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• Advantage of using Thread.Start vs QueueUserWorkItem

  - by Cheeso

  In multithreaded .NET programming, what are the decision criteria for using ThreadPool.QueueUserWorkItem versus starting my own thread via new Thread() and Thread.Start()? In a server app (let's say, an ASP.NET app or a WCF service) I think the ThreadPool is always there and available. What about in a client app, like a WinForms or WPF app? Is there a cost to spin up the thread pool? If I just want 3 or 4 threads to work for a short period on some computation, is it better to QUWI or to Thread.Start().

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• .NET Regular expressions on bytes instead of chars

  - by brickner

  Hi, I'm trying to do some parsing that will be easier using regular expressions. The input is an array (or enumeration) of bytes. I don't want to convert the bytes to chars for the following reasons: Computation efficiency Memory consumption efficiency Some non-printable bytes might be complex to convert to chars. Not all the bytes are printable. So I can't use Regex. The only solution I know, is using Boost.Regex (which works on bytes - C chars), but this is a C++ library that wrapping using C++/CLI will take considerable work. How can I use regular expressions on bytes in .NET directly, without working with .NET strings and chars? Thank you.

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• Efficiency of while(true) ServerSocket Listen

  - by Submerged

  I am wondering if a typical while(true) ServerSocket listen loop takes an entire core to wait and accept a client connection (Even when implementing runnable and using Thread .start()) I am implementing a type of distributed computing cluster and each computer needs every core it has for computation. A Master node needs to communicate with these computers (invoking static methods that modify the algorithm's functioning). The reason I need to use sockets is due to the cross platform / cross language capabilities. In some cases, PHP will be invoking these java static methods. I used a java profiler (YourKit) and I can see my running ServerSocket listen thread and it never sleeps and it's always running. Is there a better approach to do what I want? Or, will the performance hit be negligible? Please, feel free to offer any suggestion if you can think of a better way (I've tried RMI, but it isn't supported cross-language. Thanks everyone

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• Why better isolation level means better performance in SQL Server

  - by Oleg Zhylin

  When measuring performance on my query I came up with a dependency between isolation level and elapsed time that was surprising to me READUNCOMMITTED - 409024 READCOMMITTED - 368021 REPEATABLEREAD - 358019 SERIALIZABLE - 348019 Left column is table hint, and the right column is elapsed time in microseconds (sys.dm_exec_query_stats.total_elapsed_time). Why better isolation level gives better performance? This is a development machine and no concurrency whatsoever happens. I would expect READUNCOMMITTED to be the fasted due to less locking overhead. Update: I did measure this with DBCC DROPCLEANBUFFERS DBCC FREEPROCCACHE issued and Profiler confirms there're no cache hits happening. Update2: The query in question is an OLAP one and we need to run it as fast as possible. Closing the production server from outside world to get the computation done is not out of question if this gives performance benefits.

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• Has anyone ever encountered a Monad Transformer in the wild?

  - by martingw

  In my area of business - back office IT for a financial institution - it is very common for a software component to carry a global configuration around, to log it's progress, to have some kind of error handling / computation short circuit... Things that can be modelled nicely by Reader-, Writer-, Maybe-monads and the like in Haskell and composed together with monad transformers. But there seem to some drawbacks: The concept behind monad transformers is quite tricky and hard to understand, monad transformers lead to very complex type signatures, and they inflict some performance penalty. So I'm wondering: Are monad transformers best practice when dealing with those common tasks mentioned above?

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• alt-tab sort of web design

  - by sureshgl

  I am thinking of designing a web site having multiple related services. For every action of the user in a service there will be some computation going on in each of the other services. I want to display the service in action (chosen by the user) in the middle of the page in enlarged mode and rest of the services as small sized (shrunk) versions around the enlarged mode. The services which are shown in the shrunk version, should actually show what is happening in that service, at run time as if that was the chosen service. A close match to this sort of behavior I know is in Reliance BigTV, where all the small images of all the channels will be going on, and we can choose the one that we want to watch. After choosing that one image will become big and occupy the screen. Please, let me know if I can do some thing like this using css, html, ajax & php.

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• Recommend an algorithms exercise book?

  - by Parappa

  I have a little book called Problems on Algorithms by Ian Parberry which is chock full of exercises related to the study of algorithms. Can anybody recommend similar books? What I am not looking for are recommendations of good books related to algorithms or the theory of computation. Introduction to Algorithms is a good one, and of course there's the Knuth stuff. Ideally I want to know of any books that are light on instructional material and heavy on sample problems. In a nutshell, exercise books. Preferably dedicated to algorithms rather than general logic or other math problems. By the way, the Parberry book does not seem to be in print, but it is available as a PDF dowload.

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• What to know - for building a form with PHP and MySQL

  - by Tal Galili

  Hello all, I have three weeks to perform the following basic task. I wish to create an online form in PHP that will get 5 numbers and with them: save the numbers to a MySQL DB do a simple computation on the numbers and return them as an output to the visitor I already have a domain, hosting and a MySQL database+user. I don't know PHP, or SQL or databases. I do have prior knowledge in C and R (which means I have some of the basics of programming, and I am not intimidated of a new languages). My question are: Am I missing something? is there some other skill I should have for this task that I don't know of? What is a good way to learn the skills needed for the task I described? Hope the question isn't too basic/general. Thanks!

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• Haskell IO Passes to Another Function

  - by peterwkc

  This question here is related to http://stackoverflow.com/questions/3066956/haskell-input-return-tuple I wonder how we can passes the input from monad IO to another function in order to do some computation. Actually what i want is something like -- First Example test = savefile investinput -- Second Example maxinvest :: a maxinvest = liftM maximuminvest maxinvestinput maxinvestinput :: IO() maxinvestinput = do str <- readFile "C:\\Invest.txt" let cont = words str let mytuple = converttuple cont let myint = getint mytuple putStrLn "" -- Convert to Tuple converttuple :: [String] -> [(String, Integer)] converttuple [] = [] converttuple (x:y:z) = (x, read y):converttuple z -- Get Integer getint :: [(String, Integer)] -> [Integer] getint [] = [] getint (x:xs) = snd (x) : getint xs -- Search Maximum Invest maximuminvest :: (Ord a) => [a] -> a maximuminvest [] = error "Empty Invest Amount List" maximuminvest [x] = x maximuminvest (x:xs) | x > maxTail = x | otherwise = maxTail where maxTail = maximuminvest xs In the second example, the maxinvestinput is read from file and convert the data to the type maximuminvest expected. Please help. Thanks.

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• Python beginner, strange output problem

  - by Protean

  I'm having a weird problem with the following piece of code. from math import sqrt def Permute(array): result1 = [] result2 = [] if len(array) <= 1: return array for subarray in Permute(array[1:]): for i in range(len(array)): temp1 = subarray[:i]+array[0]+subarray[i:] temp2 = [0] for num in range(len(array)-1): temp2[0] += (sqrt(pow((temp1[num+1][1][0]-temp1[num][1][0]),2) + pow((temp1[num+1][1][1]-temp1[num][1][1]),2))) result1.append(temp1+temp2) return result1 a = [['A',[50,1]]] b = [['B',[1,1]]] c = [['C',[100,1]]] array = [a,b,c] result1 = Permute(array) for i in range(len(result1)): print (result1[i]) print (len(result1)) What it does is find all the permutations of the points abc and then returns them along with the sum of the distances between each ordered point. It does this; however, it also seems to report a strange additional value, 99. I figure that the 99 is coming from the computation of the distance between point a and c but I don't understand why it is appearing in the final output as it does.

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• Limiting the maximum number of concurrent requests django/apache

  - by Johan

  Hi, I have a django site that demonstrates the usage of a tool. One of my views takes a file as input and runs some fairly heavy computation trough an external python script and returns some output to the user. The tool runs fast enough to return the output in the same request though. I would however want to limit how many concurrent requests to this URL/view to keep the server from getting congested. Any tips on how i would go about doing this? The page in itself is very simple and the usage will be low.

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• Provide me with recources on Greibach & Chomsky Normal Form

  - by kristian Roger

  Hi, first please (bare with me)... :-( Im having a course at University ( Theory of Computation ) at first it was easy and simple BUT then we reach the context free grammer specialy the part where we should convert a grammer to Normal forms (Greibach & Chomsky) the thing that I couldnt understand so as usual I went to google and start searching for tutorials or videos I found many(tutorials not videos) but the problem that in tutorials they always explain things as if Im an expert or aware of every thing ... so can anyone please provide me with docs or links where they explaine the methods step by step ... (Please dont tell me to go back to my instructor because if he is useful I wont be asking your help ) thanx in advance

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• Avoid copying of data between user and kernel space and vice-versa

  - by bala1486

  Hello, I am developing a active messaging protocol for parallel computation that replaces TCP/IP. My goal is to decrease the latency of a packet. Since the environment is a LAN, i can replace TCP/IP with simpler protocol to reduce the packet latency. I am not writing any device driver and i am just trying to replace the TCP/IP stack with something simpler. Now I wanted to avoid copying of a packet's data from user space to kernel space and vice-versa. I heard of the mmap(). Is it the best way to do this? If yes, it will be nice if you can give links to some examples. I am a linux newbie and i really appreciate your help.. Thank you... Thanks, Bala

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• Repeatedly querying xml using python

  - by Jack

  I have some xml documents I need to run queries on. I've created some python scripts (using ElementTree) to do this, since I'm vaguely familiar with using it. The way it works is I run the scripts several times with different arguments, depending on what I want to find out. These files can be relatively large (10MB+) and so it takes rather a long time to parse them. On my system, just running: tree = ElementTree.parse(document) takes around 30 seconds, with a subsequent findall query only adding around a second to that. Seeing as the way I'm doing this requires me to repeatedly parse the file, I was wondering if there was some sort of caching mechanism I can use so that the ElementTree.parse computation can be reduced on subsequent queries. I realise the smart thing to do here may be to try and batch as many queries as possible together in the python script, but I was hoping there might be another way. Thanks.

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• MATLAB submatrix over variable dimensions

  - by rlbond

  I have a variable dimension matrix, X. I want a function that will get the first half of X in one dimension. I.E., I want something like this: function x = variableSubmatrix(x, d) if d == 1 switch ndims(x) case 1 x = x(1:end/2); case 2 x = x(1:end/2, :); case 3 x = x(1:end/2, :, :); (...) end elseif d == 2 switch ndims(x) case 2 x = x(:, 1:end/2); case 3 x = x(:, 1:end/2, :); (...) end elseif (...) end end I'm not quite sure how to do this. I need it to be fast, as this will be used many times in computation.

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• TSQL ID generation

  - by Markus

  Hi. I have a question regarding locking in TSQL. Suppose I have a the following table: A(int id, varchar name), where id is the primary key, but is NOT an identity column. I want to use the following pseudocode to insert a value into this table: lock (A) uniqueID = GenerateUniqueID() insert into A values (uniqueID, somename) unlock(A) How can this be accomplished in terms of TSQL? The computation of the next id should be done with the table A locked in order to avoid other sessions to do the same operation at the same time and get the same id.

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• Avoding multiple thread spawns in pthreads

  - by madman

  Hi StackOverflow, I have an application that is parallellized using pthreads. The application has a iterative routine call and a thread spawn within the rountine (pthread_create and pthread_join) to parallelize the computation intensive section in the routine. When I use an instrumenting tool like PIN to collect the statistics the tool reports statistics for several threads(no of threads x no of iterations). I beleive it is because it is spawning new set of threads each time the routine is called. How can I ensure that I create the thread only once and all successive calls use the threads that have been created first. When I do the same with OpenMP and then try to collect the statistics, I see that the threads are created only once. Is it beacause of the OpenMP runtime ? Thanks.

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• MYSQL and the LIMIT clause

  - by Lizard

  I was wondering if adding a LIMIT 1 to a query would speed up the processing? For example... I have a query that will most of the time return 1 result, but will occasionaly return 10's, 100's or even 1000's of records. But I will only ever want the first record. Would the limit 1 speed things up or make no difference? I know I could use GROUP BY to return 1 result but that would just add more computation. Any thoughts gladly accepted! Thanks

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• Inverse relationship of two variables

  - by Jam

  this one is maybe pretty stupid.. Or I am just exhausted or something, but I just cant seem to solve it.. Problem : two variables X and Y, value of Y is dependent on value of X. X can have values ranging from some value to some value (lets say from 0 to 250) and y can have different values (lets say from 0.1 to 1.0 or something..) - but it is inverse relatonship (what I mean is: if value of X is e.g. 250, then value of Y would be 0.1 and when X decreases up to 0, value of Y raises up to 1.0.. So how should I do it? lets say I have function: -- double computeValue (double X) { /computation/ return Y; } Also, is there some easy way to somehow make the scaling of the function not so linear? - For example when X raises, Y decreases slower at first but then more rapidly in the end.. (rly dont know how to say it but I hope you guys got it) Thanks in advance for this stupid question :/

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• How to make external Mathematica functions interruptible?

  - by Szabolcs

  I had an earlier question about integrating Mathematica with functions written in C++. This is a follow-up question: If the computation takes too long I'd like to be able to abort it using Evaluation Abort Evaluation. Which of the technologies suggested in the answers make it possible to have an interruptible C-based extension function? How can "interruptibility" be implemented on the C side? I need to make my function interruptible in a way which will corrupt neither it, nor the Mathematica kernel (i.e. it should be possible to call the function again from Mathematica after it has been interrupted)

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• Confusion between numpy, scipy, matplotlib and pylab

  - by goFrendiAsgard

  Numpy, scipy, matplotlib, and pylab are common terms among they who use python for scientific computation. I just learn a bit about pylab, and I got a lot of confusion. Whenever I want to import numpy, I can always do: import numpy as np I just consider, that once I do from pylab import * The numpy will be imported as well (with np alias). So basically the second one do more things compared to the first one. There are few things I want to ask. Is it right that pylab is just a wrapper for numpy, scipy and matplotlib? As np is the numpy alias, what is the scipy and matplotlib alias? (as far as I know, plt is alias of matplotlib.pyplot, but I don't know the alias for the matplotlib itself) Thanks in advance.

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• What is the purpose of OCaml's Lazy.lazy_from_val?

  - by Ricardo

  The doc of Lazy.lazy_from_val states that this function is for special cases: val lazy_from_val : 'a -> 'a t lazy_from_val v returns an already-forced suspension of v This is for special purposes only and should not be confused with lazy (v). Which cases are they talking about? If I create a pair of suspended computation from a value like: let l1 = lazy 123 let l2 = Lazy.lazy_from_val 123 What is the difference between these two? Because Lazy.lazy_is_val l1 and Lazy.lazy_is_val l2 both return true saying that the value is already forced!

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• How to implement Voting for Grails Domain Classes?

  - by userWebMobile

  I have a Book class and need to implement a yes/no voting functionality. My domain classes look like this: class Book { String title static hasMany = [votes: Vote] } class User { String name static hasMany = [votes: Vote] } class Vote { boolean yesVote static belongsTo = [user: User, book: Book] } What is the best way to implement a voting for the book class. I need the following informations: What is the average yesVote for a book over all votes (either yes or no)? How to check if a specific user has done a vote? What is the best way to implement the computation of the average yesVote such that the performance does not drop?

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• Condor job using DAG with some jobs needing to run the same host

  - by gurney alex

  I have a computation task which is split in several individual program executions, with dependencies. I'm using Condor 7 as task scheduler (with the Vanilla Universe, due do constraints on the programs beyond my reach, so no checkpointing is involved), so DAG looks like a natural solution. However some of the programs need to run on the same host. I could not find a reference on how to do this in the Condor manuals. Example DAG file: JOB A A.condor JOB B B.condor JOB C C.condor JOB D D.condor PARENT A CHILD B C PARENT B C CHILD D I need to express that B and D need to be run on the same computer node, without breaking the parallel execution of B and C. Thanks for your help.

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• Less Mathematical Approaches to Machine Learning?

  - by Ed

  Out of curiosity, I've been reading up a bit on the field of Machine Learning, and I'm surprised at the amount of computation and mathematics involved. One book I'm reading through uses advanced concepts such as Ring Theory and PDEs (note: the only thing I know about PDEs is that they use that funny looking character). This strikes me as odd considering that mathematics itself is a hard thing to "learn." Are there any branches of Machine Learning that use different approaches? I would think that a approaches relying more on logic, memory, construction of unfounded assumptions, and over-generalizations would be a better way to go, since that seems more like the way animals think. Animals don't (explicitly) calculate probabilities and statistics; at least as far as I know.

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