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• Secure password transmission over unencrypted tcp/ip

  - by academicRobot

  I'm in the designing stages of a custom tcp/ip protocol for mobile client-server communication. When not required (data is not sensitive), I'd like to avoid using SSL for overhead reasons (both in handshake latency and conserving cycles). My question is, what is the best practices way of transmitting authentication information over an unencrypted connection? Currently, I'm liking SRP or J-PAKE (they generate secure session tokens, are hash/salt friendly, and allow kicking into TLS when necessary), which I believe are both implemented in OpenSSL. However, I am a bit wary since I don't see many people using these algorithms for this purpose. Would also appreciate pointers to any materials discussing this topic in general, since I had trouble finding any.

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• Anonymous users support vs Google bot

  - by Andy

  I have a User class in my web app that represents a user currently logged in. Every time a user vists a page, a User instance is populated based on authentication data supplied in cookies. A User instance is created even if an anonymous user logs in - and a corresponding new record is created in the User table in the database. This approach allows me to save some state info for the current user regardless of its type. The problem however with this approach is the Google bot, and other non-human web organisms crawling my pages. Every time a bot starts to walk around the site, thousands of useless records will be created in the database, each of them only to be used for a single page. Question: what is the best trade off? How to support anonymous users, save their state, and don't get too much overhead because of cookieless bots?

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• In a virtual machine monitor such as VMware's ESXi Server, how are shadow page tables implemented?

  - by ali01

  My understanding is that VMMs such as VMware's ESXi Server maintain shadow page tables to map virtual page addresses of guest operating systems directly to machine (hardware) addresses. I've been told that shadow page tables are then used directly by the processor's paging hardware to allow memory access in the VM to execute without translation overhead. I would like to understand a bit more about how the shadow page table mechanism works in a VMM. Is my high level understanding above correct? What kind of data structures are used in the implementation of shadow page tables? What is the flow of control from the guest operating system all the way to the hardware? How are memory access translations made for a guest operating system before its shadow page table is populated? How is page sharing supported? Short of straight up reading the source code of an open source VMM, what resources can I look into to learn more about hardware virtualization?

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• Silverlight PRISM and 'loose' style files

  - by Suiva

  Hi, We are looking at starting a new Silverlight project using the PRISM framework (to benefit from modules etc) and I am still a little unclear about the best styling approach. What I'd ideally like is to have editable XAML files (maybe even just 1 for the whole project) containing the application's style so that they can be edited to change the look and feel of the application without having to recompile everything. Is this approach something people use? I guess it would need to load the file in at startup and apply the style which I assume wouldn't be a massive overhead. Just wondering what approaches people use Thanks for your time

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• Function calls in virtual machine killing performance

  - by GenTiradentes

  I wrote a virtual machine in C, which has a call table populated by pointers to functions that provide the functionality of the VM's opcodes. When the virtual machine is run, it first interprets a program, creating an array of indexes corresponding to the appropriate function in the call table for the opcode provided. It then loops through the array, calling each function until it reaches the end. Each instruction is extremely small, typically one line. Perfect for inlining. The problem is that the compiler doesn't know when any of the virtual machine's instructions are going to be called, as it's decided at runtime, so it can't inline them. The overhead of function calls and argument passing is killing the performance of my VM. Any ideas on how to get around this?

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• Are there any way to link my program with Wine-compiled part?

  - by seas

  I am trying to use windows dll functionality in Linux. My current solution is a compilation of a separate wine application, that uses dll and transfer requests/responses between dll and main application over IPC. This works, but is a real overhead comparing to a simple dll calls. I see that wine-compiled program usually is a bootstrapping-script and some .so, which (according to file utility) is normal linux dynamically linked library. Are there any way to link that .so directly to my application? Are there any manual?

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• Is it a good idea to close and open hibernate sessions frequently?

  - by Gaurav

  Hi, I'm developing an application which requires that state of entities be read from a database at frequent intervals or triggers. However, once hibernate reads the state, it doesn't re-read it unless I explicitly close the session and read the entity in a new session. Is it a good idea to open a session everytime I want to read the entity and then close it afterwards? How much of an overhead does this put on the application and the database (we use a c3p0 connection pool also)? Will it be enough to simply evict the entity from the session before reading it again?

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• In a process using lots of memory, how can I spawn a shell without a memory-hungry fork()?

  - by kdt

  On an embedded platform (with no swap partition), I have an application whose main process occupies most of the available physical memory. The problem is that I want to launch an external shell script from my application, but using fork() requires that there be enough memory for 2x my original process before the child process (which will ultimately execl itself to something much smaller) can be created. So is there any way to invoke a shell script from a C program without incurring the memory overhead of a fork()? I've considered workarounds such as having a secondary smaller process which is responsible for creating shells, or having a "watcher" script which I signal by touching a file or somesuch, but I'd much rather have something simpler.

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• Why do debug symbols so adversely affect the performance of threaded applications on Linux?

  - by fluffels

  Hi. I'm writing a ray tracer. Recently, I added threading to the program to exploit the additional cores on my i5 Quad Core. In a weird turn of events the debug version of the application is now running slower, but the optimized build is running faster than before I added threading. I'm passing the "-g -pg" flags to gcc for the debug build and the "-O3" flag for the optimized build. Host system: Ubuntu Linux 10.4 AMD64. I know that debug symbols add significant overhead to the program, but the relative performance has always been maintained. I.e. a faster algorithm will always run faster in both debug and optimization builds. Any idea why I'm seeing this behavior?

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• static images aren't caching with php-generated page

  - by scootklein

  Our website was just converted to being generated by mod_rewrite and php scripts. Images aren't caching in browsers when they seemingly should be. All images follow format: <img src="/images/header.png" /> I must avoid the script completely caching because the PHP parser needs to handle each page dynamically on each request; however, the download overhead of the large images is cumbersome on every single page load. I would ideally provide headers for "Cache-Control: no-cache, must-revalidate" and "Expires: some_date_in_the_past" to force revalidation of the PHP script. Why isn't the browser caching static images with consistent href values across all pages?

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• In C, is it possible do free only an array first or last position?

  - by user354959

  Hi there! I've an array, but I don't need its first (or last) position. So I point a new variable to the rest of the array, but I should free the array first/last position. For instance: p = read_csv_file(); q = p + 1; // I don't need the first CSV file field // Here I'd like to free only the first position of p return q; Otherwise I've to memcpy the array to other variable, excluding the first position, and then free the original array. Like this: p = read_csv_file(); q = (int*) malloc(sizeof(int) * (SOME_SIZE - 1)); memcpy(q, p+1, sizeof(int) * (SOME_SIZE - 1)); free(p); return q; But then I'll have the overhead of copying all the array. Is this possible to only free a single position of an array?

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• Securing input of private / protected methods?

  - by ts

  Hello, normally, all sane developers are trying to secure input of all public methods (casting to proper types, validating, sanitizing etc.) My question is: are you in your code validating also parameters passed to protected / private methods? In my opinion it is not necessary, if you securize properly parameters of public methods and return values from outside (other classes, db, user input etc...). But I am constantly facing frameworks and apps (ie. prestashop to name one) where validation is often repeated in method call, in method body and once again for securize returned value - which, I think, is creating performace overhead and is also a sign of bad design.

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• Alternatives to PropertyInfo.GetValue() for Mono?

  - by Trilok

  I have a method that has the following signature: private object GetNestedObject<y>(y objToAccess, string nestedObjectName) I'm using Reflection to get the nestedObject from the objToAccess and return it. This works well except it's really slow (I have to do this a few hundred thousand times). I came across HyperDescriptor, but since I'm running this on Linux, and Mono doesn't support TypeDescriptionProviders, I can't use it. Are there any alternatives to using getValue in this case? I could always hardcode in overrides for each type, but that is not desirable and would add a lot of maintenance overhead in my case.

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• Java: Reflection against casting when you know superclass

  - by Ema

  I don't know exactly how to define my doubt so please be patient if the question has already been asked. Let's say I have to dinamically instantiate an object. This object will surely be instance of a subclass of a known, immutable class A. I can obtain dinamically the specific implementation class. Would it be better to use reflection exactly as if I didn't know anything about the target class, or would it be preferrable/possible to do something like: A obj = (Class.forName("com.package.Sub-A")) new A(); where Sub-A extends A ? The purpose would be to avoid reflection overhead times... Thank you in advance.

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• How to change enum definition without impacting clients using it in C#

  - by Rohit

  I have the following enum defined. I have used underscores as this enum is used in logging and i don't want to incur the overhead of reflection by using custom attribute.We use very heavy logging. Now requirement is to change "LoginFailed_InvalidAttempt1" to "LoginFailed Attempt1". If i change this enum, i will have to change its value across application. I can replace underscore by a space inside logging SP. Is there any way by which i can change this without affecting whole application.Please suggest. public enum ActionType { None, Created, Modified, Activated, Inactivated, Deleted, Login, Logout, ChangePassword, ResetPassword, InvalidPassword, LoginFailed_LockedAccount, LoginFailed_InActiveAccount, LoginFailed_ExpiredAccount, ForgotPassword, LoginFailed_LockedAccount_InvalidAttempts, LoginFailed_InvalidAttempt1, LoginFailed_InvalidAttempt2, LoginFailed_InvalidAttempt3, ForgotPassword_InvalidAttempt1, ForgotPassword_InvalidAttempt2, ForgotPassword_InvalidAttempt3, SessionTimeOut, ForgotPassword_LockedAccount, LockedAccount, ReLogin, ChangePassword_Due_To_Expiration, ChangePassword_AutoExpired }

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• C# type safe and developer friendly list/collection technique

  - by Agile Noob

  I am populating a "Dictionary" with the results of an sp call. The key is the field name and the value is whatever value the sp returns for the field. This is all well and good but I'd like developers to have a predefined list of keys to access this list, for safety and documentation reasons. What I'd like to do is have something like an enum as a key for the dictionary so developers can safely access the list, but still have the ability to access the dictionary with a string key value. I am hoping to have a list of string values that I can access with an enum key AND a string key. Please make sure any suggestions are simple to implement, this is not the kind of thing I'm willing to build a lot of overhead to implement.

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• hibernate distributed 2nd level cache options

  - by ishmeister

  Not really a question but I'm looking for comments/suggestions from anyone who has experiences using one or more of the following: EhCache with RMI EhCache with JGroups EhCache with Terracotta Gigaspaces Data Grid A bit of background: our applications is read only for the most part but there is some user data that is read-write and some that is only written (and can also be reasonably inaccurate). In addition, it would be nice to have tools that enable us to flush and fill the cache at intervals or by admin intervention. Regarding the first option - are there any concerns about the overhead of RMI and performance of Java serialization?

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• when to use StringBuilder in java

  - by kostja

  It is supposed to be generally preferable to use a StringBuilder for String concatenation in Java. Is it always the case? What i mean is : Is the overhead of creating a StringBuilder object, calling the append() method and finally toString() smaller then concatenating existing Strings with + for 2 Strings already or is it only advisable for more Strings? If there is such a threshold, what does it depend on (the String length i suppose, but in which way)? And finally - would you trade the readability and conciseness of the + concatenation for the performance of the StringBuilder in smaller cases like 2, 3, 4 Strings?

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• Detecting support for a given JavaScript event?

  - by Will

  I'm interested in using the JavaScript hashchange event to monitor changes in the URL's fragment identifier. I'm aware of Really Simple History and the jQuery plugins for this. However, I've reached the conclusion that in my particular project it's not really worth the added overhead of another JS file. What I would like to do instead is take the "progressive enhancement" route. That is, I want to test whether the hashchange event is supported by the visitor's browser, and write my code to use it if it's available, as an enhancement rather than a core feature. IE 8, Firefox 3.6, and Chrome 4.1.249 support it, and that accounts for about 20% of my site's traffic. So, uh ... is there some way to test whether a browser supports a particular event? Thanks.

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• How to convert value of Generic Type Argument to a concrete type?

  - by Aleksey Bieneman

  I am trying to convert the value of the generic type parameter T value into integer after making sure that T is in fact integer: public class Test { void DoSomething<T>(T value) { var type = typeof(T); if (type == typeof(int)) { int x = (int)value; // Error 167 Cannot convert type 'T' to 'int' int y = (int)(object)value; // works though boxing and unboxing } } } Although it works through boxing and unboxing, this is an additional performance overhead and i was wandering if there's a way to do it directly. Thank you!

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• Can layouts be chosen by Grails controllers?

  - by maerics

  I'm building a CMS as a learning exercise in Grails and would like to give content managers the ability to choose between different HTML page structures (e.g. 2 column, 3 column, etc). Grails Layouts seem like a logical choice but is it possible for a Grails controller to explicitly name which layout will be used for rendering? Ideally there would be a layout option to the render method, per Ruby on Rails but I don't see anything like it. It seems like it might be possible using the applyLayout method by passing the name of the layout but this requires each GSP page to explicitly request layout (annoying overhead per-page) rather than using Layout by Convention. Any ideas?

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• Performance of jquery visible

  - by Mark Steudel

  I have a bunch of checkboxes on a page, and I only show a subset of those checkboxes at a time. I then perform some action which loops through all of the checkboxes and sees if they are checked or not: e.g. $(".delete_items").click( function() { $('.checkboxes' ).each(function(){ //do stuff } } Then I was thinking, well since the user can never interact with the hidden checkboxes, that adding :visible to checkboxes would speed up the loop e.g. $(".delete_items").click( function() { $('.checkboxes :visible' ).each(function(){ //do stuff } } But I don't know if adding :visible adds more overhead. Any thoughts?

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• All things equal what is the fastest way to output data to disk in C++?

  - by user260197

  I am running simulation code that is largely bound by CPU speed. I am not interested in pushing data in/out to a user interface, simply saving it to disk as it is computed. What would be the fastest solution that would reduce overhead? iostreams? printf? I have previously read that printf is faster. Will this depend on my code and is it impossible to get an answer without profiling? Edit: Output data needs to be in text format, whether tab or comma separated. This will require formatting, precision, etc. Running in Windows.

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• How do I find the previous/default value of a radio button in JS?

  - by royrules22

  I'm not sure if this is even possible, but I figured I'd ask before clubbing togehter an ugly solution. What I have is a group of radio buttons and I want to trigger a function if the radio button value is changed. For example say there are three buttons X Y and Z with Y default selected. If a user clicks on an unselected button (i.e. X or Z) then the function gets triggered but if he/she clicks on Y (which is already selected) nothing happens. Naturally the best solution would be to fire the function on an onchange event but IE6 doesn't fire onchange until the focus is no longer on the element which is not satisfactory. So is there a way to know what the previous value of the radio button was or at least what its default value is? I could easily create an array of default values and check against that but I'm trying to reduce the overhead. Please no jQuery suggestions ;)

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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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