They are really fast. But back to the multi-threaded issue. I asked about that and the tech advisers at Crystal said that the OS (in this case XP Pro) should take care of the multi-threading issues. When we run our program and look at the task manager, it shows four processors. The processor load is fairly well balanced between all four. Sometime if we don't load it up, it will show two of the processors with load and the other two not using much. My programs are written using normal multi-threading code. I didn't have to change anything that I wrote. Hope that helps some :-).  SignatureKnute Johnson email s/nospam/knute/ Thanks for that. I think that is good as far as to multi threading. How about single threading? For example, if my program has a thread that do a lot of computations, and these computations do not affect each other. Say, the thread is scanning through a list of records, and do some computation within each record. One way is of course to use many threads to do this. But if I just put them in one loop in one thread, will JVM and its compiler be "smart" enough to realize it and use multi CPUs to do them in parallel? The reason I am asking is that, as I remembered years back on my compiler course in school, they said good compilers can know certain codes can be paralleled, and the complier will tune the code automatically. Is that available in today's common compliers (and languages) such as in java? Hum, my question may be some what off the topic for a Java group though. :-) > The reason I am asking is that, as I remembered years back on my > compiler course in school, they said good compilers can know certain > codes can be paralleled, and the complier will tune the code > automatically. Is that available in today's common compliers (and > languages) such as in java? Not Java. There are some special-purpose compilers for languages like Fortran than /can/ do that kind of thing, but they still need plenty of help from the programmer to do it /well/. There are languages designed from the ground up to parallelise too. -- chris >One way is of course to use many threads to do this. But if I just put >them in one loop in one thread, will JVM and its compiler be "smart" >enough to realize it and use multi CPUs to do them in parallel? no. There is a fair overhead to creating a thread and starting it up. You can't do it just for tiny bits of code. Further as soon as you introduce threads you introduce terrible uncertainty as they meddle with each other's variables. It requires a much higher level of skill. It is still far from being automatic. You want your threads to have ZERO interaction ideally. See http://mindprod.com/jgloss/queue.html for stereotyped ways of interacting that have been coded by the gods. Rolling you own interactions is very difficult to get 100%. Getting it 90% is easy. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > Thanks for that. I think that is good as far as to multi threading. > [quoted text clipped - 6 lines] > them in one loop in one thread, will JVM and its compiler be "smart" > enough to realize it and use multi CPUs to do them in parallel? No. > The reason I am asking is that, as I remembered years back on my > compiler course in school, they said good compilers can know certain > codes can be paralleled, and the complier will tune the code > automatically. Is that available in today's common compliers (and > languages) such as in java? Normally, only in very expensive parallelising compliers. Even then there are only some very specific types of code which can be automatically parallelised, it normally requires specific parallel constructs to be used in the code (my only knowledge of parallelising compilers is for FORTRAN, it may be different for other languages). It's also quite easy to write code which takes longer to execute on multiple processors than it does on one... SignatureNigel Wade, System Administrator, Space Plasma Physics Group, University of Leicester, Leicester, LE1 7RH, UK E-mail : nmw@ion.le.ac.uk Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555 > How about single threading? For example, if my program has a thread > that do a lot of computations, and these computations do not affect [quoted text clipped - 14 lines] > though. > :-) A lot of people have been saying "No, there's no such thing." I also acknowledge that what you're asking for is difficult enough that I would be "surprised" to discover a compiler which can automatically insert threading when appropriate, but I don't presume to know of all the latest and greatest in compiler advancements. You might want to redirect your question to comp.compilers. - Oliver Kevin sez: > Thanks for that. I think that is good as far as to multi threading. > > How about single threading? For example, if my program has a thread > that do a lot of computations, and these computations do not affect > each other. Say, the thread is scanning through a list of records, and > do some computation within each record. There 2 kinds of parallel: single-instruction-multiple-data and multiple-instructions-multiple-data, they require different kinds of hardware. What you're talking about is SIMD (same computation on different records) and unless you happen to have a MasPar in your basement, you're SOL. These things come with specialized languages (or at least specialized C dialects) and specialized programming techiniques (e.g. "wavefront"). Your regular SMP machines and multi-core CPUs are MIMD, designed for multiple threads/processes, not for SIMD tasks. You could potentially do SIMD on them if you could bypass everything including on-chip instruction pipeline & parallelizer and get direct access to chip's cores. Definitely not in Java, for obvious reasons. > One way is of course to use many threads to do this. But if I just put > them in one loop in one thread, will JVM and its compiler be "smart" > enough to realize it and use multi CPUs to do them in parallel? Nope. > The reason I am asking is that, as I remembered years back on my > compiler course in school, they said good compilers can know certain > codes can be paralleled, and the complier will tune the code > automatically. Is that available in today's common compliers (and > languages) such as in java? IIRC Ada has "parbegin ... parend" blocks, allowing the programmer to mark code segments for parallel execution. They may become common if IBM's new playstation chip takes off (PPC "dispatcher" core + a bunch of SIMD "processing" cores). Then come parallelizing compilers... Dima SignatureThe most horrifying thing about Unix is that, no matter how many times you hit yourself over the head with it, you never quite manage to lose consciousness. It just goes on and on. -- Patrick Sobalvarro > if you could bypass everything >including on-chip instruction pipeline & parallelizer and get >direct access to chip's cores. Definitely not in Java, for >obvious reasons. most chips nowadays come with one or more specialised auxiliary processors, DSPs which are a sort of miniature array processor. These are primarily intended for video or audio processing. You might see Java using them, but only a in a native method. The most common place you see them used are in CODECs. see http://mindprod.com/jgloss/dsp.html SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. >I am just wondering that how can we make sure a java program can take >advantage of those cluster machines, or those with multi CPUs? ------ I >mean, besides write our code in a "proper" way, do we need to >"explicitly" add something into the code or tell something to the >compiler or JVM? There are two kinds of parallelism, pipelines when one CPU processing an instruction stream slightly out of order and in parallel, but logically it all works as if it had done it in order. You don't have to do anything but buy an expensive CPU to get this benefit. The other kind is like the Sun Niagara servers where they have 8 logically separate CPUs on single chip. The share some logic, but from your point of view they process 8 instructions streams fully simultaneously. This also applies to hyperthreading chips that simulate two CPUS with a great deal of shared logic. For these to buy you anything you must either: 1. have other tasks running the background outside your JVM. They may be non-java or other Java programs. Obviously you need lots of RAM for hem. 2. you use threads and those threads done just lie around waiting for each other. They are busy computing or doing i/o most of the time. Imagine for example a HTML link checker. A multithreaded one could be checking 30 links at a time. With a single thread it could only check one at a time. See http://mindprod.com/projects/htmlbrokenlink.html If you had 5 cpus, each could handle 6 threads on average. Threads are not dedicated to a CPU. A CPU, when it becomes free, just takes the next thread ready to continue executing. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > Hello, > [quoted text clipped - 7 lines] > > Thanks! Java will use multiple processors in SMP systems, as permitted by the underlying OS. It can't use other nodes in a cluster which typically requires special software which isn't built into Java. SignatureNigel Wade, System Administrator, Space Plasma Physics Group, University of Leicester, Leicester, LE1 7RH, UK E-mail : nmw@ion.le.ac.uk Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555 Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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