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• Count of memory copies in *nix systems between packet at NIC and user application?

  - by Michael_73

  Hi there, This is just a general question relating to some high-performance computing I've been wondering about. A certain low-latency messaging vendor speaks in its supporting documentation about using raw sockets to transfer the data directly from the network device to the user application and in so doing it speaks about reducing the messaging latency even further than it does anyway (in other admittedly carefully thought-out design decisions). My question is therefore to those that grok the networking stacks on Unix or Unix-like systems. How much difference are they likely to be able to realise using this method? Feel free to answer in terms of memory copies, numbers of whales rescued or areas the size of Wales ;) Their messaging is UDP-based, as I understand it, so there's no problem with establishing TCP connections etc. Any other points of interest on this topic would be gratefully thought about! Best wishes, Mike

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• Support-function in the GJK-algorithm.

  - by Marcus Johansson

  I am trying to implement the GJK-algorithm but I got stuck instantly. The problem is to implement the Support-function that isn't O(n^2). As it is now I'm computing the complete Minkowski difference, and then there is really no point in doing the GJK-algorithm. (or is it?) What I mean by Support-function is the function that returns the point in the Minkowski difference that is furthest away in a specified direction. I assume this shouldn't be O(n^2) as it is in my current implementation.

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• What to see in India as a developer and student

  - by lasseespeholt

  Hi, I'm planning a trip to India around July-August. And I want to hear Indian developers or developers traveled to India if there are something interesting to see in India (around Bombay) for developers in general. Maybe a conference etc. I have looked at http://www.conferencealerts.com/computing.htm I have studied computer science in a year now but have been a spare time and independent developer for some years now. Furthermore, I have tried finding summer courses but that is properly too late now and computer science summer courses is hard to find in India. Best regards, Lasse Espeholt

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• How to use your computer to save the world?

  - by Francisco Garcia

  Sometimes I miss the "help other people" factor within computer-related fields. However, there are little things that we all can do to make this a better place—beyond trying to eradicate annoying stuff such as Visual Basic. You could join a cloud computing network such as World Community Grid to fight cancer, write a charityware application such as Vim, improve office IT infrastructure to support telecommuting and reduce CO2 emissions, use an ebook reader to save paper, ... What else can we do to help others? Which projects can have the biggest impact?

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• gcc options for fastest code

  - by rwallace

  I'm distributing a C++ program with a makefile for the Unix version, and I'm wondering what compiler options I should use to get the fastest possible code (it falls into the category of programs that can use all the computing power they can get and still come back for more), given that I don't know in advance what hardware, operating system or gcc version the user will have, and I want above all else to make sure it at least works correctly on every major Unix-like operating system. Thus far, I have g++ -O3 -Wno-write-strings, are there any other options I should add? On Windows, the Microsoft compiler has options for things like fast calling convention and link time code generation that are worth using, are there any equivalents on gcc? (I'm assuming it will default to 64-bit on a 64-bit platform, please correct me if that's not the case.)

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• Whats the deal with python?

  - by gmatt

  My interests in programming lie mainly in algorithms, and lately I have seen many reputable researchers write a lot of their code in python. How easy and convenient is python for scientific computing? Does it have a library of algorithms that compares to matlab's? Is Python a scripting language or does it compile? Is it a great language for prototyping an algorithm? How long would it take me to learn enough of it to be productive provided I know C well and OO programming somewhat? Is it OO based? Sorry for the condensed format of questions, but I'm very curious and was hoping a more experienced programmer could help me out.

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• How to save the world from your computer?

  - by Francisco Garcia

  Sometimes I miss the "help other people" factor within computer related careers. Sure that out there I could find many great projects improving society, but that is not common. However there are little things that we all can do to make this a better place beyond trying to erradicate annoynig stuff such as Visual Basic. You could join a cloud computing network such as World Community Grid to fight cancer. Write a charityware application such as Vim, improve an office IT infrastructure to support telecommuting and reduce CO2 emissions, use an ebook reader for saving paper... what else would you? which projects do you think can have an impact?

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• Interrupting `while loop` with keyboard in Cython

  - by linello

  I want to be able to interrupt a long function with cython, using the usual CTRL+C interrupt command. My C++ long function is repeatedly called inside a while loop from Cython code, but I want to be able, during the loop, to send an "interrupt" and block the while loop. The interrupt also should wait the longFunction() to finish, so that no data are lost or kept in unknown status. This is one of my first implementation, which obviously doesn't work: computed=0; print "Computing long function..." while ( computed==0 ): try: computed = self.thisptr.aLongFunction() except (KeyboardInterrupt, SystemExit): computed=1 print '\n! Received keyboard interrupt.\n' break; (p.s. self.thisptr is the pointer to the current class which implements aLongFunction() )

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• Generating a beveled edge for a 2D polygon

  - by Metaphile

  I'm trying to programmatically generate beveled edges for geometric polygons. For example, given an array of 4 vertices defining a square, I want to generate something like this. But computing the vertices of the inner shape is baffling me. Simply creating a copy of the original shape and then scaling it down will not produce the desired result most of the time. My algorithm so far involves analyzing adjacent edges (triples of vertices; e.g., the bottom-left, top-left, and top-right vertices of a square). From there, I need to find the angle between them, and then create a vertex somewhere along that angle, depending on how deep I want the bevel to be. And because I don't have much of a math background, that's where I'm stuck. How do I find that center angle? Or is there a much simpler way of attacking this problem?

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• why don't more programming languages have builtin interfaces to the window manager?

  - by Naveen

  Programming is at the heart about automating tasks on a computer. Presumably those tasks would normally be done manually by a human. Humans use the computer through the keyboard, mouse, and interaction with the console or the window manager. But very few languages have built in functions that provide an interface to these basic computing objects. A notable exception is autohotkey, an open source language on windows, providing builtin functions that allow the following simple tasks: * Get Pixel Information * Get mouse position * Keyboard macros * Simulate key strokes * Simulate mouse click * Window management See examples on rosettacode. There have been various attempts on linux, many of which were stopped without explanation. One is the inactive tcl library: android. Search google code for android, lang:tcl

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• Binomial test in Python for very large numbers

  - by Morlock

  I need to do a binomial test in Python that allows calculation for 'n' numbers of the order of 10000. I have implemented a quick binomial_test function using scipy.misc.comb, however, it is pretty much limited around n = 1000, I guess because it reaches the biggest representable number while computing factorials or the combinatorial itself. Here is my function: from scipy.misc import comb def binomial_test(n, k): """Calculate binomial probability """ p = comb(n, k) * 0.5**k * 0.5**(n-k) return p How could I use a native python (or numpy, scipy...) function in order to calculate that binomial probability? If possible, I need scipy 0.7.2 compatible code. Many thanks!

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• Perl: Fastest way to get directory (and subdirs) size on unix - using stat() at the moment

  - by ivicas

  I am using Perl stat() function to get the size of directory and it's subdirectories. I have a list of about 20 parent directories which have few thousand recursive subdirs and every subdir has few hundred records. Main computing part of script looks like this: sub getDirSize { my $dirSize = 0; my @dirContent = <*>; my $sizeOfFilesInDir = 0; foreach my $dirContent (@dirContent) { if (-f $dirContent) { my $size = (stat($dirContent))[7]; $dirSize += $size; } elsif (-d $dirContent) { $dirSize += getDirSize($dirContent); } } return $dirSize; } The script is executing for more than one hour and I want to make it faster. I was trying with the shell du command, but the output of du (transfered to bytes) is not accurate. And it is also quite time consuming. I am working on HP-UNIX 11i v1.

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• SRAM Cell Diagram - Can someone explain this a bit more clearly? ( From COMP1917 @ UNSW: Lecture 2 o

  - by Kristina

  I've begun watching a series of first year lectures from the University of New South Wales (UNSW) in Australia, and I'm a bit perplexed by the instructors explanation of how an SRAM gate works. I realize this isn't exactly "programming-related" but since it comes from a series of lectures relating to computing and programming, I thought StackOverflow may be able to help (reddit failed me entirely). In this lecture beginning at around 32:12, Richard (the lecturer) tries to explain how a "latch gate" works within SRAM. Although his students seem to keep up, I feel I'm missing something crucial which is preventing the concept from really "clicking" in my brain. For convenience, I've added the image from the video below: Thanks in advance for any help you can provide, but if this question doesn't fit your view of "programming-related" could you please provide an alternate forum for this in a comment when you cast your close vote? Thanks!

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• Why are Asynchronous processes not called Synchronous?

  - by Balk

  So I'm a little confused by this terminology. Everyone refers to "Asynchronous" computing as running different processes on seperate threads, which gives the illusion that these processes are running at the same time. This is not the definition of the word asynchronous. a·syn·chro·nous –adjective 1. not occurring at the same time. 2. (of a computer or other electrical machine) having each operation started only after the preceding operation is completed. What am I not understanding here?

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• Mathematica & J/Link: Memory Constraints?

  - by D-Bug

  I am doing a computing-intensive benchmark using Mathematica and its J/Link Java interface. The benchmark grinds to a halt if a memory footprint of about 320 MB is reached, since this seems to be the limit and the garbage collector needs more and more time and will eventually fail. The Mathematica function ReinstallJava takes the argument command line. I tried to do ReinstallJava[CommandLine -> "java -Xmx2000m ..."] but Mathematica seems to ignore the -Xmx option completely. How can I set the -Xmx memory option for my java program? Where does the limit of 320 MB come from? Any help would be greatly appreciated.

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• What to use to wait on a indeterminate number of tasks?

  - by Scott Chamberlain

  I am still fairly new to parallel computing so I am not too sure which tool to use for the job. I have a System.Threading.Tasks.Task that needs to wait for n number number of tasks to finish before starting. The tricky part is some of its dependencies may start after this task starts (You are guaranteed to never hit 0 dependent tasks until they are all done). Here is kind of what is happening Parent thread creates somewhere between 1 and (NUMBER_OF_CPU_CORES - 1) tasks. Parent thread creates task to be run when all of the worker tasks are finished. Parent thread creates a monitoring thread Monitoring thread may kill a worker task or spawn a new task depending on load. I can figure out everything up to step 4. How do I get the task from step 2 to wait to run until any new worker threads created in step 4 finish?

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• Why are mainframes still around?

  - by ThaDon

  It's a question you've probably asked or been asked several times. What's so great about Mainframes? The answer you've probably been given is "they are fast" "normal computers can't process as many 'transactions' per second as they do". Jeese, I mean it's not like Google is running a bunch of Mainframes and look how many transactions/sec they do! The question here really is "why?". When I ask this question to the mainframe devs I know, they can't answer, they simply restate "It's fast". With the advent of Cloud Computing, I can't imagine mainframes being able to compete both cost-wise and mindshare-wise (aren't all the Cobol devs going to retire at some point, or will offshore just pickup the slack?). And yet, I know a few companies that still pump out net-new Cobol/Mainframe apps, even for things we could do easily in say .NET and Java. Anyone have a real good answer as to why "The Mainframe is faster", or can point me to some good articles relating to the topic?

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• How do I profile memory usage in my project

  - by Gacek

  Are there any good, free tools to profile memory usage in C# ? Details: I have a visualization project that uses quite large collections. I would like to check which parts of this project - on the data-processing side, or on the visualization side - use most of the memory, so I could optimize it. I know that when it comes to computing size of the collection the case is quite simple and I can do it on my own. But there are also certain elements for which I cannot estimate the memory usage so easily. The memory usage is quite big, for example processing a file of size 35 MB my program uses a little bit more than 250 MB of RAM.

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• How to interpret situations where Math.Acos() reports invalid input?

  - by Sean Ochoa

  Hey all. I'm computing the angle between two vectors, and sometimes Math.Acos() returns NaN when it's input is out of bounds (-1 input && input 1) for a cosine. What does that mean, exactly? Would someone be able to explain what's happening? Any help is appreciated! Here's me method: public double AngleBetween(vector b) { var dotProd = this.Dot(b); var lenProd = this.Len*b.Len; var divOperation = dotProd/lenProd; // http://msdn.microsoft.com/en-us/library/system.math.acos.aspx return Math.Acos(divOperation) * (180.0 / Math.PI); }

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• Query about the service or technology behind gmail service

  - by user1726908

  I am a final year computer science student. I am studying in hyderabad, andhra pradesh, india. I have come to know that the gmail is a cloud service. I am very much interested in learning more about cloud computing. This technology has been puzzling,tickling,increasing my curiosity and i just want to learn as much as i can about it. And through experience, i have learnt that practically doing can improve our knowledge and thirst to learn more. Thus, I would like to know "what are the security measures which you have taken to keep the cloud service like gmail secure and authentic? What is the architecture of the service? What are the technologies used in building it? What are the different levels of security applied in general for building a private cloud?"

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• Partially parse C++ for a domain-specific language

  - by PierreBdR

  I would like to create a domain specific language as an augmented-C++ language. I will need mostly two types of contructs: Top-level constructs for specialized types or declarations In-code constructs, i.e. to add primitives to make functions calls or idiom easier The language will be used for scientific computing purposes, and will ultimately be translated into plain C++. C++ has been chosen as it seems to offer a good compromise between: ease of use, efficiency and availability of a wide range of libraries. A previous attempt using flex and bison failed due to the complexity of the C++ syntax. The existing parser can still fail on some constructs. So we want to start over, but on better bases. Do you know about similar projects? And if you attempted to do so, what tools would you use? What would be the main pitfalls? Would you have recommendations in term of syntax?

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• Faking a Single Address Space

  - by dsimcha

  I have a large scientific computing task that parallelizes very well with SMP, but at too fine grained a level to be easily parallelized via explicit message passing. I'd like to parallelize it across address spaces and physical machines. Is it feasible to create a scheduler that would parallelize already multithreaded code across multiple physical computers under the following conditions: The code is already multithreaded and can scale pretty well on SMP configurations. The fact that not all of the threads are running in the same address space or on the same physical machine must be transparent to the program, even if this comes at a significant performance penalty in some use cases. You may assume that all of the physical machines involved are running operating systems and CPU architectures that are binary compatible. Things like locks and atomic operations may be slow (having network latency to deal with and all) but must "just work".

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• How much RAM used by Python dict or list?

  - by Who8MyLunch

  My problem: I am writing a simple Python tool to help me visualize my data as a function of many parameters. Each change in parameters involves a non-trivial amount of time, so I would like to cache each step's resulting imagery and supporting data in a dictionary. But then I worry that this dictionary could grow too large over time. Most of my data is in the form of Numpy arrays. My question: How would one go about computing the total number of bytes used by a Python dictionary. The dictionary itself may contain lists and other dictionaries, each of which contain data stored in Numpy arrays. Ideas?

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• What's the fastest way to get directory and subdirs size on unix using Perl?

  - by ivicas

  I am using Perl stat() function to get the size of directory and its subdirectories. I have a list of about 20 parent directories which have few thousand recursive subdirs and every subdir has few hundred records. Main computing part of script looks like this: sub getDirSize { my $dirSize = 0; my @dirContent = <*>; my $sizeOfFilesInDir = 0; foreach my $dirContent (@dirContent) { if (-f $dirContent) { my $size = (stat($dirContent))[7]; $dirSize += $size; } elsif (-d $dirContent) { $dirSize += getDirSize($dirContent); } } return $dirSize; } The script is executing for more than one hour and I want to make it faster. I was trying with the shell du command, but the output of du (transfered to bytes) is not accurate. And it is also quite time consuming. I am working on HP-UNIX 11i v1.

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• Is there any Application Server Frameworks for other languages/platforms than JavaEE and .NET?

  - by Jonas

  I'm a CS student and has rare experience from the enterprise software industry. When I'm reading about enterprise software platforms, I mostly read about these two: Java Enterprise Edition, JavaEE .NET and Windows Communication Foundation By "enterprise software platforms" I mean frameworks and application servers with support for the same characteristics as J2EE and WCF has: [JavaEE] provide functionality to deploy fault-tolerant, distributed, multi-tier Java software, based largely on modular components running on an application server. WCF is designed in accordance with service oriented architecture principles to support distributed computing where services are consumed by consumers. Clients can consume multiple services and services can be consumed by multiple clients. Services are loosely coupled to each other. Is there any alternatives to these two "enterprise software platforms"? Isn't any other programming languages used in a bigger rate for this problem area? I.e Why isn't there any popular application servers for C++/Qt?

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