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  • What to do for a 1 million concurrent web application? [duplicate]

    - by Amit Singh
    This question already has an answer here: How do you do load testing and capacity planning for web sites? 3 answers There are few things that I would like to know here. What server configuration do I need. And if I am deploying it on EC2 how many VMs do I need and what should be their configuration. What options do I have to do a load testing for 1 million concurrent users. Any pointer to (for php) how to code or what to keep in mind for such application. This is for sure that I don't exactly know what to ask because this is my first application on this scale. But one thing is clear that this application should pass a load test of 1 million concurrent request.

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  • What is a fairly accepted endpoints to concurrent calls ratio for a PBX?

    - by Logan Bibby
    Hey guys! Not too positive about the relevance of this question to Server Fault, but I'll let you guys poke around at it anyway. :) I'm trying to figure out what an accepted ratio of endpoints to concurrent calls on the "average" office PBX. I'm defining concurrent calls as any call being placed, including internal calls. I'm not going to be considering call centers into this ratio, I'll be looking at those differently, anyway. To give you guys a little history behind the question: I need this ratio to be able to figure out decent specs for a virtual PBX service I'm going to be rolling out within the upcoming months. The ratio will determine the number of trunks needed per PBX system. -- Logan

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  • Is FreeBSD better than CentOS for firing 40k concurrent connections (for Jmeter)?

    - by blacklotus
    Hi, I am trying to run Jmeter to simulate 40k concurrent users and stress test a particular system. Putting aside the possibility that Jmeter may not be able to push such a high number (although I have read that it is at least possible to handle 10k concurrent threads on a very powerful machine), is FreeBSD a better OS as compare to CentOS to be used for my Jmeter machine? Reason for asking this is that, I have found articles on FreeBSD for tuning and optimizing for maximum outbound connections, but seem to have little luck with CentOS. Personally however, I am more familiar with CentOS and would like to stick with it if possible. Any input is greatly appreciated!

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  • python can't start a new thread

    - by Giorgos Komnino
    I am building a multi threading application. I have setup a threadPool. [ A Queue of size N and N Workers that get data from the queue] When all tasks are done I use tasks.join() where tasks is the queue . The application seems to run smoothly until suddently at some point (after 20 minutes in example) it terminates with the error thread.error: can't start new thread Any ideas? Edit: The threads are daemon Threads and the code is like: while True: t0 = time.time() keyword_statuses = DBSession.query(KeywordStatus).filter(KeywordStatus.status==0).options(joinedload(KeywordStatus.keyword)).with_lockmode("update").limit(100) if keyword_statuses.count() == 0: DBSession.commit() break for kw_status in keyword_statuses: kw_status.status = 1 DBSession.commit() t0 = time.time() w = SWorker(threads_no=32, network_server='http://192.168.1.242:8180/', keywords=keyword_statuses, cities=cities, saver=MySqlRawSave(DBSession), loglevel='debug') w.work() print 'finished' When the daemon threads are killed? When the application finishes or when the work() finishes? Look at the thread pool and the worker (it's from a recipe ) from Queue import Queue from threading import Thread, Event, current_thread import time event = Event() class Worker(Thread): """Thread executing tasks from a given tasks queue""" def __init__(self, tasks): Thread.__init__(self) self.tasks = tasks self.daemon = True self.start() def run(self): '''Start processing tasks from the queue''' while True: event.wait() #time.sleep(0.1) try: func, args, callback = self.tasks.get() except Exception, e: print str(e) return else: if callback is None: func(args) else: callback(func(args)) self.tasks.task_done() class ThreadPool: """Pool of threads consuming tasks from a queue""" def __init__(self, num_threads): self.tasks = Queue(num_threads) for _ in range(num_threads): Worker(self.tasks) def add_task(self, func, args=None, callback=None): ''''Add a task to the queue''' self.tasks.put((func, args, callback)) def wait_completion(self): '''Wait for completion of all the tasks in the queue''' self.tasks.join() def broadcast_block_event(self): '''blocks running threads''' event.clear() def broadcast_unblock_event(self): '''unblocks running threads''' event.set() def get_event(self): '''returns the event object''' return event

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  • Producer consumer with qualifications

    - by tgguy
    I am new to clojure and am trying to understand how to properly use its concurrency features, so any critique/suggestions is appreciated. So I am trying to write a small test program in clojure that works as follows: there 5 producers and 2 consumers a producer waits for a random time and then pushes a number onto a shared queue. a consumer should pull a number off the queue as soon as the queue is nonempty and then sleep for a short time to simulate doing work the consumers should block when the queue is empty producers should block when the queue has more than 4 items in it to prevent it from growing huge Here is my plan for each step above: the producers and consumers will be agents that don't really care for their state (just nil values or something); i just use the agents to send-off a "consumer" or "producer" function to do at some time. Then the shared queue will be (def queue (ref [])). Perhaps this should be an atom though? in the "producer" agent function, simply (Thread/sleep (rand-int 1000)) and then (dosync (alter queue conj (rand-int 100))) to push onto the queue. I am thinking to make the consumer agents watch the queue for changes with add-watcher. Not sure about this though..it will wake up the consumers on any change, even if the change came from a consumer pulling something off (possibly making it empty) . Perhaps checking for this in the watcher function is sufficient. Another problem I see is that if all consumers are busy, then what happens when a producer adds something new to the queue? Does the watched event get queued up on some consumer agent or does it disappear? see above I really don't know how to do this. I heard that clojure's seque may be useful, but I couldn't find enough doc on how to use it and my initial testing didn't seem to work (sorry don't have the code on me anymore)

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  • How do I subscribe to a MSMQ queue but only "peek" the message in .Net?

    - by lemkepf
    We have a MSMQ Queue setup that receives messages and is processed by an application. We'd like to have another process subscribe to the Queue and just read the message and log it's contents. I have this in place already, the problem is it's constantly peeking the queue. CPU on the server when this is running is around 40%. The mqsvc.exe runs at 30% and this app runs at 10%. I'd rather have something that just waits for a message to come in, get's notified of it, and then logs it without constantly pooling the server. Dim lastid As String Dim objQueue As MessageQueue Dim strQueueName As String Public Sub Main() objQueue = New MessageQueue(strQueueName, QueueAccessMode.SendAndReceive) Dim propertyFilter As New MessagePropertyFilter propertyFilter.ArrivedTime = True propertyFilter.Body = True propertyFilter.Id = True propertyFilter.LookupId = True objQueue.MessageReadPropertyFilter = propertyFilter objQueue.Formatter = New ActiveXMessageFormatter AddHandler objQueue.PeekCompleted, AddressOf MessageFound objQueue.BeginPeek() end main Public Sub MessageFound(ByVal s As Object, ByVal args As PeekCompletedEventArgs) Dim oQueue As MessageQueue Dim oMessage As Message ' Retrieve the queue from which the message originated oQueue = CType(s, MessageQueue) oMessage = oQueue.EndPeek(args.AsyncResult) If oMessage.LookupId <> lastid Then ' Process the message here lastid = oMessage.LookupId ' let's write it out log.write(oMessage) End If objQueue.BeginPeek() End Sub

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  • What is the fastest collection in c# to implement a prioritizing queue?

    - by Nathan Smith
    I need to implement a queue for messages on a game server so it needs to as fast as possible. The queue will have a maxiumem size. I need to prioritize messages once the queue is full by working backwards and removing a lower priority message (if one exists) before adding the new message. The appliation is asynchronous so access to the queue needs to be locked. I'm currently implementing it using a LinkedList as the underlying storage but have concerns that searching and removing nodes will keep it locked for too long. Heres the basic code I have at the moment: public class ActionQueue { private LinkedList<ClientAction> _actions = new LinkedList<ClientAction>(); private int _maxSize; /// <summary> /// Initializes a new instance of the ActionQueue class. /// </summary> public ActionQueue(int maxSize) { _maxSize = maxSize; } public int Count { get { return _actions.Count; } } public void Enqueue(ClientAction action) { lock (_actions) { if (Count < _maxSize) _actions.AddLast(action); else { LinkedListNode<ClientAction> node = _actions.Last; while (node != null) { if (node.Value.Priority < action.Priority) { _actions.Remove(node); _actions.AddLast(action); break; } } } } } public ClientAction Dequeue() { ClientAction action = null; lock (_actions) { action = _actions.First.Value; _actions.RemoveFirst(); } return action; } }

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  • NServiceBus & MSMQ: How To Change the Default Permissions on the Queue?

    - by Amy T
    My team is on our first attempt at using NServiceBus (v2.0), using MSMQ as the backing storage. We're getting stuck on queue permissions. We're using it in a Web Forms application, where the user account the website runs under is not an administrator on the machine. When NServiceBus creates the MSMQ queue, it gives the local administrators group full control, and the local everyone and anonymous groups permissions to send messages. But then later, as part of initializing the queue, NServiceBus tries to read all of its messages. That's where we run into the permissions error. Since the website isn't running as an administrator, it's not allowed to read messages. How are other people dealing with this? Do your applications run as administrators? Or do you create the MSMQ queue in your code first, giving it the permissions you need, so that NServiceBus doesn't have to create it? Or is there a bit of configuration we're missing? Or are we likely writing our code that uses NServiceBus incorrectly to be running into this?

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  • A Method for Reducing Contention and Overhead in Worker Queues for Multithreaded Java Applications

    - by Janice J. Heiss
    A java.net article, rich in practical resources, by IBM India Labs’ Sathiskumar Palaniappan, Kavitha Varadarajan, and Jayashree Viswanathan, explores the challenge of writing code in a way that that effectively makes use of the resources of modern multicore processors and multiprocessor servers.As the article states: “Many server applications, such as Web servers, application servers, database servers, file servers, and mail servers, maintain worker queues and thread pools to handle large numbers of short tasks that arrive from remote sources. In general, a ‘worker queue’ holds all the short tasks that need to be executed, and the threads in the thread pool retrieve the tasks from the worker queue and complete the tasks. Since multiple threads act on the worker queue, adding tasks to and deleting tasks from the worker queue needs to be synchronized, which introduces contention in the worker queue.” The article goes on to explain ways that developers can reduce contention by maintaining one queue per thread. It also demonstrates a work-stealing technique that helps in effectively utilizing the CPU in multicore systems. Read the rest of the article here.

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  • How to measure the time HTTP requests spend sitting in the accept-queue?

    - by David Jones
    I am using Apache2 on Ubuntu 9.10, and I am trying to tune my configuration for a web application to reduce latency of responses to HTTP requests. During a moderately heavy load on my small server, there are 24 apache2 processes handling requests. Additional requests get queued. Using "netstat", I see 24 connections are ESTABLISHED and 125 connections are TIME_WAIT. I am trying to figure out if that is considered a reasonable backlog. Most requests get serviced in a fraction of a second, so I am assuming requests move through the accept-queue fairly quickly, probably within 1 or 2 seconds, but I would like to be more certain. Can anyone recommend an easy way to measure the time an HTTP request sits in the accept-queue? The suggestions I have come across so far seem to start the clock after the apache2 worker accepts the connection. I'm trying to quantify the accept-queue delay before that. thanks in advance, David Jones

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  • JMS - How do message selectors work with multiple queue and topic consumers?

    - by Stephen Harmon
    Say you have a JMS queue, and multiple consumers are watching the queue for messages. You want one of the consumers to get all of a particular type of message, so you decide to employ message selectors. For example, you define a property to go in your JMS message header named, "targetConsumer." Your message selector, which you apply to the consumer known as, "A," is something like "WHERE targetConsumer = "CONSUMER_A." It's clear that consumer A will now just grab messages with the property set like it is in in the example. Will the other consumers have awareness of that, though? IOW, will another consumer, unconstrained by a message selector, grab the "CONSUMER_A" messages, if it looks at the queue before Consumer A? Do I need to apply message selectors like, "WHERE targetConsumer < "CONSUMER_A" to the others? I am RTFMing and gathering empirical data now, but was hoping someone might know off the top of their head.

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  • What is the absolute fastest way to implement a concurrent queue with ONLY one consumer and one producer?

    - by JohnPristine
    java.util.concurrent.ConcurrentLinkedQueue comes to mind, but is it really optimum for this two-thread scenario? I am looking for the minimum latency possible on both sides (producer and consumer). If the queue is empty you can immediately return null AND if the queue is full you can immediately discard the entry you are offering. Does ConcurrentLinkedQueue use super fast and light locks (AtomicBoolean) ? Has anyone benchmarked ConcurrentLinkedQueue or knows about the ultimate fastest way of doing that? Additional Details: I imagine the queue should be a fair one, meaning the consumer should not make the consumer wait any longer than it needs (by front-running it) and vice-versa.

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  • Upstart: cannot run as root

    - by Ronni Egeriis
    I have made this upstart script, which starts a Node.js service. But all of the sudden the service has stopped, and upstart has failed to restart it. Now that I am trying to start it manually, it fails to recognize my service: start: Unknown job: queue The script is properly placed in /etc/init, and should have the correct rights: -rw-r--r-- 1 root root 200 Aug 7 13:30 queue.conf When I check the config file with init-checkconf however, it says that it is not able to run as root: root@production1:~# init-checkconf /etc/init/queue.conf ERROR: cannot run as root What causes this error and how do I solve it? Debug info: Ubuntu 12.04.3 LTS root@production1:~# service --version service ver. 0.91-ubuntu1 Edit Here's queue.conf: description "Echo.it command queue" author "Ronni Egeriis Persson <[email protected]>" stop on shutdown respawn respawn 20 5 exec sudo -u beanstalk /usr/bin/node /var/www/queue/index.js >> /var/log/queue.log 2>&1 The command sudo -u beanstalk /usr/bin/node /var/www/queue/index.js >> /var/log/queue.log 2>&1 works fine when run manually.

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  • How do I prevent the concurrent execution of a javascript function?

    - by RyanV
    I am making a ticker similar to the "From the AP" one at The Huffington Post, using jQuery. The ticker rotates through a ul, either by user command (clicking an arrow) or by an auto-scroll. Each list-item is display:none by default. It is revealed by the addition of a "showHeadline" class which is display:list-item. HTML for the UL Looks like this: <ul class="news" id="news"> <li class="tickerTitle showHeadline">Test Entry</li> <li class="tickerTitle">Test Entry2</li> <li class="tickerTitle">Test Entry3</li> </ul> When the user clicks the right arrow, or the auto-scroll setTimeout goes off, it runs a tickForward() function: function tickForward(){ var $active = $('#news li.showHeadline'); var $next = $active.next(); if($next.length==0) $next = $('#news li:first'); $active.stop(true, true); $active.fadeOut('slow', function() {$active.removeClass('showHeadline');}); setTimeout(function(){$next.fadeIn('slow', function(){$next.addClass('showHeadline');})}, 1000); if(isPaused == true){ } else{ startScroll() } }; This is heavily inspired by Jon Raasch's A Simple jQuery Slideshow. Basically, find what's visible, what should be visible next, make the visible thing fade and remove the class that marks it as visible, then fade in the next thing and add the class that makes it visible. Now, everything is hunky-dory if the auto-scroll is running, kicking off tickForward() once every three seconds. But if the user clicks the arrow button repeatedly, it creates two negative conditions: Rather than advance quickly through the list for just the number of clicks made, it continues scrolling at a faster-than-normal rate indefinitely. It can produce a situation where two (or more) list items are given the .showHeadline class, so there's overlap on the list. I can see these happening (especially #2) because the tickForward() function can run concurrently with itself, producing different sets of $active and $next. So I think my question is: What would be the best way to prevent concurrent execution of the tickForward() method? Some things I have tried or considered: Setting a Flag: When tickForward() runs, it sets an isRunning flag to true, and sets it back to false right before it ends. The logic for the event handler is set to only call tickForward() if isRunning is false. I tried a simple implementation of this, and isRunning never appeared to be changed. The jQuery queue(): I think it would be useful to queue up the tickForward() commands, so if you clicked it five times quickly, it would still run as commanded but wouldn't run concurrently. However, in my cursory reading on the subject, it appears that a queue has to be attached to the object its queue applies to, and since my tickForward() method affects multiple lis, I don't know where I'd attach it.

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  • Connect to ibm mq with jms . Specify the channel and queue manager

    - by bhargav
    How do i specify which queue manager to connect to in my system properties. Here is the code: Properties properties = new Properties(); properties.setProperty("java.naming.factory.initial", "com.ibm.mq.jms.context.WMQInitialContextFactory"); properties.setProperty("java.naming.provider.url", "localhost:1414/SYSTEM.DEF.SVRCONN"); Context context = new InitialContext(properties); factory= (QueueConnectionFactory)context.lookup("TESTOUT"); context always gets TEST que only not able to connect to TESTOUT queue

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  • Is there a better way to count the messages in an Message Queue (MSMQ)?

    - by Damovisa
    I'm currently doing it like this: MessageQueue queue = new MessageQueue(".\Private$\myqueue"); MessageEnumerator messageEnumerator = queue.GetMessageEnumerator2(); int i = 0; while (messageEnumerator.MoveNext()) { i++; } return i; But for obvious reasons, it just feels wrong - I shouldn't have to iterate through every message just to get a count, should I? Is there a better way?

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  • i don't receive mail notification...Nagios Core 4

    - by alessio
    I have a problem with automatically mail notification in Nagios Core 4 installed on ubuntu 12 .04 lts server... i have tried to send mail with nagios user and root user with the command: echo "test" | mail -s "test mail" [email protected] and i received mail correctly... but i don't receive any automatically mail notification... i don't know how can i do to resolve this issue! :( these are my configuration files (commands.cfg, contacts.cfg, nagios.log, mail.log): commands.cfg (the path /usr/bin/mail is the right path): # 'notify-host-by-email' command definition define command{ command_name notify-host-by-email command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nHost: $HOSTNAME$\nState: $HOSTSTATE$\nAddress: $HOSTADDRESS$\nInfo: $HOSTOUTPUT$\n\nDate/Time: $LONGDATETIME$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Host Alert: $HOSTNAME$ is $HOSTSTATE$ **" $CONTACTEMAIL$ } # 'notify-service-by-email' command definition define command{ command_name notify-service-by-email command_line /usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$ } # 'process-host-perfdata' command definition define command{ command_name process-host-perfdata command_line /usr/bin/printf "%b" "$LASTHOSTCHECK$\t$HOSTNAME$\t$HOSTSTATE$\t$HOSTATTEMPT$\t$HOSTSTATETYPE$\t$HOSTEXECUTIONTIME$\t$HOSTOUTPUT$\t$HOSTPERFDATA$\n" >> /usr/local/nagios/var/host-perfdata.out } # 'process-service-perfdata' command definition define command{ command_name process-service-perfdata command_line /usr/bin/printf "%b" "$LASTSERVICECHECK$\t$HOSTNAME$\t$SERVICEDESC$\t$SERVICESTATE$\t$SERVICEATTEMPT$\t$SERVICESTATETYPE$\t$SERVICEEXECUTIONTIME$\t$SERVICELATENCY$\t$SERVICEOUTPUT$\t$SERVICEPERFDATA$\n" >> /usr/local/nagios/var/service-perfdata.out } contacts.cfg: define contact{ contact_name supporto alias Supporto Clienti DEA service_notification_period 24x7 host_notification_period 24x7 service_notification_options w,u,c,r host_notification_options d,r service_notification_commands notify-service-by-email host_notification_commands notify-host-by-email email [email protected] } define contactgroup{ contactgroup_name admins alias Nagios Administrators members supporto } nagios.log: [1401871412] SERVICE ALERT: fileserver;Current Users;OK;SOFT;2;USERS OK - 1 users currently logged in [1401871953] SERVICE ALERT: backups;Nagios Status;WARNING;SOFT;1;NAGIOS WARNING: 36 processes, status log updated 541 seconds ago [1401872133] SERVICE ALERT: backups;Nagios Status;OK;SOFT;2;NAGIOS OK: 36 processes, status log updated 180 seconds ago [1401872321] SERVICE ALERT: posta;Swap Usage;CRITICAL;SOFT;1;CRITICAL - Plugin timed out after 10 seconds [1401872322] SERVICE ALERT: fileserver;Current Users;CRITICAL;SOFT;1;CRITICAL - Plugin timed out after 10 seconds [1401872420] SERVICE ALERT: archivio;Disk Space;CRITICAL;SOFT;1;CRITICAL - Plugin timed out after 10 seconds [1401872492] SERVICE ALERT: fileserver;Current Users;OK;SOFT;2;USERS OK - 1 users currently logged in [1401872492] SERVICE ALERT: posta;Swap Usage;OK;SOFT;2;SWAP OK: 100% free (1984 MB out of 1984 MB) [1401872590] SERVICE ALERT: archivio;Disk Space;OK;SOFT;2;DISK OK [1401872931] Auto-save of retention data completed successfully. [1401873333] SERVICE ALERT: backups;Nagios Status;WARNING;SOFT;1;NAGIOS WARNING: 36 processes, status log updated 402 seconds ago [1401873513] SERVICE ALERT: backups;Nagios Status;OK;SOFT;2;NAGIOS OK: 36 processes, status log updated 180 seconds ago mail.log (i think that the problem is here but i don't know how to resolve it): Jun 4 10:00:01 backups sm-msp-queue[6109]: My unqualified host name (backups) unknown; sleeping for retry Jun 4 10:01:01 backups sm-msp-queue[6109]: unable to qualify my own domain name (backups) -- using short name Jun 4 10:20:01 backups sm-msp-queue[7247]: My unqualified host name (backups) unknown; sleeping for retry Jun 4 10:21:01 backups sm-msp-queue[7247]: unable to qualify my own domain name (backups) -- using short name Jun 4 10:40:01 backups sm-msp-queue[8327]: My unqualified host name (backups) unknown; sleeping for retry Jun 4 10:41:01 backups sm-msp-queue[8327]: unable to qualify my own domain name (backups) -- using short name Jun 4 11:00:01 backups sm-msp-queue[9549]: My unqualified host name (backups) unknown; sleeping for retry Jun 4 11:01:01 backups sm-msp-queue[9549]: unable to qualify my own domain name (backups) -- using short name Jun 4 11:20:01 backups sm-msp-queue[10678]: My unqualified host name (backups) unknown; sleeping for retry Jun 4 11:21:01 backups sm-msp-queue[10678]: unable to qualify my own domain name (backups) -- using short name i'm at the last step and i want to finish this Nagios Core! :) Any help be appreciate!:) host definition (this host have the disk almost full and it is in hard state but non notification) : define host{ use generic-host ; Name of host template to use host_name posta alias Server Posta ESA address 10.10.2.102 parents xen1, xen2 icon_image redhat.png statusmap_image redhat.gd2 } service definition: define service{ use generic-service host_name xen1, maestro, xen2, posta, nas002, serv2, esasrvmi02, esaubuntumi service_description Disk Space check_command ssh_all_disks!10%!5% } Notification is allowed for the contact definition you gave, but is it also allowed at the the service level ? sorry but i don't understand this thing! :(

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  • AWS .NET SDK v2: setting up queues and topics

    - by Elton Stoneman
    Originally posted on: http://geekswithblogs.net/EltonStoneman/archive/2013/10/13/aws-.net-sdk-v2-setting-up-queues-and-topics.aspxFollowing on from my last post, reading from SQS queues with the new SDK is easy stuff, but linking a Simple Notification Service topic to an SQS queue is a bit more involved. The AWS model for topics and subscriptions is a bit more advanced than in Azure Service Bus. SNS lets you have subscribers on multiple different channels, so you can send a message which gets relayed to email address, mobile apps and SQS queues all in one go. As the topic owner, when you request a subscription on any channel, the owner needs to confirm they’re happy for you to send them messages. With email subscriptions, the user gets a confirmation request from Amazon which they need to reply to before they start getting messages. With SQS, you need to grant the topic permission to write to the queue. If you own both the topic and the queue, you can do it all in code with the .NET SDK. Let’s say you want to create a new topic, a new queue as a topic subscriber, and link the two together. Creating the topic is easy with the SNS client (which has an expanded name, AmazonSimpleNotificationServiceClient, compare to the SQS class which is just called QueueClient): var request = new CreateTopicRequest(); request.Name = TopicName; var response = _snsClient.CreateTopic(request); TopicArn = response.TopicArn; In the response from AWS (which I’m assuming is successful), you get an ARN – Amazon Resource Name – which is the unique identifier for the topic. We create the queue using the same code from my last post, AWS .NET SDK v2: the message-pump pattern, and then we need to subscribe the queue to the topic. The topic creates the subscription request: var response = _snsClient.Subscribe(new SubscribeRequest { TopicArn = TopicArn, Protocol = "sqs", Endpoint = _queueClient.QueueArn }); That response will give you an ARN for the subscription, which you’ll need if you want to set attributes like RawMessageDelivery. Then the SQS client needs to confirm the subscription by allowing the topic to send messages to it. The SDK doesn’t give you a nice mechanism for doing that, so I’ve extended my AWS wrapper with a method that encapsulates it: internal void AllowSnsToSendMessages(TopicClient topicClient) { var policy = Policies.AllowSendFormat.Replace("%QueueArn%", QueueArn).Replace("%TopicArn%", topicClient.TopicArn); var request = new SetQueueAttributesRequest(); request.Attributes.Add("Policy", policy); request.QueueUrl = QueueUrl; var response = _sqsClient.SetQueueAttributes(request); } That builds up a policy statement, which gets added to the queue as an attribute, and specifies that the topic is allowed to send messages to the queue. The statement itself is a JSON block which contains the ARN of the queue, the ARN of the topic, and an Allow effect for the sqs:SendMessage action: public const string AllowSendFormat= @"{ ""Statement"": [ { ""Sid"": ""MySQSPolicy001"", ""Effect"": ""Allow"", ""Principal"": { ""AWS"": ""*"" }, ""Action"": ""sqs:SendMessage"", ""Resource"": ""%QueueArn%"", ""Condition"": { ""ArnEquals"": { ""aws:SourceArn"": ""%TopicArn%"" } } } ] }"; There’s a new gist with an updated QueueClient and a new TopicClient here: Wrappers for the SQS and SNS clients in the AWS SDK for .NET v2. Both clients have an Ensure() method which creates the resource, so if you want to create a topic and a subscription you can use:  var topicClient = new TopicClient(“BigNews”, “ImListening”); And the topic client has a Subscribe() method, which calls into the message pump on the queue client: topicClient.Subscribe(x=>Log.Debug(x.Body)); var message = {}; //etc. topicClient.Publish(message); So you can isolate all the fiddly bits and use SQS and SNS with a similar interface to the Azure SDK.

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  • Does a high run queue length average result in poor performance for a web server?

    - by Domino
    I'm trying to narrow down the list of suspects of web servers that perform moderately well most of the time with occasional bouts of poor performance. I'm analyzing the data collected and summarized by sar. I've noticed a few things, one of which is high number of tasks in the run queue. 10:15:01 AM runq-sz plist-sz ldavg-1 ldavg-5 ldavg-15 blocked 10:25:01 AM 2 150 0.05 0.05 0.06 0 10:35:01 AM 4 149 0.08 0.12 0.09 0 10:45:01 AM 6 150 0.13 0.19 0.15 0 10:55:01 AM 1 150 0.08 0.10 0.13 0 11:05:01 AM 4 150 0.20 0.35 0.23 0 11:15:01 AM 3 149 0.02 0.09 0.15 0 11:25:01 AM 7 149 0.04 0.05 0.11 0 11:35:01 AM 4 150 0.14 0.15 0.13 0 11:45:01 AM 6 150 0.27 0.18 0.16 0 11:55:01 AM 5 150 0.08 0.10 0.13 0 12:05:01 PM 3 149 0.35 0.40 0.26 0 12:15:01 PM 19 155 0.02 0.10 0.16 1 12:25:01 PM 2 150 0.00 0.07 0.12 0 12:35:02 PM 3 151 0.58 0.24 0.17 0 12:45:01 PM 8 150 0.02 0.13 0.15 0 12:55:01 PM 6 149 0.81 0.29 0.18 0 01:05:01 PM 3 148 0.00 0.09 0.13 0 01:15:01 PM 7 149 0.00 0.04 0.11 0 I believe these are 10 minute averages. Is this an indicator that the web server is not performing as fast as it could if the average run queue length was lower?

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  • C#/.NET Little Wonders: The ConcurrentDictionary

    - by James Michael Hare
    Once again we consider some of the lesser known classes and keywords of C#.  In this series of posts, we will discuss how the concurrent collections have been developed to help alleviate these multi-threading concerns.  Last week’s post began with a general introduction and discussed the ConcurrentStack<T> and ConcurrentQueue<T>.  Today's post discusses the ConcurrentDictionary<T> (originally I had intended to discuss ConcurrentBag this week as well, but ConcurrentDictionary had enough information to create a very full post on its own!).  Finally next week, we shall close with a discussion of the ConcurrentBag<T> and BlockingCollection<T>. For more of the "Little Wonders" posts, see the index here. Recap As you'll recall from the previous post, the original collections were object-based containers that accomplished synchronization through a Synchronized member.  While these were convenient because you didn't have to worry about writing your own synchronization logic, they were a bit too finely grained and if you needed to perform multiple operations under one lock, the automatic synchronization didn't buy much. With the advent of .NET 2.0, the original collections were succeeded by the generic collections which are fully type-safe, but eschew automatic synchronization.  This cuts both ways in that you have a lot more control as a developer over when and how fine-grained you want to synchronize, but on the other hand if you just want simple synchronization it creates more work. With .NET 4.0, we get the best of both worlds in generic collections.  A new breed of collections was born called the concurrent collections in the System.Collections.Concurrent namespace.  These amazing collections are fine-tuned to have best overall performance for situations requiring concurrent access.  They are not meant to replace the generic collections, but to simply be an alternative to creating your own locking mechanisms. Among those concurrent collections were the ConcurrentStack<T> and ConcurrentQueue<T> which provide classic LIFO and FIFO collections with a concurrent twist.  As we saw, some of the traditional methods that required calls to be made in a certain order (like checking for not IsEmpty before calling Pop()) were replaced in favor of an umbrella operation that combined both under one lock (like TryPop()). Now, let's take a look at the next in our series of concurrent collections!For some excellent information on the performance of the concurrent collections and how they perform compared to a traditional brute-force locking strategy, see this wonderful whitepaper by the Microsoft Parallel Computing Platform team here. ConcurrentDictionary – the fully thread-safe dictionary The ConcurrentDictionary<TKey,TValue> is the thread-safe counterpart to the generic Dictionary<TKey, TValue> collection.  Obviously, both are designed for quick – O(1) – lookups of data based on a key.  If you think of algorithms where you need lightning fast lookups of data and don’t care whether the data is maintained in any particular ordering or not, the unsorted dictionaries are generally the best way to go. Note: as a side note, there are sorted implementations of IDictionary, namely SortedDictionary and SortedList which are stored as an ordered tree and a ordered list respectively.  While these are not as fast as the non-sorted dictionaries – they are O(log2 n) – they are a great combination of both speed and ordering -- and still greatly outperform a linear search. Now, once again keep in mind that if all you need to do is load a collection once and then allow multi-threaded reading you do not need any locking.  Examples of this tend to be situations where you load a lookup or translation table once at program start, then keep it in memory for read-only reference.  In such cases locking is completely non-productive. However, most of the time when we need a concurrent dictionary we are interleaving both reads and updates.  This is where the ConcurrentDictionary really shines!  It achieves its thread-safety with no common lock to improve efficiency.  It actually uses a series of locks to provide concurrent updates, and has lockless reads!  This means that the ConcurrentDictionary gets even more efficient the higher the ratio of reads-to-writes you have. ConcurrentDictionary and Dictionary differences For the most part, the ConcurrentDictionary<TKey,TValue> behaves like it’s Dictionary<TKey,TValue> counterpart with a few differences.  Some notable examples of which are: Add() does not exist in the concurrent dictionary. This means you must use TryAdd(), AddOrUpdate(), or GetOrAdd().  It also means that you can’t use a collection initializer with the concurrent dictionary. TryAdd() replaced Add() to attempt atomic, safe adds. Because Add() only succeeds if the item doesn’t already exist, we need an atomic operation to check if the item exists, and if not add it while still under an atomic lock. TryUpdate() was added to attempt atomic, safe updates. If we want to update an item, we must make sure it exists first and that the original value is what we expected it to be.  If all these are true, we can update the item under one atomic step. TryRemove() was added to attempt atomic, safe removes. To safely attempt to remove a value we need to see if the key exists first, this checks for existence and removes under an atomic lock. AddOrUpdate() was added to attempt an thread-safe “upsert”. There are many times where you want to insert into a dictionary if the key doesn’t exist, or update the value if it does.  This allows you to make a thread-safe add-or-update. GetOrAdd() was added to attempt an thread-safe query/insert. Sometimes, you want to query for whether an item exists in the cache, and if it doesn’t insert a starting value for it.  This allows you to get the value if it exists and insert if not. Count, Keys, Values properties take a snapshot of the dictionary. Accessing these properties may interfere with add and update performance and should be used with caution. ToArray() returns a static snapshot of the dictionary. That is, the dictionary is locked, and then copied to an array as a O(n) operation.  GetEnumerator() is thread-safe and efficient, but allows dirty reads. Because reads require no locking, you can safely iterate over the contents of the dictionary.  The only downside is that, depending on timing, you may get dirty reads. Dirty reads during iteration The last point on GetEnumerator() bears some explanation.  Picture a scenario in which you call GetEnumerator() (or iterate using a foreach, etc.) and then, during that iteration the dictionary gets updated.  This may not sound like a big deal, but it can lead to inconsistent results if used incorrectly.  The problem is that items you already iterated over that are updated a split second after don’t show the update, but items that you iterate over that were updated a split second before do show the update.  Thus you may get a combination of items that are “stale” because you iterated before the update, and “fresh” because they were updated after GetEnumerator() but before the iteration reached them. Let’s illustrate with an example, let’s say you load up a concurrent dictionary like this: 1: // load up a dictionary. 2: var dictionary = new ConcurrentDictionary<string, int>(); 3:  4: dictionary["A"] = 1; 5: dictionary["B"] = 2; 6: dictionary["C"] = 3; 7: dictionary["D"] = 4; 8: dictionary["E"] = 5; 9: dictionary["F"] = 6; Then you have one task (using the wonderful TPL!) to iterate using dirty reads: 1: // attempt iteration in a separate thread 2: var iterationTask = new Task(() => 3: { 4: // iterates using a dirty read 5: foreach (var pair in dictionary) 6: { 7: Console.WriteLine(pair.Key + ":" + pair.Value); 8: } 9: }); And one task to attempt updates in a separate thread (probably): 1: // attempt updates in a separate thread 2: var updateTask = new Task(() => 3: { 4: // iterates, and updates the value by one 5: foreach (var pair in dictionary) 6: { 7: dictionary[pair.Key] = pair.Value + 1; 8: } 9: }); Now that we’ve done this, we can fire up both tasks and wait for them to complete: 1: // start both tasks 2: updateTask.Start(); 3: iterationTask.Start(); 4:  5: // wait for both to complete. 6: Task.WaitAll(updateTask, iterationTask); Now, if I you didn’t know about the dirty reads, you may have expected to see the iteration before the updates (such as A:1, B:2, C:3, D:4, E:5, F:6).  However, because the reads are dirty, we will quite possibly get a combination of some updated, some original.  My own run netted this result: 1: F:6 2: E:6 3: D:5 4: C:4 5: B:3 6: A:2 Note that, of course, iteration is not in order because ConcurrentDictionary, like Dictionary, is unordered.  Also note that both E and F show the value 6.  This is because the output task reached F before the update, but the updates for the rest of the items occurred before their output (probably because console output is very slow, comparatively). If we want to always guarantee that we will get a consistent snapshot to iterate over (that is, at the point we ask for it we see precisely what is in the dictionary and no subsequent updates during iteration), we should iterate over a call to ToArray() instead: 1: // attempt iteration in a separate thread 2: var iterationTask = new Task(() => 3: { 4: // iterates using a dirty read 5: foreach (var pair in dictionary.ToArray()) 6: { 7: Console.WriteLine(pair.Key + ":" + pair.Value); 8: } 9: }); The atomic Try…() methods As you can imagine TryAdd() and TryRemove() have few surprises.  Both first check the existence of the item to determine if it can be added or removed based on whether or not the key currently exists in the dictionary: 1: // try add attempts an add and returns false if it already exists 2: if (dictionary.TryAdd("G", 7)) 3: Console.WriteLine("G did not exist, now inserted with 7"); 4: else 5: Console.WriteLine("G already existed, insert failed."); TryRemove() also has the virtue of returning the value portion of the removed entry matching the given key: 1: // attempt to remove the value, if it exists it is removed and the original is returned 2: int removedValue; 3: if (dictionary.TryRemove("C", out removedValue)) 4: Console.WriteLine("Removed C and its value was " + removedValue); 5: else 6: Console.WriteLine("C did not exist, remove failed."); Now TryUpdate() is an interesting creature.  You might think from it’s name that TryUpdate() first checks for an item’s existence, and then updates if the item exists, otherwise it returns false.  Well, note quite... It turns out when you call TryUpdate() on a concurrent dictionary, you pass it not only the new value you want it to have, but also the value you expected it to have before the update.  If the item exists in the dictionary, and it has the value you expected, it will update it to the new value atomically and return true.  If the item is not in the dictionary or does not have the value you expected, it is not modified and false is returned. 1: // attempt to update the value, if it exists and if it has the expected original value 2: if (dictionary.TryUpdate("G", 42, 7)) 3: Console.WriteLine("G existed and was 7, now it's 42."); 4: else 5: Console.WriteLine("G either didn't exist, or wasn't 7."); The composite Add methods The ConcurrentDictionary also has composite add methods that can be used to perform updates and gets, with an add if the item is not existing at the time of the update or get. The first of these, AddOrUpdate(), allows you to add a new item to the dictionary if it doesn’t exist, or update the existing item if it does.  For example, let’s say you are creating a dictionary of counts of stock ticker symbols you’ve subscribed to from a market data feed: 1: public sealed class SubscriptionManager 2: { 3: private readonly ConcurrentDictionary<string, int> _subscriptions = new ConcurrentDictionary<string, int>(); 4:  5: // adds a new subscription, or increments the count of the existing one. 6: public void AddSubscription(string tickerKey) 7: { 8: // add a new subscription with count of 1, or update existing count by 1 if exists 9: var resultCount = _subscriptions.AddOrUpdate(tickerKey, 1, (symbol, count) => count + 1); 10:  11: // now check the result to see if we just incremented the count, or inserted first count 12: if (resultCount == 1) 13: { 14: // subscribe to symbol... 15: } 16: } 17: } Notice the update value factory Func delegate.  If the key does not exist in the dictionary, the add value is used (in this case 1 representing the first subscription for this symbol), but if the key already exists, it passes the key and current value to the update delegate which computes the new value to be stored in the dictionary.  The return result of this operation is the value used (in our case: 1 if added, existing value + 1 if updated). Likewise, the GetOrAdd() allows you to attempt to retrieve a value from the dictionary, and if the value does not currently exist in the dictionary it will insert a value.  This can be handy in cases where perhaps you wish to cache data, and thus you would query the cache to see if the item exists, and if it doesn’t you would put the item into the cache for the first time: 1: public sealed class PriceCache 2: { 3: private readonly ConcurrentDictionary<string, double> _cache = new ConcurrentDictionary<string, double>(); 4:  5: // adds a new subscription, or increments the count of the existing one. 6: public double QueryPrice(string tickerKey) 7: { 8: // check for the price in the cache, if it doesn't exist it will call the delegate to create value. 9: return _cache.GetOrAdd(tickerKey, symbol => GetCurrentPrice(symbol)); 10: } 11:  12: private double GetCurrentPrice(string tickerKey) 13: { 14: // do code to calculate actual true price. 15: } 16: } There are other variations of these two methods which vary whether a value is provided or a factory delegate, but otherwise they work much the same. Oddities with the composite Add methods The AddOrUpdate() and GetOrAdd() methods are totally thread-safe, on this you may rely, but they are not atomic.  It is important to note that the methods that use delegates execute those delegates outside of the lock.  This was done intentionally so that a user delegate (of which the ConcurrentDictionary has no control of course) does not take too long and lock out other threads. This is not necessarily an issue, per se, but it is something you must consider in your design.  The main thing to consider is that your delegate may get called to generate an item, but that item may not be the one returned!  Consider this scenario: A calls GetOrAdd and sees that the key does not currently exist, so it calls the delegate.  Now thread B also calls GetOrAdd and also sees that the key does not currently exist, and for whatever reason in this race condition it’s delegate completes first and it adds its new value to the dictionary.  Now A is done and goes to get the lock, and now sees that the item now exists.  In this case even though it called the delegate to create the item, it will pitch it because an item arrived between the time it attempted to create one and it attempted to add it. Let’s illustrate, assume this totally contrived example program which has a dictionary of char to int.  And in this dictionary we want to store a char and it’s ordinal (that is, A = 1, B = 2, etc).  So for our value generator, we will simply increment the previous value in a thread-safe way (perhaps using Interlocked): 1: public static class Program 2: { 3: private static int _nextNumber = 0; 4:  5: // the holder of the char to ordinal 6: private static ConcurrentDictionary<char, int> _dictionary 7: = new ConcurrentDictionary<char, int>(); 8:  9: // get the next id value 10: public static int NextId 11: { 12: get { return Interlocked.Increment(ref _nextNumber); } 13: } Then, we add a method that will perform our insert: 1: public static void Inserter() 2: { 3: for (int i = 0; i < 26; i++) 4: { 5: _dictionary.GetOrAdd((char)('A' + i), key => NextId); 6: } 7: } Finally, we run our test by starting two tasks to do this work and get the results… 1: public static void Main() 2: { 3: // 3 tasks attempting to get/insert 4: var tasks = new List<Task> 5: { 6: new Task(Inserter), 7: new Task(Inserter) 8: }; 9:  10: tasks.ForEach(t => t.Start()); 11: Task.WaitAll(tasks.ToArray()); 12:  13: foreach (var pair in _dictionary.OrderBy(p => p.Key)) 14: { 15: Console.WriteLine(pair.Key + ":" + pair.Value); 16: } 17: } If you run this with only one task, you get the expected A:1, B:2, ..., Z:26.  But running this in parallel you will get something a bit more complex.  My run netted these results: 1: A:1 2: B:3 3: C:4 4: D:5 5: E:6 6: F:7 7: G:8 8: H:9 9: I:10 10: J:11 11: K:12 12: L:13 13: M:14 14: N:15 15: O:16 16: P:17 17: Q:18 18: R:19 19: S:20 20: T:21 21: U:22 22: V:23 23: W:24 24: X:25 25: Y:26 26: Z:27 Notice that B is 3?  This is most likely because both threads attempted to call GetOrAdd() at roughly the same time and both saw that B did not exist, thus they both called the generator and one thread got back 2 and the other got back 3.  However, only one of those threads can get the lock at a time for the actual insert, and thus the one that generated the 3 won and the 3 was inserted and the 2 got discarded.  This is why on these methods your factory delegates should be careful not to have any logic that would be unsafe if the value they generate will be pitched in favor of another item generated at roughly the same time.  As such, it is probably a good idea to keep those generators as stateless as possible. Summary The ConcurrentDictionary is a very efficient and thread-safe version of the Dictionary generic collection.  It has all the benefits of type-safety that it’s generic collection counterpart does, and in addition is extremely efficient especially when there are more reads than writes concurrently. Tweet Technorati Tags: C#, .NET, Concurrent Collections, Collections, Little Wonders, Black Rabbit Coder,James Michael Hare

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