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• Enumerating combinations in a distributed manner

  - by Reyzooti

  I have a problem where I must analyse 500C5 combinations (255244687600) of something. Distributing it over a 10 node cluster where each cluster processes roughly 10^6 combinations per second means the job will be complete in about 7hours. The problem I have is distributing the 255244687600 combinations over the 10 nodes. I'd like to present each node with 25524468760, however the algorithms I'm using can only produce the combinations sequentially, I'd like to be able to pass the set of elements and a range of combination indicies eg: [0-10^7) or [10^7,2.0 10^7) etc and have the nodes themselves figure out the combinations. The algorithms I'm using at the moment are from the following: http://home.roadrunner.com/~hinnant/combinations.html A logical question I've considered using a master node, that enumerates each of the combinations and sends work to each of the nodes, however the overhead incurred in iterating the combinations from a single node and communicating back and forth work is enormous, and will subsequently lead to the master node becoming the bottleneck. Are there any good combination iterating algorithms geared up for efficient/optimal distributed enumeration?

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• C++ Virtual keyword as away to avoid including code units.

  - by user394663

  Currently we have a body of code that allows service plugins which offer forms of communication to the core e.g tcp/ip, udp/ip, usb, etc... These service plugins feedback notifier class instances to the core for further processing. In the current implementation a service project (which is a separate dynamically linked library brought in at runtime by the core via dlopen and friends) will compile against the notifier.cpp file that resides in the core source code (a separate project). This gives access to the notifiers method implementations. This works perfectly no complaints. Two alternative options: 1. put the notifier method implementations in the header file. 2. declare the notifier methods virtual and delay binding until runtime. Avoiding the issues of computational overhead what are the impacts of option 2? Are there any other options available to us? Thanks

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• Random numbers from binomial distribution

  - by Sarah

  I need to generate quickly lots of random numbers from binomial distributions for dramatically different trial sizes (most, however, will be small). I was hoping not to have to code an algorithm by hand (see, e.g., this related discussion from November), because I'm a novice programmer and don't like reinventing wheels. It appears Boost does not supply a generator for binomially distributed variates, but TR1 and GSL do. Is there a good reason to choose one over the other, or is it better that I write something customized to my situation? I don't know if this makes sense, but I'll alternate between generating numbers from uniform distributions and binomial distributions throughout the program, and I'd like for them to share the same seed and to minimize overhead. I'd love some advice or examples for what I should be considering.

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• How to optimize indexing of large number of DB records using Zend_Lucene and Zend_Paginator

  - by jdichev

  So I have this cron script that is deployed and ran using Cron on a host and indexes all the records in a database table - the index is later used both for the front end of the site and the backed operations as well. After the operation, the index is about 3-4 MB. The problem is it takes a lot of resources (CPU: 30+ and a good chunk of memory) and slows the machine down. My question is about how to optimize the operation described below: First there is a select query built using the Zend Framework API, this query is then passed to a Paginator factory that returns a paginator which I am using to balance the current number of items being indexed and not iterate over too much items. The script is iterating over the current items in the paginator object using a foreach loop until reaching the end and then it starts from the beginning after getting items for the next page. I am suspecting this overhead is caused by the Zend_Lucene but no idea how this could be improved.

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• Best way to reuse a Runnable

  - by Gandalf

  I have a class that implements Runnable and am currently using an Executor as my thread pool to run tasks (indexing documents into Lucene). executor.execute(new LuceneDocIndexer(doc, writer)); My issue is that my Runnable class creates many Lucene Field objects and I would rather reuse them then create new ones every call. What's the best way to reuse these objects (Field objects are not thread safe so I cannot simple make them static) - should I create my own ThreadFactory? I notice that after a while the program starts to degrade drastically and the only thing I can think of is it's GC overhead. I am currently trying to profile the project to be sure this is even an issue - but for now lets just assume it is.

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• Converting delimited string to multiple values in mysql

  - by epo

  I have a mysql legacy table which contains an client identifier and a list of items, the latter as a comma-delimited string. E.g. "xyz001", "foo,bar,baz". This is legacy stuff and the user insists on being able to edit a comma delimited string. They now have a requirement for a report table with the above broken into separate rows, e.g. "xyz001", "foo" "xyz001", "bar" "xyz001", "baz" Breaking the string into substrings is easily doable and I have written a procedure to do this by creating a separate table, but that requires triggers to deal with deletes, updates and inserts. This query is required rarely (say once a month) but has to be absolutely up to date when it is run, so e.g. the overhead of triggers is not warranted and scheduled tasks to create the table might not be timely enough. Is there any way to write a function to return a table or a set so that I can join the identifier with the individual items on demand?

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• 2D Game: Fast(est) way to find x closest entities for another entity - huge amount of entities, high

  - by Pygmy

  I'm working on a 2D game that has a huge amount of dynamic entities. For fun's sake, let's call them soldiers, and let's say there are 50000 of them (which I just randomly thought up, it might be much more or much less :)). All these soldiers are moving every frame according to rules - think boids / flocking / steering behaviour. For each soldier, to update it's movement I need the X soldiers that are closest to the one I'm processing. What would be the best spatial hierarchy to store them to facilitate calculations like this without too much overhead ? (All entities are updated/moved every frame, so it has to handle dynamic entities very well)

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• How can I add floats together in different orders, and always get the same total?

  - by splicer

  Let's say I have three 32-bit floating point values, a, b, and c, such that (a + b) + c != a + (b + c). Is there summation algorithm, perhaps similar to Kahan summation, that guarantees that these values can be summed in any order and always arrive at the exact same (fairly accurate) total? I'm looking for the general case (i.e. not a solution that only deals with 3 numbers). Is arbitrary precision arithmetic the only way to go? I'm dealing with very large data sets, so I'd like to avoid the overhead of using arbitrary precision arithmetic if possible. Thanks!

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• Adding a Jar to ext and using it in Eclipse

  - by Bob Breznak

  I am providing a client with a lab station image for a school program. There is a library that I'd like to directly add to the JRE so that students can use the library without needing to fiddle with adding the classpath. As this is a basic after school program, the goal is to just get students into programming with as little overhead additions as possible. At this point, I have the lib put into the jre/lib/ext and Eclipse is able to find it, however it will not allow access to any of the classes. I see it in the JRE System Library and the classes and packages are showing up there but when I go to use it, I am getting an "Access restriction" error. When I remove the JRE System Library then add it back in, everything works swimmingly. The library is accessible exactly as intended. Any ideas on how to resolve this?

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• Forward a call to a webservice to another webservice?

  - by Luhmann

  If I have an https webservice behind a firewall on a machine (A) that I cannot access, but access to a machine on the same network (B), from where I can call the webservice on machine A. What is the best way of talking with the webservice on machine A, from the outside via machine B (that I access via VPN)? I can obviously create a service with a matching interface on machine B, and call the methods on the webservice on machine A, and return the result. But I fear for the overhead. Is there another way? Can i somehow forward the request?

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• Unsafe, super-fast cross-process memory buffer?

  - by John

  Cross-process memory buffers always have some overhead, and my understanding is this is quite high. But what if you're implementing a cross-process render-buffer, this isn't critically important in the same way as other data so are there techniques we can use to get 'raw' access to a chunk of memory from multiple processes, with no safety nets apart from it not crashing? Or do modern operating systems simply not work with unabstracted memory in a way to make this possible? I'm working in C++ but the question applies to Win XP/Vista/7, MacOSX 10.5+ (& Linux less importantly).

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• Struggling to "clear" a CGLayer -- can it even be done?

  - by Joe Blow

  So I'm doing this repetitively - making a CGLayer, doing some processing, and then releasing it. This happens a lot in real time -- so surely there is a lot of overhead in making a whole new CGLayer each time? Surely it would be better to just keep lair around, and start fresh each time? However, I do not know any way, at all, to "erase" or "start from blank" a CGLayer?? Can anyone help on this? There is a function CGContextBeginPath(cc) but it's confusing: it seems to only clear out "that" path, it does not erase all of the CGLayer back to a blank canvas. how to return a CGLayer to a blank canvas????? Does anyone know? CGLayerRef lair = CGLayerCreateWithContext( UIGraphicsGetCurrentContext(), CGSizeMake(1024,768), NULL); CGContextRef cc = CGLayerGetContext(ether); // various processing here CGContextAddPath(cc, somePath); // various processing here CGLayerRelease(lair); Any ideas?!

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• Are spinlocks a good choice for a memory allocator?

  - by dsimcha

  I've suggested to the maintainers of the D programming language runtime a few times that the memory allocator/garbage collector should use spinlocks instead of regular OS critical sections. This hasn't really caught on. Here are the reasons I think spinlocks would be better: At least in synthetic benchmarks that I did, it's several times faster than OS critical sections when there's contention for the memory allocator/GC lock. Edit: Empirically, using spinlocks didn't even have measurable overhead in a single-core environment, probably because locks need to be held for such a short period of time in a memory allocator. Memory allocations and similar operations usually take a small fraction of a timeslice, and even a small fraction of the time a context switch takes, making it silly to context switch in the case of contention. A garbage collection in the implementation in question stops the world anyhow. There won't be any spinning during a collection. Are there any good reasons not to use spinlocks in a memory allocator/garbage collector implementation?

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• Unity and web service

  - by zachary

  I had this awesome idea... but I am afraid maybe it is actually a bad idea.... we use unity for dependency injection. I make interfaces from my web services using partial classes for the purpose of mocking and web services.... What I want to do is put my web services into unity and get them via dependency injection... What do you think? Is there too much overhead somewhere? Memory leaks? Is this a bad idea?

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• sql unite fields to one result

  - by none

  i know this is a "not build in" or "the way dba thinks" but a programmer aproach , How could one request from 3 fields to get the one that is not null, into a result filed. lets say we have a table with f1,f2,f3,f4,f5. lets say f2,f3,f4 are the same type. lets say the content of the table be tupples of (key1,null,null,value1,value2) (key2,null,value3,value4,value5) (key3,null,null,null,value6) now if we return the first tupple then we get (key1) we get (key1,value1,value2) if we ask for key2 we get (key1,value3,value5) if we ask for key3 we get (key1,null,value6) how is it possible to get the fields in the priority of if you have value in f2, then its set into the returned field, only then if we have value in f3 then its set into the middle returned field, only then if we have value in f4 then its set into the middle returned field the main goal is to get the result into a sigel feild and prevent the overhead work needed at the result end.

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• How do I make "simple" throughput j2ee-filter?

  - by Tommy

  I'm looking to create a filter that can give me two things: number of request pr minute, and average responsetime pr minute. I already got the individual readings, I'm just not sure how to add them up. My filter captures every request, and it records the time each request takes: public void doFilter(ServletRequest request, ...() { long start = System.currentTimeMillis(); chain.doFilter(request, response); long stop = System.currentTimeMillis(); String time = Util.getTimeDifferenceInSec(start, stop); } This information will be used to create some pretty Google Chart charts. I don't want to store the data in any database. Just a way to get current numbers out when requested As this is a high volume application; low overhead is essential. I'm assuming my applicationserver doesn't provide this information.

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• C++ performance when accessing class members

  - by Dr. Acula

  I'm writing something performance-critical and wanted to know if it could make a difference if I use: int test( int a, int b, int c ) { // Do millions of calculations with a, b, c } or class myStorage { public: int a, b, c; }; int test( myStorage values ) { // Do millions of calculations with values.a, values.b, values.c } Does this basically result in similar code? Is there an extra overhead of accessing the class members? I'm sure that this is clear to an expert in C++ so I won't try and write an unrealistic benchmark for it right now

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• what's the best (most effecient) way in asp .net to return a whole page into tabbed content?

  - by ijjo

  what i want to do is every time i click on a tab, the content area is replaced by pretty much a whole new page. i don't want a full page load so i want to do it in ajax, but i'm used to sending back small jason data via page methods. i'm not sure how i would construct a whole new page and return that via ajax and i would like to simply assign the whole content returned to a div and be done with it. what's the best way to do this with the least amount of overhead (i know there are some inefficient ways the scriptmanager does ajax)? or is it better to load the tabbed content in an iframe? fyi i'm already using jquery to call lightweight pagemethods on my asp net page and that works great.

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• How to permanently prevent specific part of a file from being committed in git?

  - by boutta

  I have cloned a remote SVN repository with git-svn. I have modified a pom.xml file in this cloned repo in a way that the code compiles. This setup is exclusive for me. Thus I don't want to push the changes back on the remote repo. Is there a way to prevent this (partial) change of a file from being committed into the repo? I'm aware of the fact, that I could use a personal branch, but this would mean certain merging overhead. Are there other ways? I've looked into this question and this one, but they are for rather temporal changes. Update: I'm also aware of the .gitignore possibilities, but this would mean to exclude the file completely.

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• Giving users a "reputation system" - Should I... ?

  - by RadiantHex

  Hi folks, I'm thinking of adding a reputation system to a web application, the site is already being used so I'm trying to be careful about my choices. I'm developing in Django/Python, thought this would be important. Reputation is generated in all actions that contribute to the site, similar to Stackoverflow's system. I know there are literally millions of ways of implementing this, and this is why I feel quite lost. Two alternatives I am not sure about are: Keep track of reasons why reputation was incremented Ignore reasons in order to reduce complexity of the site and overhead Would be happy with a few pointers, and directions. Would be very much appreciated!

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• Are there design-time watch windows for Visual Studio 2008/2010?

  - by Jeff

  There are many times when I need to test a little snippet of .net code but rebuilding and publishing the entire project or writing a suite of unit tests just seems like overkill. For example, I am writing a regular expression right now and I want to see if it the pattern is matching on the right parts. I could go and find a million other utilities that do that sort of thing, but that is not exactly my point. FireBug has an exact analogue to what I want - the FireBug console. There is a text box where the user can enter some JavaScript and FireBug will execute it on the spot and display the return value. I would love to be able to enter something like (new Regex("b+")).Replace("abc", "x") and see the results without having to do all the overhead. Does VS have anything like this?

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• jquery filtering content before specific string

  - by jeff

  Is is possible remove all content before a specific string with jquery? For instance say I wanted to strip all the text before the word "subcommittee" in this example below. How would I do that? With losses mounting among hoteliers, fishermen and others whose livelihoods have been curtailed by the spill, frustration is "rapidly escalating" along the Gulf Coast alive." Linn told a House Energy and Commerce subcommittee Monday that the amount of money BP has paid local residents for their losses has typically been about $5,000, a sum he dismissed as "a marketing ploy." Businesses such as his vacation rental company are borrowing money to pay their overhead costs, which he called "the only way we're going to keep our business alive."

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• Performance when accessing class members

  - by Dr. Acula

  I'm writing something performance-critical and wanted to know if it could make a difference if I use: int test( int a, int b, int c ) { // Do millions of calculations with a, b, c } or class myStorage { public: int a, b, c; }; int test( myStorage values ) { // Do millions of calculations with values.a, values.b, values.c } Does this basically result in similar code? Is there an extra overhead of accessing the class members? I'm sure that this is clear to an expert in C++ so I won't try and write an unrealistic benchmark for it right now

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• Memcached - how to deal with adding/deploying servers

  - by Industrial

  Hi everybody, How do you handle replacing/adding/removing memcached nodes in your production applications? I will have a number of applications that are cloned and customized due to each customers need running on one and same webserver, so i'll guess that there will be a day when some of the nodes will be changed. Here's how memcached is populated by normal: $m = new Memcached(); $servers = array( array('mem1.domain.com', 11211, 33), array('mem2.domain.com', 11211, 67) ); $m->addServers($servers); My initial idea, is to make the $servers array to be populated from the database, also cached, but file-based, done once a day or something, with the option to force an update on next run of the function that holds the $addservers call. However, I am guessing that this might add some additional overhead since disks are quite slow storage... What do you think?

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• Username correct, password incorrect?

  - by jonnnnnnnnnie

  In a login system, how can you tell if the user has entered the password incorrectly? Do you perform two SQL queries, one to find the username, and then one to find the username and matching (salted+hashed etc) password? I'm asking this because If the user entered the password incorrectly, I want to update the failed_login_attempts column I have. If you perform two queries wouldn't that increase overhead? If you did a query like this, how would you tell if the password entered was correct or not, or whether the username doesn't exist: SELECT * FROM author WHERE username = '$username' AND password = '$password' LIMIT 1 ( ^ NB: I'm keeping it simple, will use hash and salt, and will sanitize input in real one.) Something like this: $user = perform_Query() // get username and password? if ($user['username'] == $username && $user['password'] == $password) { return $user; } elseif($user['username'] == $username && $user['password'] !== $password) { // here the password doesn't match // update failed_login_attemps += 1 }

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