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• Is there a way to get number of connections in Signalr hub group?

  - by pajo

  Here is my problem, I want to track if user is online or offline and notify other clients about it. I'm using hubs and implemented both IConnected and IDisconnect interfaces. My idea was to send notification to all clients when hub detects connect or disconnect. By default when user refreshes page he will get new connection id and eventually previous connection will call disconnect notifying other clients user is offline even though he's actually online. I tired to use my own ConnectionIdFactory returning username for connection id but with multiple tabs opened at some point it will detect user connectionid disconnected and after that client side hub will try to unsuccessfully connect to the hub in endless loop wasting memory and cpu making browser almost unusable. I needed to fix it fast so I removed my factory and now I add every new connection to the group using username, so I can easily notify single user on all connections, but then I have problem of detecting if user is online or offline as I don't know how many active connection user is having. So I'm wondering is there a way to get number of connections in one group? Or if anybody has some better idea how to track when user goes offline? I'm using Signalr 0.4

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• .net IHTTPHandler Streaming SQL Binary Data

  - by Yisman

  Hello everybody I am trying to implement an ihttphandeler for streaming files. files may be tiny thumbnails or gigantic movies the binaries r stored in sql server i looked at a lot of code online but something does not make sense isnt streaming supposed to read the data piece by piece and move it over the line? most of the code seems to first read the whole field from mssql to memory and then use streaming for the output writing wouldnt it b more eficient to actually stream from disk directly to http byte by byte (or buffered chunks?) heres my code so far but cant figure out the correct combination of the sqlreader mode and the stream object and the writing system Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest context.Response.BufferOutput = False Dim FileField=safeparam(context.Request.QueryString("FileField")) Dim FileTable=safeparam(context.Request.QueryString("FileTable")) Dim KeyField=safeparam(context.Request.QueryString("KeyField")) Dim FileKey=safeparam(context.Request.QueryString("FileKey")) Using connection As New SqlConnection(ConfigurationManager.ConnectionStrings("Main").ConnectionString) Using command As New SqlCommand("SELECT " & FileField & "Bytes," & FileField & "Type FROM " & FileTable & " WHERE " & KeyField & "=" & FileKey, connection) command.CommandType = Data.CommandType.Text enbd using end using end sub please be aware that this sql command also returns the file extension (pdf,jpg,doc...) in the second field of the query thank you all very much

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• Elegant way of parsing Data files for Simulation

  - by sc_ray

  I am working on this project where I need to read in a lot of data from .dat files and use the data to perform simulations. The data in my .dat file looks as follows: DeviceID InteractingDeviceID InteractionStartTime InteractionEndTime 1 2 1101 1105 1,2 1101 and 1105 are tab delimited and it means Device 1 interacted with Device 2 at 1101 ms and ended the interaction at 1105ms. I have a trace data sets that compile thousands of such interactions and my job is to analyze these interactions. The first step is to parse the file. The language of choice is C++. The approach I was thinking of taking was to read the file, for every line that's read create a Device Object. This Device object will contain the property DeviceId and an array/vector of structs, that will contain a list of all the devices the given DeviceId interacted with over the course of the simulation.The struct will contain the Interacting Device Id, Interaction Start Time and Interaction End Time. I have a two fold question here: Is my approach correct? If I am on the right track, how do I rapidly parse these tab delimited data files and create Device objects without excessive memory overhead using C++? A push in the right direction will be much appreciated. Thanks

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• Wait on multiple condition variables on Linux without unnecessary sleeps?

  - by Joseph Garvin

  I'm writing a latency sensitive app that in effect wants to wait on multiple condition variables at once. I've read before of several ways to get this functionality on Linux (apparently this is builtin on Windows), but none of them seem suitable for my app. The methods I know of are: Have one thread wait on each of the condition variables you want to wait on, which when woken will signal a single condition variable which you wait on instead. Cycling through multiple condition variables with a timed wait. Writing dummy bytes to files or pipes instead, and polling on those. #1 & #2 are unsuitable because they cause unnecessary sleeping. With #1, you have to wait for the dummy thread to wake up, then signal the real thread, then for the real thread to wake up, instead of the real thread just waking up to begin with -- the extra scheduler quantum spent on this actually matters for my app, and I'd prefer not to have to use a full fledged RTOS. #2 is even worse, you potentially spend N * timeout time asleep, or your timeout will be 0 in which case you never sleep (endlessly burning CPU and starving other threads is also bad). For #3, pipes are problematic because if the thread being 'signaled' is busy or even crashes (I'm in fact dealing with separate process rather than threads -- the mutexes and conditions would be stored in shared memory), then the writing thread will be stuck because the pipe's buffer will be full, as will any other clients. Files are problematic because you'd be growing it endlessly the longer the app ran. Is there a better way to do this? Curious for answers appropriate for Solaris as well.

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• MYSQL not running on Ubuntu OS - Error 2002.

  - by mgj

  Hi, I am a novice to mysql DB. I am trying to run the MYSQL Server on Ubuntu 10.04. Through Synaptic Package Manager I am have installed the mysql version: mysql-client-5.1 I wonder that how was the database password set for the mysql-client software that I installed through the above way.It would be nice if you could enlighten me on this. When I tried running this database, I encountered the error given below: mohnish@mohnish-laptop:/var/lib$ mysql ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) mohnish@mohnish-laptop:/var/lib$ I referred to a similar question posted by another user. I didn't find a solution through the proposed answers. For instance when I tried the solutions posted for the similar question I got the following: mohnish@mohnish-laptop:/var/lib$ service start mysqld start: unrecognized service mohnish@mohnish-laptop:/var/lib$ ps -u mysql ERROR: User name does not exist. ********* simple selection ********* ********* selection by list ********* -A all processes -C by command name -N negate selection -G by real group ID (supports names) -a all w/ tty except session leaders -U by real user ID (supports names) -d all except session leaders -g by session OR by effective group name -e all processes -p by process ID T all processes on this terminal -s processes in the sessions given a all w/ tty, including other users -t by tty g OBSOLETE -- DO NOT USE -u by effective user ID (supports names) r only running processes U processes for specified users x processes w/o controlling ttys t by tty *********** output format ********** *********** long options *********** -o,o user-defined -f full --Group --User --pid --cols --ppid -j,j job control s signal --group --user --sid --rows --info -O,O preloaded -o v virtual memory --cumulative --format --deselect -l,l long u user-oriented --sort --tty --forest --version -F extra full X registers --heading --no-heading --context ********* misc options ********* -V,V show version L list format codes f ASCII art forest -m,m,-L,-T,H threads S children in sum -y change -l format -M,Z security data c true command name -c scheduling class -w,w wide output n numeric WCHAN,UID -H process hierarchy mohnish@mohnish-laptop:/var/lib$ which mysql /usr/bin/mysql mohnish@mohnish-laptop:/var/lib$ mysql ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) I even tried referring to http://forums.mysql.com/read.php?11,27769,84713#msg-84713 but couldn't find anything useful. Please let me know how I could tackle this error. Thank you very much..

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• Using ember-resource with couchdb - how can i save my documents?

  - by Thomas Herrmann

  I am implementing an application using ember.js and couchdb. I choose ember-resource as database access layer because it nicely supports nested JSON documents. Since couchdb uses the attribute _rev for optimistic locking in every document, this attribute has to be updated in my application after saving the data to the couchdb. My idea to implement this is to reload the data right after saving to the database and get the new _rev back with the rest of the document. Here is my code for this: // Since we use CouchDB, we have to make sure that we invalidate and re-fetch // every document right after saving it. CouchDB uses an optimistic locking // scheme based on the attribute "_rev" in the documents, so we reload it in // order to have the correct _rev value. didSave: function() { this._super.apply(this, arguments); this.forceReload(); }, // reload resource after save is done, expire to make reload really do something forceReload: function() { this.expire(); // Everything OK up to this location Ember.run.next(this, function() { this.fetch() // Sub-Document is reset here, and *not* refetched! .fail(function(error) { App.displayError(error); }) .done(function() { App.log("App.Resource.forceReload fetch done, got revision " + self.get('_rev')); }); }); } This works for most cases, but if i have a nested model, the sub-model is replaced with the old version of the data just before the fetch is executed! Interestingly enough, the correct (updated) data is stored in the database and the wrong (old) data is in the memory model after the fetch, although the _rev attribut is correct (as well as all attributes of the main object). Here is a part of my object definition: App.TaskDefinition = App.Resource.define({ url: App.dbPrefix + 'courseware', schema: { id: String, _rev: String, type: String, name: String, comment: String, task: { type: 'App.Task', nested: true } } }); App.Task = App.Resource.define({ schema: { id: String, title: String, description: String, startImmediate: Boolean, holdOnComment: Boolean, ..... // other attributes and sub-objects } }); Any ideas where the problem might be? Thank's a lot for any suggestion! Kind regards, Thomas

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• Please critique this method

  - by Jakob

  Hi I've been looking around the net for some tab button close functionality, but all those solutions had some complicated eventhandler, and i wanted to try and keep it simple, but I might have broken good code ethics doing so, so please review this method and tell me what is wrong. public void AddCloseItem(string header, object content){ //Create tabitem with header and content StackPanel headerPanel = new StackPanel() { Orientation = Orientation.Horizontal, Height = 14}; headerPanel.Children.Add(new TextBlock() { Text = header }); Button closeBtn = new Button() { Content = new Image() { Source = new BitmapImage(new Uri("images/cross.png", UriKind.Relative)) }, Margin = new Thickness() { Left = 10 } }; headerPanel.Children.Add(closeBtn); TabItem newTabItem = new TabItem() { Header = headerPanel, Content = content }; //Add close button functionality closeBtn.Tag = newTabItem; closeBtn.Click += new RoutedEventHandler(closeBtn_Click); //Add item to list this.Add(newTabItem); } void closeBtn_Click(object sender, RoutedEventArgs e) { this.Remove((TabItem)((Button)sender).Tag); } So what I'm doing is storing the tabitem in the btn.Tag property, and then when the button is clicked i just remove the tabitem from my observablecollection, and the UI is updated appropriately. Am I using too much memory saving the tabitem to the Tag property?

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• how to implement a really efficient bitvector sorting in python

  - by xiao

  Hello guys! Actually this is an interesting topic from programming pearls, sorting 10 digits telephone numbers in a limited memory with an efficient algorithm. You can find the whole story here What I am interested in is just how fast the implementation could be in python. I have done a naive implementation with the module bitvector. The code is as following: from BitVector import BitVector import timeit import random import time import sys def sort(input_li): return sorted(input_li) def vec_sort(input_li): bv = BitVector( size = len(input_li) ) for i in input_li: bv[i] = 1 res_li = [] for i in range(len(bv)): if bv[i]: res_li.append(i) return res_li if __name__ == "__main__": test_data = range(int(sys.argv[1])) print 'test_data size is:', sys.argv[1] random.shuffle(test_data) start = time.time() sort(test_data) elapsed = (time.time() - start) print "sort function takes " + str(elapsed) start = time.time() vec_sort(test_data) elapsed = (time.time() - start) print "sort function takes " + str(elapsed) start = time.time() vec_sort(test_data) elapsed = (time.time() - start) print "vec_sort function takes " + str(elapsed) I have tested from array size 100 to 10,000,000 in my macbook(2GHz Intel Core 2 Duo 2GB SDRAM), the result is as following: test_data size is: 1000 sort function takes 0.000274896621704 vec_sort function takes 0.00383687019348 test_data size is: 10000 sort function takes 0.00380706787109 vec_sort function takes 0.0371489524841 test_data size is: 100000 sort function takes 0.0520560741425 vec_sort function takes 0.374383926392 test_data size is: 1000000 sort function takes 0.867373943329 vec_sort function takes 3.80475401878 test_data size is: 10000000 sort function takes 12.9204008579 vec_sort function takes 38.8053860664 What disappoints me is that even when the test_data size is 100,000,000, the sort function is still faster than vec_sort. Is there any way to accelerate the vec_sort function?

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• How do I store a string longer than 4000 characters in an Oracle Database using Java/JDBC?

  - by Ventrue

  I’m not sure how to use Java/JDBC to insert a very long string into an Oracle database. I have a String which is greater than 4000 characters, lets say it’s 6000. I want to take this string and store it in an Oracle database. The way to do this seems to be with the CLOB datatype. Okay, so I declared the column as description CLOB. Now, when it comes time to actually insert the data, I have a prepared statement pstmt. It looks like pstmt = conn.prepareStatement(“INSERT INTO Table VALUES(?)”). So I want to use the method pstmt.setClob(). However, I don’t know how to create a Clob object with my String in it; there’s no constructor (presumably because it can be potentially much larger than available memory). How do I put my String into a Clob? Keep in mind I’m not a very experienced programmer; please try to keep the explanations as simple as possible. Efficiency, good practices, etc. are not a concern here, I just want the absolute easiest solution. I’d like to avoid downloading other packages if it all possible; right now I’m just using JDK 1.4 and what is labelled “ojdbc14.jar”. I've looked around a bit but I haven't been able to follow any of the explanations I've found. If you have a solution that doesn’t use Clobs, I’d be open to that as well, but it has to be one column.

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• document.onclick settimeout function javascript help

  - by Jamex

  Hi, I have a document.onclick function that I would like to have a delay. I can't seem to get the syntax right. my original code is <script type="text/javascript"> document.onclick=check; function check(e){do something} I tried the below, but that code is incorrect, the function did not execute and nothing happened. <script type="text/javascript"> document.onclick=setTimeout("check", 1000); function check(e){do something} I tried the next set, the function got executed, but no delay. <script type="text/javascript"> setTimeout(document.onclick=check, 1000); function check(e){do something} what is the correct syntax for this code. TIA Edit: The solutions were all good, my problem was that I use the function check to obtain the id of the element being clicked on. But after the delay, there is no "memory" of what was being clicked on, so the rest of the function does not get executed. Jimr wrote the short code to preserve clicked event. The code that is working is // Delay execution of event handler function "f" by "time" ms. document.onclick = makeDelayedHandler(check, 250); function makeDelayedHandler( f, time) { return function( e ) {setTimeout(function() {f( e );}, time ); }; } function check(e){ var click = (e && e.target) || (event && event.srcElement); . . . Thank you all.

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• Divide and conquer of large objects for GC performance

  - by Aperion

  At my work we're discussing different approaches to cleaning up a large amount of managed ~50-100MB memory.There are two approaches on the table (read: two senior devs can't agree) and not having the experience the rest of the team is unsure of what approach is more desirable, performance or maintainability. The data being collected is many small items, ~30000 which in turn contains other items, all objects are managed. There is a lot of references between these objects including event handlers but not to outside objects. We'll call this large group of objects and references as a single entity called a blob. Approach #1: Make sure all references to objects in the blob are severed and let the GC handle the blob and all the connections. Approach #2: Implement IDisposable on these objects then call dispose on these objects and set references to Nothing and remove handlers. The theory behind the second approach is since the large longer lived objects take longer to cleanup in the GC. So, by cutting the large objects into smaller bite size morsels the garbage collector will processes them faster, thus a performance gain. So I think the basic question is this: Does breaking apart large groups of interconnected objects optimize data for garbage collection or is better to keep them together and rely on the garbage collection algorithms to processes the data for you? I feel this is a case of pre-optimization, but I do not know enough of the GC to know what does help or hinder it.

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• Apache module, is it possible to have asynchronous processing

  - by prashant2361

  Hi, I have a requirement where I need to send continous updates to my clients. Client is browser in this case. We have some data which updates every sec, so once client connects to our server, we maintain a persistent connection and keep pushing data to the client. I am looking for suggestions of this implementation at the server end. Basically what I need is this: 1. client connects to server. I maintain the socket and metadata about the socket. metadata contains what updates need to be send to this client 2. server process now waits for new client connections 3. One other process will have the list of all the sockets opened and will go through each of them and send the updates if required. Can we do something like this in apache module: 1. apache process gets the new connection. It maintains the state for the connection. It keeps the state in some global memory and returns back to root process to signify that it is done so that it can accept the new connection 2. the apache process though has returned the status to root process but it is also executing parallely where it going through its global store and sending updates to the client, if any. So can a apache process do these things: 1. Have more than one connection associated with it 2. Asynchronously waiting for new connection and at the same time processing the previous connections? Regards Prashant

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• Django caching seems to be causing problems

  - by Issy

  Hey guys, i have just implemented the Django Cache Local Memory back end in some my code, however it seems to be causing a problem. I get the following error when trying to view the site (With Debug On): Traceback (most recent call last): File "/usr/lib/python2.6/dist-packages/django/core/servers/basehttp.py", line 279, in run self.result = application(self.environ, self.start_response) File "/usr/lib/python2.6/dist-packages/django/core/servers/basehttp.py", line 651, in __call__ return self.application(environ, start_response) File "/usr/lib/python2.6/dist-packages/django/core/handlers/wsgi.py", line 245, in __call__ response = middleware_method(request, response) File "/usr/lib/python2.6/dist-packages/django/middleware/cache.py", line 91, in process_response patch_response_headers(response, timeout) File "/usr/lib/python2.6/dist-packages/django/utils/cache.py", line 112, in patch_response_headers response['Expires'] = http_date(time.time() + cache_timeout) TypeError: unsupported operand type(s) for +: 'float' and 'str' I have checked my code, for caching everything seems to be ok. For example, i have the following in my middleware. MIDDLEWARE_CLASSES = ( 'django.contrib.sessions.middleware.SessionMiddleware', 'django.contrib.auth.middleware.AuthenticationMiddleware', 'django.middleware.cache.UpdateCacheMiddleware', 'django.middleware.common.CommonMiddleware', 'django.middleware.cache.FetchFromCacheMiddleware', 'django.contrib.flatpages.middleware.FlatpageFallbackMiddleware', ) My settings for Cache: CACHE_BACKEND = 'locmem://' CACHE_MIDDLEWARE_SECONDS = '3600' CACHE_MIDDLEWARE_KEY_PREFIX = 'za' CACHE_MIDDLEWARE_ANONYMOUS_ONLY = True And some of my code (template tag): def get_featured_images(): """ provides featured images """ cache_key = 'featured_images' images = cache.get(cache_key) if images is None: images = FeaturedImage.objects.all().filter(enabled=True)[:5] cache.set(cache_key, images) return {'images': images} Any idea what could be the problem, from the error message below it looks like there's an issue in django's cache.py?

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• Getting a nicely formatted timestamp without lots of overhead?

  - by Brad Hein

  In my app I have a textView which contains real-time messages from my app, as things happen, messages get printed to this text box. Each message is time-stamped with HH:MM:SS. Up to now, I had also been chasing what seemed to be a memory leak, but as it turns out, it's just my time-stamp formatting method (see below), It apparently produces thousands of objects that later get gc'd. For 1-10 messages per second, I was seeing 500k-2MB of garbage collected every second by the GC while this method was in place. After removing it, no more garbage problem (its back to a nice interval of about 30 seconds, and only a few k of junk typically) So I'm looking for a new, more lightweight method for producing a HH:MM:SS timestamp string :) Old code: /** * Returns a string containing the current time stamp. * @return - a string. */ public static String currentTimeStamp() { String ret = ""; Date d = new Date(); SimpleDateFormat timeStampFormatter = new SimpleDateFormat("hh:mm:ss"); ret = timeStampFormatter.format(d); return ret; }

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• How to accommodate for the iPhone 4 screen resolution?

  - by dontWatchMyProfile

  This is a programming question! Read on before you vote to close! According to Apple, the iPhone 4 has a new and better screen resolution: 3.5-inch (diagonal) widescreen Multi-Touch display 960-by-640-pixel resolution at 326 ppi This little detail affects our apps in a heavy way. Most of the demo apps on the net have one thing in common: They position views in the believe that the screen has a fixed size of 320 x 480 pixels. So what most -if not all- developers do is: They designed everything in such a way, that a touchable area is -for example- 50 x 50 pixels big. Just enough to tap it. Things have been positioned relative to the upper left, to reach a specific position on screen - let's say the center, or somewhere at the bottom. Edit: It seems Apple has integrated an switch that allows to tell if an app is highRes or not. Nice. When we develop high-resolution apps, they probably won't work on older devices. And if they do, they would suffer a lot from 4-times the size of any image, having to scale them down in memory.

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• Associating an object with another object for GC clearup

  - by thecoop

  Is there any way of associating an object instance (object A) with a second object (object B) in a generalised way, so that when B gets collected A becomes eligable for collection? The same behaviour that would happen if B had an instance variable pointing to A, but without explicitly changing the class definition of B, and being able to do this in a dynamic way? The same sort of effect could be done by using the Component.Disposed event in a funky way, but I don't want to make B disposable EDIT I'm basically creating a cache of objects that are associated with a single 'root' object, and I don't want the cache to be static, as there can be lots of root objects using different caches, so lots of memory will be used up when a root object is no longer used but the cached objects are still around. So, I want a collection of cached objects to be associated with each 'root' object, without changing the root object definition. Sort of like metadata of an extra object reference attached to each root object instance. That way, each collection will get collected when the root object is collected, and not hang around like they would if a static cache was used.

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• Accessing Members of Containing Objects from Contained Objects.

  - by Bunkai.Satori

  If I have several levels of object containment (one object defines and instantiates another object which define and instantiate another object..), is it possible to get access to upper, containing - object variables and functions, please? Example: class CObjectOne { public: CObjectOne::CObjectOne() { Create(); }; void Create(); std::vector<ObjectTwo>vObejctsTwo; int nVariableOne; } bool CObjectOne::Create() { CObjectTwo ObjectTwo(this); vObjectsTwo.push_back(ObjectTwo); } class CObjectTwo { public: CObjectTwo::CObjectTwo(CObjectOne* pObject) { pObjectOne = pObject; Create(); }; void Create(); CObjectOne* GetObjectOne(){return pObjectOne;}; std::vector<CObjectTrhee>vObjectsTrhee; CObjectOne* pObjectOne; int nVariableTwo; } bool CObjectTwo::Create() { CObjectThree ObjectThree(this); vObjectsThree.push_back(ObjectThree); } class CObjectThree { public: CObjectThree::CObjectThree(CObjectTwo* pObject) { pObjectTwo = pObject; Create(); }; void Create(); CObjectTwo* GetObjectTwo(){return pObjectTwo;}; std::vector<CObjectsFour>vObjectsFour; CObjectTwo* pObjectTwo; int nVariableThree; } bool CObjectThree::Create() { CObjectFour ObjectFour(this); vObjectsFour.push_back(ObjectFour); } main() { CObjectOne myObject1; } Say, that from within CObjectThree I need to access nVariableOne in CObjectOne. I would like to do it as follows: int nValue = vObjectThree[index].GetObjectTwo()->GetObjectOne()->nVariable1; However, after compiling and running my application, I get Memory Access Violation error. What is wrong with the code above(it is example, and might contain spelling mistakes)? Do I have to create the objects dynamically instead of statically? Is there any other way how to achieve variables stored in containing objects from withing contained objects?

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• Putting BigDecimal data into HSQLDB test database using DbUnit

  - by Denise

  Hi everyone, I'm using Hibernate JPA in my backend. I am writing a unit test using JUnit and DBUnit to insert a set of data into an in-memory HSQL database. My dataset contains: <order_line order_line_id="1" quantity="2" discount_price="0.3"/> Which maps to an OrderLine Java object where the discount_price column is defined as: @Column(name = "discount_price", precision = 12, scale = 2) private BigDecimal discountPrice; However, when I run my test case and assert that the discount price returned equals 0.3, the assertion fails and says that the stored value is 0. If I change the discount_price in the dataset to be 0.9, it rounds up to 1. I've checked to make sure HSQLDB isn't doing the rounding and it definitely isn't because I can insert an order line object using Java code with a value like 5.3 and it works fine. To me, it seems like DBUtils is for some reason rounding the number I've defined. Is there a way I can force this to not happen? Can anyone explain why it might be doing this? Thanks!

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• Several Objective-C objects become Invalid for no reason, sometimes.

  - by farnsworth

  - (void)loadLocations { NSString *url = @"<URL to a text file>"; NSStringEncoding enc = NSUTF8StringEncoding; NSString *locationString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString:url] usedEncoding:&enc error:nil]; NSArray *lines = [locationString componentsSeparatedByString:@"\n"]; for (int i=0; i<[lines count]; i++) { NSString *line = [lines objectAtIndex:i]; NSArray *components = [line componentsSeparatedByString:@", "]; Restaurant *res = [byID objectForKey:[components objectAtIndex:0]]; if (res) { NSString *resAddress = [components objectAtIndex:3]; NSArray *loc = [NSArray arrayWithObjects:[components objectAtIndex:1], [components objectAtIndex:2]]; [res.locationCoords setObject:loc forKey:resAddress]; } else { NSLog([[components objectAtIndex:0] stringByAppendingString:@" res id not found."]); } } } There are a few weird things happening here. First, at the two lines where the NSArray lines is used, this message is printed to the console- *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSCFDictionary count]: method sent to an uninitialized mutable dictionary object' which is strange since lines is definitely not an NSMutableDictionary, definitely is initialized, and because the app doesn't crash. Also, at random points in the loop, all of the variables that the debugger can see will become Invalid. Local variables, property variables, everything. Then after a couple lines they will go back to their original values. setObject:forKey never has an effect on res.locationCoords, which is an NSMutableDictionary. I'm sure that res, res.locationCoords, and byId are initialized. I also tried adding a retain or copy to lines, same thing. I'm sure there's a basic memory management principle I'm missing here but I'm at a loss.

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• how to implement a sparse_vector class

  - by Neil G

  I am implementing a templated sparse_vector class. It's like a vector, but it only stores elements that are different from their default constructed value. So, sparse_vector would store the index-value pairs for all indices whose value is not T(). I am basing my implementation on existing sparse vectors in numeric libraries-- though mine will handle non-numeric types T as well. I looked at boost::numeric::ublas::coordinate_vector and eigen::SparseVector. Both store: size_t* indices_; // a dynamic array T* values_; // a dynamic array int size_; int capacity_; Why don't they simply use vector<pair<size_t, T>> data_; My main question is what are the pros and cons of both systems, and which is ultimately better? The vector of pairs manages size_ and capacity_ for you, and simplifies the accompanying iterator classes; it also has one memory block instead of two, so it incurs half the reallocations, and might have better locality of reference. The other solution might search more quickly since the cache lines fill up with only index data during a search. There might also be some alignment advantages if T is an 8-byte type? It seems to me that vector of pairs is the better solution, yet both containers chose the other solution. Why?

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• Special technology needed for browser based chat?

  - by orokusaki

  On this post, I read about the usage of XMPP. Is this sort of thing necessary, and more importantly, my main question expanded: Can a chat server and client be built efficiently using only standard HTTP and browser technologies (such as PHP and JS, or RoR and JS, etc)? Or, is it best to stick with old protocols like XMPP find a way to integrate them with my application? I looked into CampFire via LiveHTTPHeaders and Firebug for about 5 minutes, and it appears to use Ajax to send a request which is never answered until another chat happens. Is this just CampFire opening a new thread on the server to listen for an update and then returning a response to the request when the thread hears an update? I noticed that they're requesting on a specific port (8043 if memory serves me) which makes me think that they're doing something more complex than just what I mentioned. Also, the URL requested started with /tcp/ which I found interesting. Note: I don't expect to ever have more than 150 users live-chatting in all the rooms combined at the same time. I understand that if I was building a hosted pay for chat service like CampFire with thousands of concurrent users, it would behoove me to invest time in researching special technologies vs trying to reinvent the wheel in a simple way in my app. Also, if you're going to do it with server polling, how often would you personally poll to maximize response without slamming the server?

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• Segmentation in Linux : Segmentation & Paging are redundant?

  - by claws

  Hello, I'm reading "Understanding Linux Kernel". This is the snippet that explains how Linux uses Segmentation which I didn't understand. Segmentation has been included in 80 x 86 microprocessors to encourage programmers to split their applications into logically related entities, such as subroutines or global and local data areas. However, Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be used to separate the physical address spaces of processes: segmentation can assign a different linear address space to each process, while paging can map the same linear address space into different physical address spaces. Linux prefers paging to segmentation for the following reasons: Memory management is simpler when all processes use the same segment register values that is, when they share the same set of linear addresses. One of the design objectives of Linux is portability to a wide range of architectures; RISC architectures in particular have limited support for segmentation. All Linux processes running in User Mode use the same pair of segments to address instructions and data. These segments are called user code segment and user data segment , respectively. Similarly, all Linux processes running in Kernel Mode use the same pair of segments to address instructions and data: they are called kernel code segment and kernel data segment , respectively. Table 2-3 shows the values of the Segment Descriptor fields for these four crucial segments. I'm unable to understand 1st and last paragraph.

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• Thoughts on GoGrid vs EC2

  - by Jason

  I am currently hosting my SaaS application at GoGrid (Microsoft stack). Here's what I have: Database Server - physical box, 12 GB RAM, 2 X Quad Core CPU (2.13 GHz Xeon E5506) 2 Web / App servers - cloud servers, 2 GB RAM, 2 VCPUs 300 GB monthly bandwidth I am paying around $900 / month for this. My web / app servers are busting at the seams and need to be upgraded to 4 GB of RAM. I also need a firewall, and GoGrid just added this service for an additional $200. After the upgrade, I will be paying around $1,400. I started looking at Amazon EC2, specifically this config: Database server - "High Memory Double Extra Large Instance" - 34 GB RAM, 13 EC2 compute units 2 Web / App servers - "Large Instance" - 7.5 GB RAM, 4 EC2 compute units If I go with 1 year reserved instances, my upfront cost would be $4,500 and my monthly would be $700. This comes to $1,075 / month when amortized. Amazon also includes a firewall for free. Here are my questions: Do any of you have experience running a database (especially SQL Server) on an EC2 instance? How did it perform compared to a dedicated machine? One of my major concerns is with disk I/O. Amazon's description of a compute unit is fairly vague. Any ideas on how the CPU performance on the database servers would compare? I am hoping that the Amazon solution will provide significantly better performance than my current or even improved GoGrid setup. Having a virtual database server would also be nice in terms of availability. Right now I would be in serious trouble if I had any hardware issues. Thanks for any insight...

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• Doubts about .NET Garbage Collector

  - by Smjert

  I've read some docs about the .NET Garbage Collector but i still have some doubts (examples in C#): 1)Does GC.Collect() call a partial or a full collection? 2)Does a partial collection block the execution of the "victim" application? If yes.. then i suppose this is a very "light" things to do since i'm running a game server that uses 2-3GB of memory and i "never" have execution stops (or i can't see them..). 3)I've read about GC roots but still can't understand how exactly they works. Suppose that this is the code (C#): MyClass1: [...] public List<MyClass2> classList = new List<MyClass2>(); [...] Main: main() { MyClass1 a = new MyClass1(); MyClass2 b = new MyClass2(); a.classList.Add(b); b = null; DoSomeLongWork(); } Will b ever be eligible to be garbage collected(before the DoSomeLongWork finishes)? The reference to b that classList contains, can it be considered a root? Or a root is only the first reference to the instance? (i mean, b is the root reference because the instantiation happens there).

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• ColdFusion 8 Slow Performance

  - by JoeBob

  We have started a new CF8 app and it is running dog slow. A test where we go around ColdFusion (queries within a database utility) show normal speed (80ms). CF8 returns the same query in something like 60 to 80 seconds! I have been looking online and seeing lots of posts about CF8 and performance problems, but don't get any overall sense of a solution; just lots of people trying things and saying that they didn't have the problem with CF7. We are also seeing instability on the server, and some errors relating to garbage collection and the memory heap. We have a number of other applications running on CF8 and they perform adequately...our programmer is not an expert or a guru, he just plugs away. We have isolated this down to a single query that takes forever to return, so it is not a complicated test. Are there any known CF8 problems or obvious tweaks that we should consider trying? If we have to start over and learn a new environment, I will never make deadline. JoeBob

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