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• Why is domU faster than dom0 on IO?

  - by Paco

  I have installed debian 7 on a physical machine. This is the configuration of the machine: 3 hard drives using RAID 5 Strip element size: 1M Read policy: Adaptive read ahead Write policy: Write Through /boot 200 MB ext2 / 15 GB ext3 SWAP 10GB LVM rest (~500GB) emphasized text I installed postgresql, created a big database (over 1GB). I have an SQL request that takes a lot of time to run (a SELECT statement, so it only reads data from the database). This request takes approximately 5.5 seconds to run. Then, I installed XEN, created a domU, with another debian distro. On this OS, I also installed postgresql, with the same database. The same SQL request takes only 2.5 seconds to run. I checked the kernel on both dom0 and domU. uname-a returns "Linux debian 3.2.0-4-amd64 #1 SMP Debian 3.2.41-2+deb7u2 x86_64 GNU/Linux" on both systems. I checked the kernel parameters, which are approximately the same. For those that are relevant, I changed their values to make them match on both systems using sysctl. I saw no changes (the requests still take the same amount of time). After this, I checked the file systems. I used ext3 on domU. Still no changes. I installed hdparm, and ran hdparm -Tt on both systems, on all my partitions on both systems, and I get similar results. Now, I am stuck, I don't know what is different, and what could be the cause of such a big difference. Additional Info: Debian runs on a Dell server PowerEdge 2950 postgresql: 9.1.9 (both dom0 and domU) xen-linux-system: 3.2.0 xen-hypervisor: 4.1 Thanks EDIT: As Krzysztof Ksiezyk suggested, it might be due to some file caching system. I ran the dd command to test both the read and write speed. Here is domU: root@test1:~# dd if=/dev/zero of=/root/dd count=5MB bs=1MB ^C2020+0 records in 2020+0 records out 2020000000 bytes (2.0 GB) copied, 18.8289 s, 107 MB/s root@test1:~# dd if=/root/dd of=/dev/null count=5MB bs=1MB 2020+0 records in 2020+0 records out 2020000000 bytes (2.0 GB) copied, 15.0549 s, 134 MB/s And here is dom0: root@debian:~# dd if=/dev/zero of=/root/dd count=5MB bs=1MB ^C1693+0 records in 1693+0 records out 1693000000 bytes (1.7 GB) copied, 8.87281 s, 191 MB/s root@debian:~# dd if=/root/dd of=/dev/null count=5MB bs=1MB 1693+0 records in 1693+0 records out 1693000000 bytes (1.7 GB) copied, 0.501509 s, 3.4 GB/s What can be the cause of this caching system? And how can we "fix" it? Can we apply it to dom0? EDIT 2: I switched my virtual disk type. To do so I followed this article. I did a dd if=/dev/vg0/test1-disk of=/mnt/test1-disk.img bs=16M Then in /etc/xen/test1.cfg, I changed the disk parameter to use file: instead of phy: it should have removed the file caching, but I still get the same numbers (domU being much faster for Postgres)

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• Environment variable for volume names in Windows?

  - by Shinrai

  I'm trying to write a batch file that will look at the volume label of the current drive and report if it's not equivalent to a certain string. Is there a default variable in the shell for this? Can I define one? Am I SOL and I'll actually have to do some (shudder) programming? EDIT: If this is possible in PowerShell that would work fine. (For the curious, we ship our machines with software cloning as a rapid bootable backup solution since most of our customers are daytraders and aren't interested in RAID due to urgency of getting-the-hell-back-to-work-right-away if there's a software corruption problem, and we want to make it immediately obvious if they're booting to the backup drive unintentionally, like say the primary failed entirely. The hope was just to write a simple batch file that would autostart on boot and throw a warning in the event of a problem.)

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• Places to store basic data

  - by Ella

  I am using PHP. I'm building a fully modular CMS, which is destined for the public. Some people might view this as a framework, but I intend to write a set of extensions for it, extensions that will make it a CMS :P Because it's completely modular I have a problem figuring out how to load extensions. Practically I need to get the list of active extensions, so I can load them inside my base class. I load them by reading some file headers, which contain a "dependency" field. That field decides the order in which I have to instantiate the objects. The problem is that when the CMS starts I have no database interface, because that's an extension too, so I can't store the active extensions list in the database :) You might ask how are extensions activated in the first place. Well - in the administration interface, which is an extension as well (obviously on first install of the CMS there will be some extensions active by default). Could writing that list inside a text file be a solution? The problem is that a lot of hosts are not very nice with scripts when they write files. And since this CMS is public I might have a problem here?

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• Samba does not reload user group members

  - by xato

  I am running a simple samba server setup where users connect to a share which contains folders for specific user groups. The folders are chmod 2770, so only users which are in the correct group can read/write in them. The problem is that if I change group memberships (i.e. remove user from group / add user to group; changes are in sync between clients and server!) samba does not automatically reload the group memberships for the user, so they can still write to groups that they are no longer a member of etc. I either have to reconnect to the share or to restart samba to apply the changes. Is there any way to prevent group caching and/or enable group membership reload in samba? My smb.conf: https://gist.github.com/anonymous/ca7c10a3b3e2168d7a03

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• VIM: How do you get the last ex command you used?

  - by TrevorBoydSmith

  I find that sometimes I write a really long ex mode command that do lots of stuff. They are sort of "mini-scripts" that I write in the text editor then I start ex mode and copy them into the ex line and execute. But then I always end up editing in the ex mode and then I find it difficult to get the changes i did in ex mode back to my text editing session. Using the keyboard, how do you copy the last ex command you used and paste it into your text editor? (Note: This is sort of the opposite of this question "how do I copy/paste in vim ex mode" where the user asks "how do you copy from the text editor and paste INTO the ex mode?". My question is the opposite because I wish to copy from the ex mode and paste into my text editor.)

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• Why is the use of abstractions (such as LINQ) so taboo?

  - by Matthew Patrick Cashatt

  I am an independent contractor and, as such, I interview 3-4 times a year for new gigs. I am in the midst of that cycle now and got turned down for an opportunity even though I felt like the interview went well. The same thing has happened to me a couple of times this year. Now, I am not a perfect guy and I don't expect to be a good fit for every organization. That said, my batting average is lower than usual so I politely asked my last interviewer for some constructive feedback, and he delivered! The main thing, according to the interviewer, was that I seemed to lean too much towards the use of abstractions (such as LINQ) rather than towards lower-level, organically grown algorithms. On the surface, this makes sense--in fact, it made the other rejections make sense too because I blabbed about LINQ in those interviews as well and it didn't seem that the interviewers knew much about LINQ (even though they were .NET guys). So now I am left with this question: If we are supposed to be "standing on the shoulders of giants" and using abstractions that are available to us (like LINQ), then why do some folks consider it so taboo? Doesn't it make sense to pull code "off the shelf" if it accomplishes the same goals without extra cost? It would seem to me that LINQ, even if it is an abstraction, is simply an abstraction of all the same algorithms one would write to accomplish exactly the same end. Only a performance test could tell you if your custom approach was better, but if something like LINQ met the requirements, why bother writing your own classes in the first place? I don't mean to focus on LINQ here. I am sure that the JAVA world has something comparable, I just would like to know why some folks get so uncomfortable with the idea of using an abstraction that they themselves did not write. UPDATE As Euphoric pointed out, there isn't anything comparable to LINQ in the Java world. So, if you are developing on the .NET stack, why not always try and make use of it? Is it possible that people just don't fully understand what it does?

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• when installing software on a mac, how do companies prevent trial software from continueing?

  - by sososo

  i know on windows they write to the registry, do Mac's have some sort of registry?

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• TechEd North America 2012-Day 4 #msTechEd #teched

  - by Marco Russo (SQLBI)

  I hadn’t time yesterday to write a blog post before of the beginning of the day and I recover now that I’m already back to Europe. I spent many hours at the Microsoft booth meeting people who asked questions about Tabular, Power View and PowerPivot. I have to say that I’m really glad so many people started using PowerPivot and of the large interest around Tabular. During my BISM: Multidimensional vs. Tabular I’ve been helped by Alberto during the demo (we joked about he was the demo monkey) and the feedback received are good. I tried to compare the strength and weakness of the two modeling options without spending time describing the area in which they are similar. During the days before I had many discussions about scenarios based on snapshots and I added a few slides to the presentation in order to cover this area that I thought was marginal but seems to be a very common one. To know more, watch the presentation when it will be available or wait for some article I’ll write in the future describing these patterns. In less than 10 days, I’ll be at TechEd Europe and I’m really looking forward to meet other Tabular developers and PowerPivot users there!

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• C# Threading Background Process - Programming - How to?

  - by Magic

  Hello...I have been given the horrible task of doing this. Launch the website Take a screenshot Fill in the form details, click on Next Take a screenshot ... ... ... Rinse. Repeat. Now, with various combinations, this comes up to 300 screenshots. And I have to do this for 4 different browsers. Chrome, Firefox, IE 6 and IE 7. I cannot use tools which will capture the screenshot and store them, such as, SnagIT. I need to take a screenshot, copy it to a Word Document and take the second screenshot and take it to a Word Document. I thought, I will write a tiny utility which will help me do this. Here is the requirement spec that I put up for it - An executable which once launched seats itself in the System Tray. While it is active, all instances of Key Press (Print Scrn), it should write the contents to a Word Document as defined (either a default path or a user defined one). Save the document periodically. Now, my question is - if I am going to develop this using C# (Winforms application), how do I go about doing this. I can do a fair bit of C# programming and I am willing to learn. But I am not able to locate the references for how to do a background process so that it runs in the background. And while it runs, it has to capture the Print Scrn command. Can you folks point me to the right material where I can learn this? Theoretical references should suffice. But if there are practical references, then nothing like it. Thanks!

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• Entry level engineer question regarding memory mangement

  - by Ealianis

  It has been a few months since I started my position as an entry level software developer. Now that I am past some learning curves (e.g. the language, jargon, syntax of VB and C#) I'm starting to focus on more esoteric topics, as to write better software. A simple question I presented to a fellow coworker was responded with "I'm focusing on the wrong things." While I respect this coworker I do disagree that this is a "wrong thing" to focus upon. Here was the code (in VB) and followed by the question. Note: The Function GenerateAlert() returns an integer. Dim alertID as Integer = GenerateAlert() _errorDictionary.Add(argErrorID, NewErrorInfo(Now(), alertID)) vs... _errorDictionary.Add(argErrorID, New ErrorInfo(Now(), GenerateAlert())) I originally wrote the ladder and rewrote it with the "Dim alertID" so that someone else might find it easier to read. But here was my concern and question. "Should one write this with the Dim AlertID, it would in fact take up more memory; finite but more, and should this method be called many times could it lead to an issue? How will .NET handle this object AlertID. Outside of .NET should one manually dispose of the object after use (near the end of the sub)." I want to ensure I become a knowledgeable programmer that does not just rely upon garbage collection. Am I over thinking this? Am I focusing on the wrong things?

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• The idea of functionN in Scala / Functionaljava

  - by Luke Murphy

  From brain driven development It turns out, that every Function you’ll ever define in Scala, will become an instance of an Implementation which will feature a certain Function Trait. There is a whole bunch of that Function Traits, ranging from Function1 up to Function22. Since Functions are Objects in Scala and Scala is a statically typed language, it has to provide an appropriate type for every Function which comes with a different number of arguments. If you define a Function with two arguments, the compiler picks Function2 as the underlying type. Also, from Michael Froh's blog You need to make FunctionN classes for each number of parameters that you want? Yes, but you define the classes once and then you use them forever, or ideally they're already defined in a library (e.g. Functional Java defines classes F, F2, ..., F8, and the Scala standard library defines classes Function1, ..., Function22) So we have a list of function traits (Scala), and a list of interfaces (Functional-java) to enable us to have first class funtions. I am trying to understand exactly why this is the case. I know, in Java for example, when I write a method say, public int add(int a, int b){ return a + b; } That I cannot go ahead and write add(3,4,5); ( error would be something like : method add cannot be applied to give types ) We simply have to define an interface/trait for functions with different parameters, because of static typing?

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• vsftpd, virtual users and permissions. Avoid using chmod 777?

  - by Jakobud

  I am running vsftpd with Virtual Users (managed through a MySQL db). Each users home/default directory is owned by vsftpd:vsftpd. I need to give a user read/write permissions to some website files, owned by apache:apache so they can make some changes. I did a bind mount for the web directory to a directory in the ftp user's home/default directory. When logging in, the user is not able to write to the web folder, unless I set files to 777. Is it possible to set this up with making the directory and it's files 777? The web directory needs to be apache:apache in order for apache to work with it.

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• Screenshot is black in Mac OS X SL

  - by kirbez.ya.ru

  When I press cmd+shift+3 in desktop saves black 1400x900 file Command: defaults write com.apple.screencapture disable-shadow -bool true Does not work

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• Simple website with a GPL V3 Framework

  - by sineverba

  I write web-based software and simple website ("Home", "Who we are", "Contact"). For a simple website I'm using a covered GPL v3 framework. The user surf the website, send an email, take info, etc. I repeat: simple website, not a Joomla or Wordpress. 1) Will the website be covered with the GPL? I don't modify the framework. I'm using his classes in other classes... (OOP). 2) For the point 1, if yes, do I need to add (e.g. in the footer) name of framework and his link? 3) I must permit download of entire website to study code (nothing that a programmer has interest in)? E.g. placing it in Github? 4) If 2 is NO, how you can "understand" that we use that framework? In effect no php lines are exposed to the browser... You cannot understand that when you push "Send email" the site is calling $this->send($email). If you write me an email "Are you using XXX framework"? I can answer NO.

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• Should I use OpenGL or DX11 for my game?

  - by Sundareswaran Senthilvel

  I'm planning to write a game from scratch (a BIG Game, for commercial purpose). I'm aware that there are certain compute libraries like OpenCL, AMD APP SDK, C++ AMP as well as DirectCompute - both from MS (NOT interested in CUDA) are available in the market. I'm planning to write the game from the scratch, which includes the following engines... Physics Engine AI Engine Main Game Engine (... and if anything is missed). I'm aware that, there are some free physics engine libraries in the market. Not sure about free AI engine libraries. I'm bit confused in choosing between the OpenCL, AMD APP SDK, and C++ AMP libraries (as already mentioned i'm NOT interested in CUDA). I want my game to be published in Windows/Android/Mac OSX. It means it should be a cross-platform game. I will be having "one source code" that i'll compile for various platforms like Windows/Android/Mac OSX, and any others if i missed. Note: Since I'm NOT a Java guy, kindly do NOT suggest me the Java Language. For Graphics language should i use OpenGL or DirectX 11? I have heard that OpenGL runs on a single core, and not sure of DirectX 11. Between OpenGL and DirectX which one should i follow? or else, are there any other graphics language that i need to start with? I want to make use of the parallelism in GPU as well as CPU.

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• Installing 10.04 server on HP xw9400 Workstation with RAID 5

  - by Dave Long

  I have a workstation that was given to me that is a friggen powerhouse, so I figured I would set it up as my development and demo server. This is my first experience installing Ubuntu onto a RAID array and so far it has not been a fun one. I have been following the Advanced Installation guide for installing Ubuntu 10.04 server, and it says that there will be an option on the Partition Disks screen to manually create the partitions, but the only options I have are: Configure iSCSI volumes Undo changes to partitions Finish partitioning and write changes to disk Just before I got to that screen I got a message that said: One or more drives containing Serial ATA RAID configurations have been found. Do you wish to activate these RAID devices? It doesn't matter whether I answer yes or no to that, I still get the same Partition Disks screen. When I try to select Finish partitioning and write changes to disk I just get the No root file system error. Has anyone else experienced this, and how do I get past it? Can I not run Ubuntu on this machine?

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• Event handler generation in Visual Studio 2012

  - by Jalpesh P. Vadgama

  This post will be a part of Visual Studio 2012 feature series There are lots of new features there in visual studio 2012. Event handler generation is one of them. In earlier version of visual studio there was no way to create event handler from source view directly.  Now visual studio 2012 have event handler generation functionality. So if you are editing an event view in source view intellisense will display add new event handler template and once you click on it. It will create a new event handler in the cs file. It will also put a eventhandler name against event name so you don’t need to write that. So, let’s take a simple example of button click event so once I write onclick attribute their smart intellisense will pop up . Now once you click on <Create New Event> It will create event handler in .cs file like following. It will also put submitButton_Click on onClick attribute. Hope you liked it. Stay tuned for more. Till then happy programming..

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• How can I set deadline as the I/O scheduler for USB Flash devices by using udev rules?

  - by ????

  I have set CFQ as the default I/O scheduler. I often get bad performance when I write data into a Flash device. This is resolved if I use deadline as the I/O scheduler for USB Flash devices. I can't always change the scheduler manually, right? I think writing udev rules is a good idea. Can someone please write rules for me? I want: When I plug in a USB device, detect the type of the device. If it is a portable USB hard disk, do nothing (I think if a device has more than one partitions, it always a portable hard disk. If it is a USB Flash device, set deadline as it's scheduler.

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• Should I impersonate PHP via FastCGI?

  - by AKeller

  I am installing the latest version of PHP onto IIS 7.5 via FastCGI, and all of the instructions say that FastCGI should impersonate the calling client by setting fastcgi.impersonate = 1 If my website will have this configuration dedicated application pool application pool identity of ApplicationPoolIdentity anonymous authentication only (as IUSR) why do I want to impersonate? I come from an ASP.NET background, where the IUSR gets read-only permissions and the application pool identity gets any write permissions. Giving write access to the IUSR usually opens the door for WebDAV vulnerabilities. So I hesitate to let PHP run as the IUSR. I can't find many people asking this question (1 | 2) so I think I must be missing something. Can someone clarify this for me?

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• Any website/software where i can add some text rows on daily basis

  - by Moorage

  I have few notes or text like few remembering lines on daily or weekly basis. I want to write it. but i should be able to see at any backdate /monthly or yearly. The date-time should also be stored when i enter text. is it possible EDIT: I will explain clearly what i exactly want. Suppose while working on internet 1)i find "ABC is good for BCD". now i want to add that text to some online site where i can see later 2)Now i can add those type of things any time and on internet i can see that in tabular form like click to see montly list , yearly or weekly 3)Other thing is i should be able to add text as easy as possible like in firefox if i can press some shotcut and enter something and it gets added there rather than opening the site and then write The to do list do that sort of thing but they dont have creation date , rather they filter based on due date.

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• Multithreaded Windows FOR batch command

  - by Axarydax

  Hi, do you know if there is a simple way to run FOR command in batch file on multiple threads? What's the point of having 4 cores if I can't run my tasks in 4 parallel threads? For example, if I am optimizing PNGs with PNGOUT, the command I would use is for %i in (*.png) do pngout "%i" But this is highly paralellizable task in which the sub-tasks do not depend on each other at all. To run this in 4 'queues' I'd write something like for -thread 4 %i in (*.png) do pngout "%i" Do I need to write my own for-like app that would be able to do this or is there available free solution?

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• Jquery calculator

  - by Nemanja

  I want to make calculator for summation. I have this jquery code: <script type="text/javascript"> $(document).ready(function() { $("#calculate").click(function() { if ($("#input").val() != '' && $("#input").val() != undefined) { $("#result").html("Result is: " + parseInt($("#input").val()) * 30 ); } else { $("#result").html("Please enter some value"); } }); }); </script> <div id="calculator"> <br /> <input type="text" style="left:20px;" id="input" /> <div id="result"></div> <input type="button" id="calculate" value="calculate" style="left:20px;" /> </div> I want something like this: I write number 5, then input + from keyboard and write another value. Then to click on summation button and to get value. Can anyone help me with this? Thank you very much!

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• Windows 7: Use VirtualStore technology as user for own benefit?

  - by mfn

  I've researched a bit and came to the understanding that VirtualStore is part of the new UAC feature in Vista/W7 which is the file system part of the transparent data redirection and redirects write access to folders like program files to C:\User\<username>\AppData\Local\VirtualStore\ in lack of applications respecting the LUA principles. Now I'm interested if that kind of transparent redirection can also be used as a power to the user. Here's an example which comes to my mind: I install any kind of software to e.g. D:\Whatever\ThisAndThisApp\ and I set up things that, after initial installation, any write access to this folder is transparently redirected to e.g. D:\MyOwnVirtualStore\Whatever\ThisAndThisApp\file_only_writable_here.txt. Is this thinking too far or can I actually use that power of VirtualStore as a user on Windows 7? I'm using the Professional version of that matters.

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• How can I be certain that my code is flawless? [duplicate]

  - by David

  This question already has an answer here: Theoretically bug-free programs 5 answers I have just completed an exercise from my textbook which wanted me to write a program to check if a number is prime or not. I have tested it and seems to work fine, but how can I be certain that it will work for every prime number? public boolean isPrime(int n) { int divisor = 2; int limit = n-1 ; if (n == 2) { return true; } else { int mod = 0; while (divisor <= limit) { mod = n % divisor; if (mod == 0) { return false; } divisor++; } if (mod > 0) { return true; } } return false; } Note that this question is not a duplicate of Theoretically Bug Free Programs because that question asks about whether one can write bug free programs in the face of the the limitative results such as Turing's proof of the incomputability of halting, Rice's theorem and Godel's incompleteness theorems. This question asks how a program can be shown to be bug free.

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• How do you handle the task of changing the schema of a production MySQL database?

  - by Continuation

  One of the biggest complaints I have heard about MySQL is that it locks up a table if you try to change its schema like adding a column or adding an index. By "locking up the table" does it mean I can neither read nor write to the table? Sometimes for hours? That seems a pretty severe limitations. I was going to use MySQL for my new project but this gives me pause. Is there a workaround for this? How do you handle the task of changing the schema of your production MySQL database? By the way someone told me Postgresql doesn't have this problem. Is that true - I can both read and write to a Postgresql table while changing its schema? Is there any performance penalty incurred? Would love to hear your experiences.

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