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• Linux server apache httpd processes take i/o wait to close to 100% and lock down server

  - by user3682065

  For about 5 days now, and seemingly out of the blue, my linux server has started locking up from time to time. The pattern is always the same as far as I can tell from top and iotop commands around the time it starts happening: One or more httpd processes (usually one) hang and start using up 100% of CPU power, the %wa goes close to 100% and in the iotop I see several httpd processes with 99.99% in the IO column. I'm also running an SVN server on this machine through apache and the one way that I've been consistently able to reproduce this is to do an SVN commit of new files or an SVN update from the repository on this server (I am the only one using this SVN repository). This will always reproduce this scenario successfully, but until very recently I had no problems at all checking in/out of SVN. But sometimes it just happens for no detectable reason at all it seems. So it seems like there is some issue with my Apache that leads it to have processes use up a lot of read/write upon certain triggers. I was wondering if anyone could help me uncover that issue. EDIT: OK now it's happening again: This is top: [root@server ~]# top top - 10:56:54 up 2:59, 5 users, load average: 171.46, 70.35, 27.01 Tasks: 328 total, 2 running, 326 sleeping, 0 stopped, 0 zombie Cpu(s): 1.9%us, 2.0%sy, 0.0%ni, 0.0%id, 96.1%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 2021144k total, 1968192k used, 52952k free, 2500k buffers Swap: 4194288k total, 2938584k used, 1255704k free, 39008k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 10390 apache 20 0 2774m 936m 6200 D 2.0 47.4 1:52.27 httpd 2149 root 20 0 927m 13m 1040 S 0.7 0.7 1:50.46 namecoind 11 root 20 0 0 0 0 R 0.3 0.0 0:30.10 events/0 23 root 20 0 0 0 0 S 0.3 0.0 0:17.88 kblockd/1 2049 root 20 0 382m 4932 2880 D 0.3 0.2 0:03.67 httpd 2144 root 20 0 1702m 69m 1164 S 0.3 3.5 5:19.68 bitcoind 6325 root 20 0 15164 1100 656 R 0.3 0.1 0:11.09 top 10311 apache 20 0 387m 9496 7320 D 0.3 0.5 0:01.89 httpd 10313 apache 20 0 391m 10m 7364 D 0.3 0.5 0:02.40 httpd 10466 apache 20 0 399m 12m 7392 D 0.3 0.7 0:02.41 httpd 10599 apache 20 0 391m 9324 7340 D 0.3 0.5 0:00.15 httpd 10628 apache 20 0 384m 7620 4052 D 0.3 0.4 0:00.01 httpd 10633 apache 20 0 384m 7048 3504 D 0.3 0.3 0:00.01 httpd 10634 apache 20 0 384m 8012 4048 D 0.3 0.4 0:00.02 httpd 10638 apache 20 0 400m 22m 9.8m D 0.3 1.1 0:01.93 httpd 10640 apache 20 0 385m 8288 4028 D 0.3 0.4 0:00.03 httpd 10641 apache 20 0 401m 21m 6376 D 0.3 1.1 0:01.45 httpd 10759 apache 20 0 385m 8816 3480 D 0.3 0.4 0:01.45 httpd 10773 apache 20 0 384m 8044 3464 D 0.3 0.4 0:00.02 httpd This is an iotop snapshot: Total DISK READ: 5.93 M/s | Total DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND 10732 be/4 apache 3.76 K/s 0.00 B/s 0.00 % 58.48 % httpd 876 be/3 root 0.00 B/s 52.68 K/s 0.00 % 52.98 % [jbd2/dm-1-8] 10906 be/4 root 124.17 K/s 0.00 B/s 0.00 % 23.03 % sh -c [ -x /usr/local/psa/admin/sbin/backupmng ] && /usr/local/psa/admin/sbin/backupmng >/dev/null 2>&1 2156 be/4 root 206.94 K/s 0.00 B/s 0.00 % 21.15 % bitcoind 10904 be/4 mysql 0.00 B/s 0.00 B/s 0.00 % 18.94 % mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 10773 be/4 apache 7.53 K/s 0.00 B/s 0.00 % 14.77 % httpd 10641 be/4 apache 15.05 K/s 0.00 B/s 0.00 % 11.57 % httpd 10399 be/4 apache 1057.29 K/s 0.00 B/s 43.16 % 10.56 % httpd 10682 be/4 sw-cp-se 158.03 K/s 0.00 B/s 0.00 % 7.45 % sw-engine-cgi -c /usr/local/psa/admin/conf/php.ini -d auto_prepend_file=auth.php3 -u psaadm 10774 be/4 apache 3.76 K/s 0.00 B/s 0.00 % 6.53 % httpd 10624 be/4 apache 0.00 B/s 0.00 B/s 0.00 % 5.53 % httpd 10356 be/4 apache 899.26 K/s 0.00 B/s 35.52 % 4.01 % httpd 10795 be/4 apache 0.00 B/s 0.00 B/s 0.00 % 3.93 % httpd 10804 be/4 apache 7.53 K/s 0.00 B/s 0.00 % 3.08 % httpd 4379 be/4 root 2.89 M/s 0.00 B/s 99.99 % 0.00 % namecoind 10619 be/4 apache 462.80 K/s 0.00 B/s 7.80 % 0.00 % httpd 10636 be/4 apache 3.76 K/s 0.00 B/s 0.00 % 0.00 % httpd 10716 be/4 mysql 105.35 K/s 0.00 B/s 5.92 % 0.00 % mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 1988 be/4 root 18.81 K/s 0.00 B/s 0.00 % 0.00 % spamd_full.sock I also ran lsof -p for pid 10390 which was way up top under the top command and this is the bottom line where I can sort of see what request this was and it says CLOSE_WAIT: httpd 10390 apache 34u IPv6 315879 0t0 TCP default-domain.com:https->crawl-66-249-65-91.googlebot.com:42907 (CLOSE_WAIT) I'm still not sure what exactly is causing this all to happen though? I killed that service but %wa and load average remain high, I also stopped mysqld and other services. It really only goes down once I stop httpd altogether, and even then I can't start it without finding remaining hanging httpd processes via "netstat -tulpn", killing those or doing "killall -9 httpd" and after waiting a while for it to cycle through all those then doing /etc/init.d/httpd start

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• how to install vlc player from tar file

  - by mohit

  After getting vlc-2.0.3tar.gz I extracted it and then tried the ./install, but it gave me an error. Then i did ./configure and got this.. configure: error: No package 'dbus-1' found. Then tried this: apt-get install dbus-1 But I got this: E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable) E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it? What do I do next?

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• unable to add/remove program in Ubuntu 12.04 LTS?

  - by Manish Kumar Chauhan

  ** my problem is as following: unable to add/remove any program using either update-manager or Synaptic Package Manager or terminal update-manager is asking for partial upgrade and while updating software-center 5.6.2 catalog , there is no progress beyond the line "this is may take a moment" synaptic is unable to obtain an exclusive lock, similarly can't do terminal command sudo apt-get update if i try to break down the lock using the command sudo fuser -cuk /var/lib/dpkg/lock; sudo rm -f /var/lib/dpkg/lock it turns off my monitor display and i have to restart the whole system. note: this whole trouble started ,when i found ubuntu software-center missing after adding a repository and reinstalled it. **

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• How can I drag-drop files-photos to blu ray burner?

  - by CCWOW

  I found it not possible to format a bluray disk and store misc files to it, why

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• Did Acronis True Image Home 2012 clone operation "lock" my BitLocker disk?

  - by Thomas Eyde

  After trying to clone my primary drive, which requires a reboot to proceed, BitLocker detected that the boot sector had changed and required the recovery keys. Not a good situation to be in on Sunday evening when it is your IT-department who hold the keys. After talking with the guys at IT, I have tree suspects on why BitLocker locked me out: The newly added, second hard drive in my machine. The changed boot-order in BIOS. The changes True Image did in order to do its things on boot. As an end-user, I should never have to end up in situations like these without any warning. I would like to know what really happened, so I can inform Acronis on how they can improve their product to avoid this situation in the future.

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• 11.10 runs really choppy and slow, but not if I choose "Gnome Classic"

  - by Ingram

  I'd like to use Unity with Ubuntu 11.10, but everything from Unity runs really choppy and slow. I have an ATI graphics card and I have installed the drivers through "Additional Drivers". The drivers work, as I can play 3d games flawlessly. When I drag the mouse box around or drag windows around, it is really choppy. I previously had Ubuntu 10.10 and everything worked fine. I installed gnome 3 on 11.10 and it does the same thing Unity does, very choppy and slow graphics. However, if I choose Gnome Classic, everything is fine. I can drag the mouse box all around with no problems. I can drag windows around and it looks and feels great. Is this a bug with Unity? Are others experiencing this? Or is there something I can do to fix this?

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• Number of routers in small community lock up and require reboot.

  - by Anthony Hiscox

  I live in a small town which has one primary ISP. Lately I have noticed that a number of wireless routers have been locking up and requiring a reboot before allowing any connections. This has affected two of my routers, my work router, and a few others. In all cases wired continued to function as usual. Often wireless clients can see the SSID but simply won't connect. I can only think of a few possibilities and was hoping someone here might be able to point me in the right direction: Our ISP is well known to be flaky, something they are doing is causing this, what that might be I have no clue it as seems to affect the wireless only. There's a power issue in town, given our remote location and reputation for crap electrical, this seems reasonable. Only one router was plugged in to a UPS, and I'm not sure of the quality. There is some bug in all the different firmware for every one of these routers (all different). That doesn't seem reasonable, unless; it's an unknown (or known) exploit or DoS of some sort being launched by a massive team of ninjas hell bent on forcing us all to be tethered to our walls by ethernet cables or; it's just been a coincidence and I'm just paranoid (this has some weight, I mean read 4 again). Anyone else experience similar issues and have some tips?

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• Weebly Websites SEO

  - by etangins

  From what I understand Weebly uses the drag and drop interface and even when looking at the code, it doesn't show the full content, but rather shows {content} which is where the drag and drop parts of the content are put. Does their drag and drop content, such as text have the same effect as a <p> or <h1> tag would on a website? Is the weebly drag and drop less optimized for SEO? Does using Weebly have an adverse affect on SEO compared with building from scratch if I do include keywords, alternate text etc...?

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• How to identify cause of system lock on debian squeeze?

  - by David Sanders

  I have a desktop system at work with debian squeeze installed. Every couple days or so, when I come in in the morning, the system appears to be asleep and won't wake up. I suspect it goes into power save mode and then doesn't receive the wake up signal when I click the mouse or touch the keyboard. Does anyone know where I should look in order to start tracking this down? Like log files, etc? I doubt it, but maybe the fact that I'm using a mac aluminum usb keyboard and a wireless usb mouse (that has an off switch) has something to do with it? Maybe, when I turn my mouse off at the end of the day, the system loses track of the device somehow? I dunno :P. Thanks in advance.

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• Unable to remove fs icons - package

  - by Muhammad

  I tried to remove fs icons from my system and i get the error E: Could not get lock /var/lib/dpkg/lock - open (11 Resource temporarily unavailable) E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it? what is the problem???..... Note: After downloading the package i couldn't fin it in my Downloads. I used the following command sudo apt-get remove fs-icons-ubuntu

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• Lock down PC so only a few programs can be used?

  - by user1413

  I'm interested in locking down my PC so that only a few programs can be used. For example, when I'm writing a paper I only want to be able to use Word. No Firefox or DVD player because I will be distracted. I want to have various modes (one for writing a paper, another for doing taxes) that will prevent me from using any program except for the ones I need to perform a task. Is there a program that will do that? Can be free or commercial.

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• Why does Explorer lock up indefinitely when I try to access a particular file?

  - by NoCatharsis

  I'm working in XP Pro and I've never seen this problem. When I try to open a file, Explorer locks up and I have to shut it down from the task manager. When I right-click, the only options in the context menu are Print, Open With..., and Send To. No Delete, no cut/copy/paste, etc.

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• How can I lock a dictionary in debian server installed with ngix?

  - by Tin Aung Linn

  I tried so many methods and get stick hours with this.I edit /etc/nginx/nginx.conf and write these lines. location /home/user/domains/example.com/public_html/lockfolder/ { auth_basic "Restricted"; auth_basic_user_file /home/user/domains/example.com/.htpasswd; } and I use crypt(3) encryption to make passwd with the command mkpasswd.Then I did with the given procedure user:encryptedpasswd in .htpasswd. But things does not work as said.Let me know if anyone know how I can exactly make configure for my purpose! Thanks you.

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• How do I edit keyboard preferences from the command line?

  - by jumpnett

  I want to swap the Caps Lock and Escape key as specified in this answer: Use the keyboard preferences to swap Caps Lock and Escape - seriously, how often do you use Caps Lock? Using vim you will be using Escape all the time, and having it available on the home row makes a huge difference. With the standard Ubuntu desktop, go through the menus: System - Preferences - Keyboard - Layouts tab. Then hit the "Layout Options" button, click on the triangle next to "Caps Lock key behaviour" and select "Swap ESC and CapsLock". but, I'm using Ubuntu Server with no gui, so how would I do this from the command line?

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• Why does xvid encoding lag/lock up windows 7?

  - by acidzombie24

  It seems to encode just fine so you can see the results http://www.sendspace.com/file/msku4q If you look at the mouse cursor you'll see firefox locks up once i click it. Calculator seems fine but when i try to move it, it locks up. The resource monitor and task manager are up so you can see if the CPU is being used up. It isnt as you can see <30% was used.

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• Asp.Net C# JQuery base Web Application for Rapid Web Presence

  visinia can be used expand to create a website through module drag drop

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• why doesnt ubuntu 13.04 prompt me for a password after suspend?

  - by mark

  I am on 13.04 and waking from suspend takes me straight to desktop without a password prompt even though it is set to ask for a password in the brightness and lock settings. Also I recently tried to lock my computer,going to the power menu in the top right corner and hitting lock,it does not do anything,I am still on the desktop..(just tried crtl alt l to lock,it does work) but not the way i did it though,but that is not important to me ,I need the password prompt after suspend.. Password prompts works only when booted from a shutdown...

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• Change style display for cells with Javascript

  - by Ronny

  Hi, I want to do something like this: user selects one radio button (lock,delete or compare). I want to show to him only the relevant column from the table. (each option has different column). The table is ajax. I guess i need to change the display style for every cell but i don't know how. Here is example: Here i want to change the display of the cells function ButtonForTbl(value) { var x=document.getElementById("audithead").rows[0].cells; if (value == "lock"){ document.getElementById('lock').checked = true; //something like for(...)lockCell.style.display='' //something like for(...)deleteCell.style.display='none' //something like for(...)compareCell.style.display='none' } else if(value == "delete"){ document.getElementById('delete').checked = true; //something like for(...)lockCell.style.display='none' //something like for(...)deleteCell.style.display='' //something like for(...)compareCell.style.display='none' } else{ document.getElementById('compare').checked = true; } } I guess i need something like that: for (i = 0; i < deleteCell.length; i++) deleteCell[i].style.display='' = true ; The table: oCell = oRow.insertCell(-1); oCell.setAttribute('id','comCell' ); oCell.setAttribute('align', 'center'); oCell.innerHTML = "<input type='checkbox' id='com' value='"+ ind + "'name='com[]'>"; oCell = oRow.insertCell(-1); oCell.setAttribute('id','lockCell' ); oCell.setAttribute('align', 'center'); oCell.innerHTML = "<input type='checkbox' id='lock' value='"+ ind + "'name='lock[]'>"; Radio buttons: <input type="radio" value="compare" id="compare" name="choose" onclick="ButtonForTbl(this.value)"/> Compare&nbsp; <input type="radio" value="delete" id="delete" name="choose" onclick="ButtonForTbl(this.value)"/> Delete&nbsp; <input type="radio" value="lock" id="lock" name="choose" onclick="ButtonForTbl(this.value)"/> Lock<br/> The table html: <table class="auditable"> <thead id="audithead"> <tr><td></td></tr> </thead> <tbody id="auditTblBody"> </tbody> </table> EDIT: Full row is like that: <tr> <td align="center" id="lockCell" style="display: none;"> <input type="checkbox" onclick="" name="lock[]" value="1500" id="lock"></td> <td align="center" id="delCell" style="display: none;"> <input type="checkbox" name="del[]" value="1500"></td> <td align="center" id="comCell"> <input type="checkbox" onclick="setChecks(this)" name="com[]" value="1500" id="com"></td> <td width="65px">100% 1/1</td><td width="105px">2011-01-10 17:47:37</td> </tr> Thank you so much!

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• html5 uploader + jquery drag & drop: how to store file data with FormData?

  - by lauthiamkok

  I am making a html5 drag and drop uploader with jquery, below is my code so far, the problem is that I get an empty array without any data. Is this line incorrect to store the file data - fd.append('file', $thisfile);? $('#div').on( 'dragover', function(e) { e.preventDefault(); e.stopPropagation(); } ); $('#div').on( 'dragenter', function(e) { e.preventDefault(); e.stopPropagation(); } ); $('#div').on( 'drop', function(e){ if(e.originalEvent.dataTransfer){ if(e.originalEvent.dataTransfer.files.length) { e.preventDefault(); e.stopPropagation(); // The file list. var fileList = e.originalEvent.dataTransfer.files; //console.log(fileList); // Loop the ajax post. for (var i = 0; i < fileList.length; i++) { var $thisfile = fileList[i]; console.log($thisfile); // HTML5 form data object. var fd = new FormData(); //console.log(fd); fd.append('file', $thisfile); /* var file = {name: fileList[i].name, type: fileList[i].type, size:fileList[i].size}; $.each(file, function(key, value) { fd.append('file['+key+']', value); }) */ $.ajax({ url: "upload.php", type: "POST", data: fd, processData: false, contentType: false, success: function(response) { // .. do something }, error: function(jqXHR, textStatus, errorMessage) { console.log(errorMessage); // Optional } }); } /*UPLOAD FILES HERE*/ upload(e.originalEvent.dataTransfer.files); } } } ); function upload(files){ console.log('Upload '+files.length+' File(s).'); }; then if I use another method is that to make the file data into an array inside the jquery code, var file = {name: fileList[i].name, type: fileList[i].type, size:fileList[i].size}; $.each(file, function(key, value) { fd.append('file['+key+']', value); }); but where is the tmp_name data inside e.originalEvent.dataTransfer.files[i]? php, print_r($_POST); $uploaddir = './uploads/'; $file = $uploaddir . basename($_POST['file']['name']); if (move_uploaded_file($_POST['file']['tmp_name'], $file)) { echo "success"; } else { echo "error"; } as you can see that tmp_name is needed to upload the file via php... html, <div id="div">Drop here</div>

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• How can I get the main thread to sleep while waiting for a delgate to be called?

  - by Erik B

  Consider a class with these methods: - (id) initWithFrame: (CGRect) frame { if (!(self = [super init])) return nil; webView = [[UIWebView alloc] initWithFrame:frame]; [webView setDelegate:self]; lock = [[NSConditionLock alloc] initWithCondition:LOCK_WAIT]; return self; } - (void) setHTML: (NSString *) html { [lock lockWhenCondition:LOCK_WAIT]; [webView loadHTMLString:html baseURL:nil]; [lock unlock]; } - (void)webViewDidFinishLoad:(UIWebView *)aWebView { [lock lockWhenCondition:LOCK_WAIT]; // Locking to be able to unlock and change the condition. [lock unlockWithCondition:LOCK_GO]; } - (NSString *) stringByEvaluatingJavaScriptFromString: (NSString *) jsCommand { [lock lockWhenCondition:LOCK_GO]; return [webView stringByEvaluatingJavaScriptFromString:jsCommand]; [lock unlock]; } Let's call this class SynchronousUIWebView. From the main thread I execute: webView = [[SynchronousUIWebView alloc] initWithFrame:frame]; [webView setHTML:html]; [webView stringByEvaluatingJavaScriptFromString:jsCommand]; The problem seems to be that the delegate is not called until I leave the current call stack, which I don't since I'm waiting for the delegate call to happen, aka deadlock. To me it seems like the delegate call is pushed to a queue that is called when the current call is done. So the question is can I modify this behavior? Note: The reason this is needed is that I can't execute the JavaScript until the HTML has loaded.

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• Should I use IDisposable for purely managed resources?

  - by John Gietzen

  Here is the scenario: I have an object called a Transaction that needs to make sure that only one entity has permission to edit it at any given time. In order to facilitate a long-lived lock, I have the class generating a token object that can be used to make the edits. You would use it like this: var transaction = new Transaction(); using (var tlock = transaction.Lock()) { transaction.Update(data, tlock); } Now, I want the TransactionLock class to implement IDisposable so that its usage can be clear. But, I don't have any unmanaged resources to dispose. however, the TransctionLock object itself is a sort of "unmanaged resource" in the sense that the CLR doesn't know how to properly finalize it. All of this would be fine and dandy, I would just use IDisposable and be done with it. However, my issue comes when I try to do this in the finalizer: ~TransactionLock() { this.Dispose(false); } I want the finalizer to release the transaction from the lock, if possible. How, in the finalizer, do I detect if the parent transaction (this.transaction) has already been finalized? Is there a better pattern I should be using? The Transaction class looks something like this: public sealed class Transaction { private readonly object lockMutex = new object(); private TransactionLock currentLock; public TransactionLock Lock() { lock (this.lockMutex) { if (this.currentLock != null) throw new InvalidOperationException(/* ... */); this.currentLock = new TransactionLock(this); return this.currentLock; } } public void Update(object data, TransactionLock tlock) { lock (this.lockMutex) { this.ValidateLock(tlock); // ... } } internal void ValidateLock(TransactionLock tlock) { if (this.currentLock == null) throw new InvalidOperationException(/* ... */); if (this.currentLock != tlock) throw new InvalidOperationException(/* ... */); } internal void Unlock(TransactionLock tlock) { lock (this.lockMutex) { this.ValidateLock(tlock); this.currentLock = null; } } }

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• Limiting object allocation over multiple threads

  - by John

  I have an application which retrieves and caches the results of a clients query. The client then requests different chunks of data and the application sends the relevant results and removes them from the cache. A new requirement for this application is that there needs to be a run-time configurable maximum number of results which may be cached. I've taken the naive approach and implemented this by using a counter under a lock which is incremented every time a result is cached and decremented whenever a result is removed from the cache. Unfortunately, this has drastically reduced the applications performance when processing a large number of concurrent requests. I have tried both a critical section lock and spin-lock; the performance improves a bit with a spin-lock, but is still unacceptably slow. Is there a better way to solve this problem which may improve performance? Right now I have a thread pool that services requests and each request is tied to a Request object which stores that cached results for that particular request. Here is a simplified pseudo code version of my current implementation: void ResultCallback( Result result, Request *request ) { lock totalResultsCached lock cachedLimit if( totalResultsCached + 1 > cachedLimit ) { unlock cachedLimit unlock totalResultsCached //cancel the request return; } ++totalResultsCached; unlock cachedLimit unlock totalResultsCached request.add(result) } void SendResults( int resultsToSend, Request *request ) { while ( resultsToSend > 0 ) { send(request.remove()) lock totalResultsCached --totalResultsCached unlock totalResultsCached --resultsToSend; } }

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• Is multithreading the right way to go for my case?

  - by Julien Lebosquain

  Hello, I'm currently designing a multi-client / server application. I'm using plain good old sockets because WCF or similar technology is not what I need. Let me explain: it isn't the classical case of a client simply calling a service; all clients can 'interact' with each other by sending a packet to the server, which will then do some action, and possible re-dispatch an answer message to one or more clients. Although doable with WCF, the application will get pretty complex with hundreds of different messages. For each connected client, I'm of course using asynchronous methods to send and receive bytes. I've got the messages fully working, everything's fine. Except that for each line of code I'm writing, my head just burns because of multithreading issues. Since there could be around 200 clients connected at the same time, I chose to go the fully multithreaded way: each received message on a socket is immediately processed on the thread pool thread it was received, not on a single consumer thread. Since each client can interact with other clients, and indirectly with shared objects on the server, I must protect almost every object that is mutable. I first went with a ReaderWriterLockSlim for each resource that must be protected, but quickly noticed that there are more writes overall than reads in the server application, and switched to the well-known Monitor to simplify the code. So far, so good. Each resource is protected, I have helper classes that I must use to get a lock and its protected resource, so I can't use an object without getting a lock. Moreover, each client has its own lock that is entered as soon as a packet is received from its socket. It's done to prevent other clients from making changes to the state of this client while it has some messages being processed, which is something that will happen frequently. Now, I don't just need to protect resources from concurrent accesses. I must keep every client in sync with the server for some collections I have. One tricky part that I'm currently struggling with is the following: I have a collection of clients. Each client has its own unique ID. When a client connects, it must receive the IDs of every connected client, and each one of them must be notified of the newcomer's ID. When a client disconnects, every other client must know it so that its ID is no longer valid for them. Every client must always have, at a given time, the same clients collection as the server so that I can assume that everybody knows everybody. This way if I'm sending a message to client #1 telling "Client #2 has done something", I know that it will always be correctly interpreted: Client 1 will never wonder "but who is Client 2 anyway?". My first attempt for handling the connection of a new client (let's call it X) was this pseudo-code (remember that newClient is already locked here): lock (clients) { foreach (var client in clients) { lock (client) { client.Send("newClient with id X has connected"); } } clients.Add(newClient); newClient.Send("the list of other clients"); } Now imagine that in the same time, another client has sent a packet that translates into a message that must be broadcasted to every connected client, the pseudo-code will be something like this (remember that the current client - let's call it Y - is already locked here): lock (clients) { foreach (var client in clients) { lock (client) { client.Send("something"); } } } An obvious deadlock occurs here: on one thread X is locked, the clients lock has been entered, started looping through the clients, and at one moment must get Y's lock... which is already acquired on the second thread, itself waiting for the clients collection lock to be released! This is not the only case like this in the server application. There are other collections which must be kept in sync with the clients, some properties on a client can be changed by another one, etc. I tried other types of locks, lock-free mechanisms and a bunch of other things. Either there were obvious deadlocks when I'm using too much locks for safety, or obvious race conditions otherwise. When I finally find a good middle point between the two, it usually comes with very subtle race conditions / dead locks and other multi-threading issues... my head hurts very quickly since for any single line of code I'm writing I have to review almost the whole application to ensure everything will behave correctly with any number of threads. So here's my final question: how would you resolve this specific case, the general case, and more importantly: aren't I going the wrong way here? I have little problems with the .NET framework, C#, simple concurrency or algorithms in general. Still, I'm lost here. I know I could use only one thread processing the incoming requests and everything will be fine. However, that won't scale well at all with more clients... But I'm thinking more and more to go this simple way. What do you think? Thanks in advance to you, StackOverflow people which have taken the time to read this huge question. I really had to explain the whole context if I want to get some help.

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• C programming: hashtable insertion/search

  - by Ricardo Campos

  Hello i have a problem with my hash table its implemented like this: #define HT_SIZE 10 typedef struct _list_t_ { char key[20]; char string[20]; char prevValue[20]; struct _list_t_ *next; } list_t; typedef struct _hash_table_t_ { int size; /* the size of the table */ list_t ***table; /* first */ sem_t lock; } hash_table_t; I have a Linked list with 3 pointers because i want a hash table with several partitions (shards), here is my initialization of my Hash table: hash_table_t *create_hash_table(int NUM_SERVER_THREADS, int num_shards){ hash_table_t *new_table; int j,i; if (HT_SIZE<1) return NULL; /* invalid size for table */ /* Attempt to allocate memory for the hashtable structure */ new_table = (hash_table_t*)malloc(sizeof(hash_table_t)*HT_SIZE); /* Attempt to allocate memory for the table itself */ new_table->table = (list_t ***)calloc(1,sizeof(list_t **)); /* Initialize the elements of the table */ for(j=0; j<num_shards; j++){ new_table->table[j] = (list_t **)calloc(1,sizeof(list_t *)); for(i=0; i<HT_SIZE; i++){ new_table->table[j][i] = (list_t *)calloc(1,sizeof(list_t )); } } /* Set the table's size */ new_table->size = HT_SIZE; sem_init(&new_table->lock, 0, 1); return new_table; } Here is my search function to search in the hash table list_t *lookup_string(hash_table_t *hashtable, char *key, int shardId){ list_t *list ; int hashval = hash(key); /* Go to the correct list based on the hash value and see if key is * in the list. If it is, return return a pointer to the list element. * If it isn't, the item isn't in the table, so return NULL. */ sem_wait(&hashtable->lock); for(list = hashtable->table[shardId][hashval]; list != NULL; list =list->next) { if (strcmp(key, list->key) == 0){ sem_post(&hashtable->lock); return list; } } sem_post(&hashtable->lock); return NULL; } And my insert function: char *add_string(hash_table_t *hashtable, char *str,char *key, int shardId){ list_t *new_list; list_t *current_list; unsigned int hashval = hash(key); /*printf("|%d|%d|%s|\n",hashval,shardId,key);*/ /* Lock for concurrency */ sem_wait(&hashtable->lock); /* Attempt to allocate memory for list */ new_list = (list_t*)malloc(sizeof(list_t)); /* Does item already exist? */ sem_post(&hashtable->lock); current_list = lookup_string(hashtable, key,shardId); sem_wait(&hashtable->lock); /* item already exists, don't insert it again. */ if (current_list != NULL){ strcpy(new_list->prevValue,current_list->string); strcpy(new_list->string,str); strcpy(new_list->key,key); new_list->next = hashtable->table[shardId][hashval]; hashtable->table[shardId][hashval] = new_list; sem_post(&hashtable->lock); return new_list->prevValue; } /* Insert into list */ strcpy(new_list->string,str); strcpy(new_list->key,key); new_list->next = hashtable->table[shardId][hashval]; hashtable->table[shardId][hashval] = new_list; /* Unlock */ sem_post(&hashtable->lock); return new_list->prevValue; } My main class runs some of tests by executing the insertion / reading / delete from the elements of the hash table the problem is when i have more than 4 partitions/shards the tests stop at the first reading element saying it returned the wrong value NULL on the search function, when its less than 4 it runs perfectly well and passes all the tests. You can see my main.c in here if you want to give a look: http://hostcode.sourceforge.net/view/1105 My complete Hash table code: http://hostcode.sourceforge.net/view/1103 And other functions where hash table code is executed: .c file http://hostcode.sourceforge.net/view/1104 .h file http://hostcode.sourceforge.net/view/1106 Thank for you time, i appreciate any help you can give to me this is a college important project that I'm trying to solve and I'm stuck here for 2 days.

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• Is this a valid pattern for raising events in C#?

  - by Will Vousden

  Update: For the benefit of anyone reading this, since .NET 4, the lock is unnecessary due to changes in synchronization of auto-generated events, so I just use this now: public static void Raise<T>(this EventHandler<T> handler, object sender, T e) where T : EventArgs { if (handler != null) { handlerCopy(sender, e); } } And to raise it: SomeEvent.Raise(this, new FooEventArgs()); Having been reading one of Jon Skeet's articles on multithreading, I've tried to encapsulate the approach he advocates to raising an event in an extension method like so (with a similar generic version): public static void Raise(this EventHandler handler, object @lock, object sender, EventArgs e) { EventHandler handlerCopy; lock (@lock) { handlerCopy = handler; } if (handlerCopy != null) { handlerCopy(sender, e); } } This can then be called like so: protected virtual void OnSomeEvent(EventArgs e) { this.someEvent.Raise(this.eventLock, this, e); } Are there any problems with doing this? Also, I'm a little confused about the necessity of the lock in the first place. As I understand it, the delegate is copied in the example in the article to avoid the possibility of it changing (and becoming null) between the null check and the delegate call. However, I was under the impression that access/assignment of this kind is atomic, so why is the lock necessary? Update: With regards to Mark Simpson's comment below, I threw together a test: static class Program { private static Action foo; private static Action bar; private static Action test; static void Main(string[] args) { foo = () => Console.WriteLine("Foo"); bar = () => Console.WriteLine("Bar"); test += foo; test += bar; test.Test(); Console.ReadKey(true); } public static void Test(this Action action) { action(); test -= foo; Console.WriteLine(); action(); } } This outputs: Foo Bar Foo Bar This illustrates that the delegate parameter to the method (action) does not mirror the argument that was passed into it (test), which is kind of expected, I guess. My question is will this affect the validity of the lock in the context of my Raise extension method? Update: Here is the code I'm now using. It's not quite as elegant as I'd have liked, but it seems to work: public static void Raise<T>(this object sender, ref EventHandler<T> handler, object eventLock, T e) where T : EventArgs { EventHandler<T> copy; lock (eventLock) { copy = handler; } if (copy != null) { copy(sender, e); } }

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