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• quartz throws deadlocks under load

  - by Khandelwal

  We are using Quartz with Spring and our configuration is throwing deadlocks when quartz has more than 1 thread configured. I'm starting to believe that it's because we don't have our quartz configured correctly with Spring, but I can't find enough documentation on how to configure the two to play nicely. We are running on both Windows and Linux environments - pointing at MSSQL and Oracle DBs. With both OS, using either DB, we can throw the following deadlock errors... We're consistently throwing these deadlock errors. We run under heavy load, inserting hundreds of quartz triggers in a matter of minutes. 2010-03-17 18:52:31,737 [] [] ERROR [DFScheduler_Worker-42] core.ErrorLogger core.ErrorLogger (QuartzScheduler.java:2185) - An error occured while marking executed job complete. job= 'BPM.6e41a6567f0000020100362a51dc7a50' org.quartz.JobPersistenceException: Couldn't remove trigger: Transaction (Process ID 87) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction. [See nested exception: com.microsoft.sqlserver.jdbc.SQLServerException: Transaction (Process ID 87) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.] at org.quartz.impl.jdbcjobstore.JobStoreSupport.removeTrigger(JobStoreSupport.java:1469)at org.quartz.impl.jdbcjobstore.JobStoreSupport.triggeredJobComplete(JobStoreSupport.java:2978)at org.quartz.impl.jdbcjobstore.JobStoreSupport$39.execute(JobStoreSupport.java:2962) at org.quartz.impl.jdbcjobstore.JobStoreSupport$41.execute(JobStoreSupport.java:3713)at org.quartz.impl.jdbcjobstore.JobStoreSupport.executeInNonManagedTXLock(JobStoreSupport.java:3747) at org.quartz.impl.jdbcjobstore.JobStoreSupport.executeInNonManagedTXLock(JobStoreSupport.java:3709)at org.quartz.impl.jdbcjobstore.JobStoreSupport.triggeredJobComplete(JobStoreSupport.java:2958)at org.quartz.core.QuartzScheduler.notifyJobStoreJobComplete(QuartzScheduler.java:1727)at org.quartz.core.JobRunShell.run(JobRunShell.java:273) at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:534) Caused by: com.microsoft.sqlserver.jdbc.SQLServerException: Transaction (Process ID 87) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction. at com.microsoft.sqlserver.jdbc.SQLServerException.makeFromDatabaseError(Unknown Source at com.microsoft.sqlserver.jdbc.SQLServerStatement.getNextResult(Unknown Source) at com.microsoft.sqlserver.jdbc.SQLServerPreparedStatement.doExecutePreparedStatement(Unknown Source) at ... org.quartz.impl.jdbcjobstore.StdJDBCDelegate.deleteSimpleTrigger(StdJDBCDelegate.java:1820) at org.quartz.impl.jdbcjobstore.JobStoreSupport.deleteTriggerAndChildren(JobStoreSupport.java:1345 at org.quartz.impl.jdbcjobstore.JobStoreSupport.removeTrigger(JobStoreSupport.java:1453 ... 9 more

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• Increasing deadlocks with NoLock

  - by Dave Ballantyne

  One on my personnel pet issues is with inappropriate use of the NOLOCK hint (and read uncommitted) .  Dont get me wrong, I have used it in exceptional circumstances , but as a general statement it is a bad thing.  Mostly , when NOLOCK, is used the discussion is around a single statement,  “it runs faster with nolock for XYZ reason”,  however ,IMO, this is quite a shorted sighted view.  What about the Transaction ? What about other concurrent users ?  What is good for one statement in isolation , does not mean that it is good for the system as a whole.  I have seen on a number of occasions deadlocks happen, when tasks that would of(and should of) be blocked continue to execute, only for a deadlock to occur at a later data writing (INSERT,UPDATE,DELETE) statement.  Writers will block writers regardless of isolation level. By Way of (fairly contrived ) example , lets generate some dummy tables and populate with some data drop table a go drop table b go Create Table a ( col1 integer ) go insert into a values(1) insert into a values(2) go Create Table b ( col1 integer ) go insert into b values(1) insert into b values(2) go   Now make two connections. In connection one execute set transaction isolation level read committed BEGIN TRAN Select * from a Select * from b delete from a In connection two execute set transaction isolation level read committed BEGIN TRAN Select * from a Select * from b delete from b Right now the ‘select from a’ in connection two is being blocked by the ‘delete from a’ in connection one.  This is ,IMO, quite a healthy and natural thing to be happening , some see this as a ‘slow down’, a drop in performance.  So, lets reach for our ‘NOLOCK’ magic pill.  Cancel the blocked query and ROLLBACK both transactions, then in connection one execute set transaction isolation level read uncommitted BEGIN TRAN Select * from a Select * from b delete from b and then in connection two execute set transaction isolation level read uncommitted BEGIN TRAN Select * from a Select * from b delete from a We have now solved out performance problem , no more blocking.  Lets finish the work required by the transaction, in connection one , execute delete from a Oh, ‘ performance problem’ again , its now being blocked. Still, lets complete the work in connection two…. delete from b DEADLOCK!!  It is important to be clear about the role of the select statements.  They do not participate within the deadlock, but are preventing code executing that would of.   Additionally, without the select readers to block, a deadlock would occur on the deletes with READ COMMITTED. Naturally, other isolation levels will exhibit different behaviour as to where and when they will and wont block,  and I would encourage you to read BOL and satisfy yourself that you really do NEED to NOLOCK.

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• Gracefully Handling Deadlocks

  - by Derek Dieter

  In some situations, deadlocks may need to be dealt with not by changing the source of the deadlock, but by changing handling the deadlock gracefully. An example of this may be an external subscription that runs on a schedule deadlocking with another process. If the subscription deadlocks then it would be ok to [...]

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• Gracefully Dealing with Deadlocks

  - by Derek Dieter

  In some situations, deadlocks may need to be dealt with not by changing the source of the deadlock, but by changing handling the deadlock gracefully. An example of this may be an external subscription that runs on a schedule deadlocking with another process. If the subscription deadlocks then it would be ok to [...]

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• Check for new mails in Hotmail using OpenPop.NET

  - by deadlock

  I was advised to use OpenPop lib to fetch my mail from hotmail. But I could'nt find any better way to check for new mail except disconnecting and reconnecting again. And there, I found a problem, Hotmail doesn't allow more than 1 pop3 login each 15 minutes. I want to know if it's possible to get the new mail without disconnecting and reconnecting to their pop3 server. Here is my code: Timer checker = new Timer(); checker.Interval = 1000; checker.Tick += new EventHandler(delegate { Pop3Client client = new Pop3Client(); client.Connect("pop3.live.com", 995, true); client.Authenticate("[email protected]", "myPassword"); label1.Text = client.GetMessageCount().ToString(); client.Disconnect(); }); checker.Start();

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• enable profiler / Traceon after SQL server restart automatically

  - by Nikhil

  hi, My client is facing some deadlocks while using our application. I want to track all the deadlocks for my research and to solve the deadlocks. i am currently run the SQL profiler for the event deadlock graph to capture the deadlock scenario. the actual problem is that the SQL server gets restarted every day at 2 am, and the profiler stops capturing the events after the restart. by the time i come to office at start the profiler at say 10 am, there could be deadlocks which i could have missed between 2 am and 10 am. so i am looking for a way so i can capture the deadlocks without me starting manually. i thought i could use TRACEON(1204,-1) so that the deadlock events get captured in the SQL Server error logs. But i found that the TRACE capturing too gets disabled after the restart. Is there a way i can capture the deadlocks either by SQL profiler or by using TRACEON without me manually starting the capturing? Nikhil

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• Questions re: Eclipse Jobs API

  - by BenCole

  Similar to http://stackoverflow.com/questions/8738160/eclipse-jobs-api-for-a-stand-alone-swing-project This question mentions the Jobs API from the Eclipse IDE: ...The disadvantage of the pre-3.0 approach was that the user had to wait until an operation completed before the UI became responsive again. The UI still provided the user the ability to cancel the currently running operation but no other work could be done until the operation completed. Some operations were performed in the background (resource decoration and JDT file indexing are two such examples) but these operations were restricted in the sense that they could not modify the workspace. If a background operation did try to modify the workspace, the UI thread would be blocked if the user explicitly performed an operation that modified the workspace and, even worse, the user would not be able to cancel the operation. A further complication with concurrency was that the interaction between the independent locking mechanisms of different plug-ins often resulted in deadlock situations. Because of the independent nature of the locks, there was no way for Eclipse to recover from the deadlock, which forced users to kill the application... ...The functionality provided by the workspace locking mechanism can be broken down into the following three aspects: Resource locking to ensure multiple operations did not concurrently modify the same resource Resource change batching to ensure UI stability during an operation Identification of an appropriate time to perform incremental building With the introduction of the Jobs API, these areas have been divided into separate mechanisms and a few additional facilities have been added. The following list summarizes the facilities added. Job class: support for performing operations or other work in the background. ISchedulingRule interface: support for determining which jobs can run concurrently. WorkspaceJob and two IWorkspace#run() methods: support for batching of delta change notifications. Background auto-build: running of incremental build at a time when no other running operations are affecting resources. ILock interface: support for deadlock detection and recovery. Job properties for configuring user feedback for jobs run in the background. The rest of this article provides examples of how to use the above-mentioned facilities... In regards to above API, is this an implementation of a particular design pattern? Which one?

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• What do you look for when debugging deadlocks?

  - by Michael K

  Recently I've been working on projects that heavily use threading. I think that I'm OK at designing them; use stateless design as much as possible, lock access to all resources that more than one thread needs, etc. My experience in functional programming has helped that immensely. However, when reading other people's thread code, I get confused. I am debugging a deadlock right now, and since the coding style and design are different from my personal style, I am having a difficult time seeing potential deadlock conditions. What do you look for when debugging deadlocks?

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• 100k+ Records and sp_xml_preparedocument

  - by Jonn

  I've been encountering a seeming deadlock with one of my tables and the only place I can trace it back to is a stored procedure that uses sp_xml_preparedocument on a list of data. The data inserted, btw, consists of a 100k+ records on average. Is it possible that it is causing the deadlock? What other pitfalls does using sp_xml_preparedocument have?

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• SQL Server 2008 error message from store procedure

  - by George2

  Hello everyone, When we met with such error message from store procedure, Message 1205, Level 13, State 52, the process Pr_FooV2, Line 9 Services (Process ID 111) and another process is deadlock in the lock | communication buffer resources, and has been chosen as the deadlock victim. Rerun the transaction. I am wondering whether such messages are stored in log files? I searched log folder of my SQL Server 2008 installation root (in my environment, it is C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\Log), but can not find such files. thanks in avdance, George

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• I have data about deadlocks, but I can't understand why they occur

  - by Alex

  I am receiving a lot of deadlocks in my big web application. http://stackoverflow.com/questions/2941233/how-to-automatically-re-run-deadlocked-transaction-asp-net-mvc-sql-server Here I wanted to re-run deadlocked transactions, but I was told to get rid of the deadlocks - it's much better, than trying to catch the deadlocks. So I spent the whole day with SQL Profiler, setting the tracing keys etc. And this is what I got. There's a Users table. I have a very high usable page with the following query (it's not the only query, but it's the one that causes troubles) UPDATE Users SET views = views + 1 WHERE ID IN (SELECT AuthorID FROM Articles WHERE ArticleID = @ArticleID) And then there's the following query in ALL pages: User = DB.Users.SingleOrDefault(u => u.Password == password && u.Name == username); That's where I get User from cookies. Very often a deadlock occurs and this second Linq-to-SQL query is chosen as a victim, so it's not run, and users of my site see an error screen. I read a lot about deadlocks... And I don't understand why this is causing a deadlock. So obviously both of this queries run very often. At least once a second. Maybe even more often (300-400 users online). So they can be run at the same time very easily, but why does it cause a deadlock? Please help. Thank you

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• I have data about deadlocks, but I can't understand why they occur (MS SQL/ASP.NET MVC)

  - by Alex

  I am receiving a lot of deadlocks in my big web application. http://stackoverflow.com/questions/2941233/how-to-automatically-re-run-deadlocked-transaction-asp-net-mvc-sql-server Here I wanted to re-run deadlocked transactions, but I was told to get rid of the deadlocks - it's much better, than trying to catch the deadlocks. So I spent the whole day with SQL profiler, setting the tracing keys etc. And this is what I got. There's a Users table. I have a very high usable page with the following query (it's not the only query, but it's the one that causes troubles) UPDATE Users SET views = views + 1 WHERE ID IN (SELECT AuthorID FROM Articles WHERE ArticleID = @ArticleID) And then there's the following query in ALL pages: User = DB.Users.SingleOrDefault(u => u.Password == password && u.Name == username); That's where I get User from cookies. Very often a deadlock occurs and this second LINQ TO SQL query is chosen as a victim, so it's not run, and users of my site see an error screen. I read a lot about deadlocks... And I don't understand why this is causing a deadlock. So obviously both of this queries run very often. At least once a second. Maybe even more often (300-400 users online). So they can be run at the same time very easily, but why does it cause a deadlock? Please help. Thank you

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• How atomic is a SELECT INTO?

  - by leo.pasta

  Last week I got an interesting situation that prompted me to challenge a long standing assumption. I always thought that a SELECT INTO was an atomic statement, i.e. it would either complete successfully or the table would not be created. So I got very surprised when, after a “select into” query was chosen as a deadlock victim, the next execution (as the app would handle the deadlock and retry) would fail with: Msg 2714, Level 16, State 6, Line 1 There is already an object named '#test' in the database. The only hypothesis we could come up was that the “create table” part of the statement was committed independently from the actual “insert”. We can confirm that by capturing the “Transaction Log” event on Profiler (filtering by SPID0). The result is that when we run: SELECT * INTO #results FROM master.sys.objects we get the following output on Profiler: It is easy to see the two independent transactions. Although this behaviour was a surprise to me, it is very easy to workaround it if you feel the need (as we did in this case). You can either change it into independent “CREATE TABLE / INSERT SELECT” or you can enclose the SELECT INTO in an explicit transaction: SET XACT_ABORT ON BEGIN TRANSACTION SELECT * INTO #results FROM master.sys.objects COMMIT

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• WebCenter Content (WCC) Trace Sections

  - by Kevin Smith

  Kyle has a good post on how to modify the size and number of WebCenter Content (WCC) trace files. His post reminded me I have been meaning to write a post on WCC trace sections for a while. searchcache - Tells you if you query was found in the WCC search cache. searchquery - Shows the processing of the query as it is converted form what the user submitted to the end query that will be sent to the database. Shows conversion from the universal query syntax to the syntax specific to the search solution WCC is configured to use. services (verbose) - Lists the filters that are called for each service. This will let you know what filters are available for each service and will also tell you what filters are used by WCC add-on components and any custom components you have installed. The How To Component Sample has a list of filters, but it has not been updated since 7.5, so it is a little outdated now. With each new release WCC adds more filters. If you have a filter that has no code attached to it you will see output like this: services/6    09.25 06:40:26.270    IdcServer-423    Called filter event computeDocName with no filter plugins registered When a WCC add-on or custom component uses a filter you will see trace output like this: services/6    09.25 06:40:26.275    IdcServer-423    Calling filter event postValidateCheckinData on class collections.CollectionValidateCheckinData with parameter postValidateCheckinDataservices/6    09.25 06:40:26.275    IdcServer-423    Calling filter event postValidateCheckinData on class collections.CollectionFilters with parameter postValidateCheckinData As you can see from this sample output it is possible to have multiple code points using the same filter. systemdatabase - Dumps the database call AFTER it executes. This can be somewhat troublesome if you are trying to track down some weird database problems. We had a problem where WCC was getting into a deadlock situation. We turned on the systemdatabase trace section and thought we had the problem database call, but it turned out since it printed out the database call after it was executed we were looking at the database call BEFORE the one causing the deadlock. We ended up having to turn on tracing at the database level to see the database call WCC was making that was causing the deadlock. socketrequests (verbose) - dumps the actual messages received and sent over the socket connection by WCC for a service. If you have gzip enabled you will see junk on the response coming back from WCC. For debugging disable the gzip of the WCC response.Here is an example of the dump of the request for a GET_SEARCH_RESULTS service call. socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: REMOTE_USER=sysadmin.USER-AGENT=Java;.Stel socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: lent.CIS.11g.CONTENT_TYPE=text/html.HEADER socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: _ENCODING=UTF-8.REQUEST_METHOD=POST.CONTEN socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: T_LENGTH=270.HTTP_HOST=CIS.$$$$.NoHttpHead socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: ers=0.IsJava=1.IdcService=GET_SEARCH_RESUL socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: [email protected] socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: calData.SortField=dDocName.ClientEncoding= socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: UTF-8.IdcService=GET_SEARCH_RESULTS.UserTi socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: meZone=UTC.UserDateFormat=iso8601.SortDesc socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: =ASC.QueryText=dDocType..matches..`Documen socketrequests/6 09.25 06:46:02.501 IdcServer-6 request: t`.@end. userstorage, jps - Provides trace details for user authentication and authorization. Includes information on the determination of what roles and accounts a user has access to. In 11g a new trace section, jps, was added with the addition of the JpsUserProvider to communicate with WebLogic Server. The WCC developers decide when to use the verbose option for their trace output, so sometime you need to try verbose to see what different information you get. One of the things I would always have liked to see if the ability to turn on verbose output selectively for individual trace sections. When you turn on verbose output you get it for all trace sections you have enabled. This can quickly fill up your trace files with a lot of information if you have the socket trace section turned on.

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• SQL Server 2008 error message from stored procedure

  - by George2

  Hello everyone, I am using SQL Server 2008 Enterprise. When we met with such error message from stored procedure, Message 1205, Level 13, State 52, the process Pr_FooV2, Line 9 Services (Process ID 111) and another process is deadlock in the lock | communication buffer resources, and has been chosen as the deadlock victim. Rerun the transaction. I am wondering whether such messages are stored in log files? I searched log folder of my SQL Server 2008 installation root (in my environment, it is C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\Log), but can not find such files. thanks in advance, George

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• Multi-Threaded Application vs. Single Threaded Application

  Why would we use a multi threaded application vs. a single threaded application? First we must define multithreading. Multithreading is a feature of an operating system that allows programs to run subcomponents or threads in parallel. Typically most applications only need to use one thread because they do not perform time consuming tasks. The use of multiple threads allows an application to distribute long running tasks so that they can be executed in parallel. This gives the user the perceived appearance that the application is working faster due to the fact that while one thread is waiting on an IO process the remaining tasks can make use of the available CPU. The allows working threads to execute in tandem so that they can be competed sooner. Multithreading Benefits Improved responsiveness — Users usually report improved responsiveness compared to single thread applications. Faster applications — Multiple threads can lead to improved application performance. Prioritization — Threads can be assigned a priority which would allow higher priority tasks to take precedence over lower priority tasks. Single Threading Benefits Programming and debugging —These activities are easier compared to multithreaded applications due to the reduced complexity Less Overhead — Threads add overhead to an application When developing multi-threaded applications, the following must be considered. Deadlocks occur when two threads hold a monitor that the other one requires. In essence each task is blocking the other and both tasks are waiting for the other monitor to be released. This forces an application to hang or deadlock. Resource allocation is used to prevent deadlocks because the system determines if approving the resource request will render the system in an unsafe state. An unsafe state could result in a deadlock. The system only approves requests that will lead to safe states. Thread Synchronization is used when multiple threads use the same instance of an object. The threads accessing the object can then be locked and then synchronized so that each task can interact with the static object on at a time.

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• Handling Deadlocks in SQL Server

  In this excerpt from his book Troubleshooting SQL Server: A Guide for the Accidental DBA, Jonathan Kehayias provides a guide to identifying the causes, reacting to, and ultimately preventing the dreaded deadlock. What are your servers really trying to tell you? Find out with new SQL Monitor 3.0, an easy-to-use tool built for no-nonsense database professionals.For effortless insights into SQL Server, download a free trial today.

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• Suggestions for lightweight, thread-safe scheduler

  - by nirvanai

  I am trying to write a round-robin scheduler for lightweight threads (fibers). It must scale to handle as many concurrently-scheduled fibers as possible. I also need to be able to schedule fibers from threads other than the one the run loop is on, and preferably unschedule them from arbitrary threads as well (though I could live with only being able to unschedule them from the run loop). My current idea is to have a circular doubly-linked list, where each fiber is a node and the scheduler holds a reference to the current node. This is what I have so far: using Interlocked = System.Threading.Interlocked; public class Thread { internal Future current_fiber; public void RunLoop () { while (true) { var fiber = current_fiber; if (fiber == null) { // block the thread until a fiber is scheduled continue; } if (fiber.Fulfilled) fiber.Unschedule (); else fiber.Resume (); //if (current_fiber == fiber) current_fiber = fiber.next; Interlocked.CompareExchange<Future> (ref current_fiber, fiber.next, fiber); } } } public abstract class Future { public bool Fulfilled { get; protected set; } internal Future previous, next; // this must be thread-safe // it inserts this node before thread.current_fiber // (getting the exact position doesn't matter, as long as the // chosen nodes haven't been unscheduled) public void Schedule (Thread thread) { next = this; // maintain circularity, even if this is the only node previous = this; try_again: var current = Interlocked.CompareExchange<Future> (ref thread.current_fiber, this, null); if (current == null) return; var target = current.previous; while (target == null) { // current was unscheduled; negotiate for new current_fiber var potential = current.next; var actual = Interlocked.CompareExchange<Future> (ref thread.current_fiber, potential, current); current = (actual == current? potential : actual); if (current == null) goto try_again; target = current.previous; } // I would lock "current" and "target" at this point. // How can I do this w/o risk of deadlock? next = current; previous = target; target.next = this; current.previous = this; } // this would ideally be thread-safe public void Unschedule () { var prev = previous; if (prev == null) { // already unscheduled return; } previous = null; if (next == this) { next = null; return; } // Again, I would lock "prev" and "next" here // How can I do this w/o risk of deadlock? prev.next = next; next.previous = prev; } public abstract void Resume (); } As you can see, my sticking point is that I cannot ensure the order of locking, so I can't lock more than one node without risking deadlock. Or can I? I don't want to have a global lock on the Thread object, since the amount of lock contention would be extreme. Plus, I don't especially care about insertion position, so if I lock each node separately then Schedule() could use something like Monitor.TryEnter and just keep walking the list until it finds an unlocked node. Overall, I'm not invested in any particular implementation, as long as it meets the requirements I've mentioned. Any ideas would be greatly appreciated. Thanks! P.S- For the curious, this is for an open source project I'm starting at http://github.com/nirvanai/Cirrus

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• Indexed view deadlocking

  - by Dave Ballantyne

  Deadlocks can be a really tricky thing to track down the root cause of.  There are lots of articles on the subject of tracking down deadlocks, but seldom do I find that in a production system that the cause is as straightforward.  That being said,  deadlocks are always caused by process A needs a resource that process B has locked and process B has a resource that process A needs.  There may be a longer chain of processes involved, but that is the basic premise. Here is one such (much simplified) scenario that was at first non-obvious to its cause: The system has two tables,  Products and Stock.  The Products table holds the description and prices of a product whilst Stock records the current stock level. USE tempdb GO CREATE TABLE Product ( ProductID INTEGER IDENTITY PRIMARY KEY, ProductName VARCHAR(255) NOT NULL, Price MONEY NOT NULL ) GO CREATE TABLE Stock ( ProductId INTEGER PRIMARY KEY, StockLevel INTEGER NOT NULL ) GO INSERT INTO Product SELECT TOP(1000) CAST(NEWID() AS VARCHAR(255)), ABS(CAST(CAST(NEWID() AS VARBINARY(255)) AS INTEGER))%100 FROM sys.columns a CROSS JOIN sys.columns b GO INSERT INTO Stock SELECT ProductID,ABS(CAST(CAST(NEWID() AS VARBINARY(255)) AS INTEGER))%100 FROM Product There is a single stored procedure of GetStock: Create Procedure GetStock as SELECT Product.ProductID,Product.ProductName FROM dbo.Product join dbo.Stock on Stock.ProductId = Product.ProductID where Stock.StockLevel <> 0 Analysis of the system showed that this procedure was causing a performance overhead and as reads of this data was many times more than writes,  an indexed view was created to lower the overhead. CREATE VIEW vwActiveStock With schemabinding AS SELECT Product.ProductID,Product.ProductName FROM dbo.Product join dbo.Stock on Stock.ProductId = Product.ProductID where Stock.StockLevel <> 0 go CREATE UNIQUE CLUSTERED INDEX PKvwActiveStock on vwActiveStock(ProductID) This worked perfectly, performance was improved, the team name was cheered to the rafters and beers all round.  Then, after a while, something else happened… The system updating the data changed,  The update pattern of both the Stock update and the Product update used to be: BEGIN TRAN UPDATE... COMMIT BEGIN TRAN UPDATE... COMMIT BEGIN TRAN UPDATE... COMMIT It changed to: BEGIN TRAN UPDATE... UPDATE... UPDATE... COMMIT Nothing that would raise an eyebrow in even the closest of code reviews.  But after this change we saw deadlocks occuring. You can reproduce this by opening two sessions. In session 1 begin transaction Update Product set ProductName ='Test' where ProductID = 998 Then in session 2 begin transaction Update Stock set Stocklevel = 5 where ProductID = 999 Update Stock set Stocklevel = 5 where ProductID = 998 Hop back to session 1 and.. Update Product set ProductName ='Test' where ProductID = 999 Looking at the deadlock graphs we could see the contention was between two processes, one updating stock and the other updating product, but we knew that all the processes do to the tables is update them.  Period.  There are separate processes that handle the update of stock and product and never the twain shall meet, no reason why one should be requiring data from the other.  Then it struck us,  AH the indexed view. Naturally, when you make an update to any table involved in a indexed view, the view has to be updated.  When this happens, the data in all the tables have to be read, so that explains our deadlocks.  The data from stock is read when you update product and vice-versa. The fix, once you understand the problem fully, is pretty simple, the apps did not guarantee the order in which data was updated.  Luckily it was a relatively simple fix to order the updates and deadlocks went away.  Note, that there is still a *slight* risk of a deadlock occurring, if both a stock update and product update occur at *exactly* the same time.

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• what type of bug causes a program to slowly use more processor power and all of a sudden go to 100%?

  - by reinier

  Hi, I was hoping to get some good ideas as to what might be causing a really nasty bug. This is a program which is transmitting data over a socket, and also receives messages back. I could explain lots more, but I don't think this will help here. I'm just searching for hypothetical problems which can cause the following behaviour: program runs processor time slowly accumulates (till around 60%) all of a sudden (could be after 30 but also after 60 seconds) the processor time shoots to 100%. the program halts completely In my syslog it always ends on one line with a memory allocation (something similar to: myArray = new byte[16384]) in the same thread. now here is the weird part: if I set the debugger anywhere...it immediately stops on that line. So just the act of setting a breakpoint, made the thread continue (it wasn't running since I saw no log output anymore) I was thinking 'deadlock' but that would not cause 100% processor power. If anything, the opposite. Also, setting a breakpoint would not cause a deadlock to end. anyone else a theoretical suggestion as to what kind of 'construct' might cause this effect? (apart from 'bad programming') ;^) thanks EDIT: I just noticed.... by setting the sendspeed slower, the problem shows itself much later than expected. I would think around the same amount of packets send...but no the amount of packets send is much higher this way before it has the same problem.

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• Python C API from C++ app - know when to lock

  - by Alex

  Hi Everyone, I am trying to write a C++ class that calls Python methods of a class that does some I/O operations (file, stdout) at once. The problem I have ran into is that my class is called from different threads: sometimes main thread, sometimes different others. Obviously I tried to apply the approach for Python calls in multi-threaded native applications. Basically everything starts from PyEval_AcquireLock and PyEval_ReleaseLock or just global locks. According to the documentation here when a thread is already locked a deadlock ensues. When my class is called from the main thread or other one that blocks Python execution I have a deadlock. Python Cfunc1() - C++ func that creates threads internally which lead to calls in "my class", It stuck on PyEval_AcquireLock, obviously the Python is already locked, i.e. waiting for C++ Cfunc1 call to complete... It completes fine if I omit those locks. Also it completes fine when Python interpreter is ready for the next user command, i.e. when thread is calling funcs in the background - not inside of a native call I am looking for a workaround. I need to distinguish whether or not the global lock is allowed, i.e. Python is not locked and ready to receive the next command... I tried PyGIL_Ensure, unfortunately I see hang. Any known API or solution for this ? (Python 2.4)

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• Operating systems theory -- using minimum number of semaphores

  - by stackuser

  This situation is prone to deadlock of processes in an operating system and I'd like to solve it with the minimum of semaphores. Basically there are three cooperating processes that all read data from the same input device. Each process, when it gets the input device, must read two consecutive data. I want to use mutual exclusion to do this. Semaphores should be used to synchronize: P1: P2: P3: input(a1,a2) input (b1,b2) input(c1,c2) Y=a1+c1 W=b2+c2 Z=a2+b1 Print (X) X=Z-Y+W The declaration and initialization that I think would work here are: semaphore s=1 sa1 = 0, sa2 = 0, sb1 = 0, sb2 = 0, sc1 = 0, sc2 = 0 I'm sure that any kernel programmers that happen on this can knock this out in a minute or 2. Diagram of cooperating Processes and one input device: It seems like P1 and P2 would start something like: wait(s) input (a1/b1, a2/b2) signal(s)

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• Interviewing a DBA

  - by kev

  Our Company is in the Process of recuiting a DBA. I have built a group test of questions from basic questions such as Pk and Fk constraints, simple querries(fizzbuzz style) to more advanced things such as indexes, Collation, isolation levels and how to trace deadlocks. However, that is the limit of my knowledge. So my question to all the DBA's is what is the base level knowledge that all DBA's should have? We are really looking for someone that will be able to manage our replication, analyzing some of our slower running queries(that the devs can go to for help) and someone that can trace some of the deadlock issues that we are having. Any help would be most appreciated!

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• Kinetic Scroll in Maverick

  - by Shoaib

  I have Sony VAIO FW 230J laptop. I was using Lucid and upgraded my system to Maverick beta in September 2010. Although there were few issues with beta release on my system although I did not felt myself in any deadlock. The most notable experience on my ubuntu was the smooth and kinetic scrolling and very sensitive touch pad experience with finer control that was oddly normal in previous releases and even on windows 7 currently. Definitely this release of ubuntu is improved for touch UI experience. I believe that this is magic of 10.10. For my official system I waited until the final release was out. After installing (upgrading) official system to Maverick I am not feeling the same smooth and kinetic scroll experience. My laptop has Graphics: Intel DMA-X4500MHD while my official desktop has Graphics: Intel DMA-X4500HD Do I need to install or update some software package to have similar kinetic scroll experience?

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• Interviewing a DBA

  - by kev

  Our Company is in the Process of recuiting a DBA. I have built a group test of questions from basic questions such as Pk and Fk constraints, simple querries(fizzbuzz style) to more advanced things such as indexes, Collation, isolation levels and how to trace deadlocks. However, that is the limit of my knowledge. So my question to all the DBA's is what is the base level knowledge that all DBA's should have? We are really looking for someone that will be able to manage our replication, analyzing some of our slower running queries(that the devs can go to for help) and someone that can trace some of the deadlock issues that we are having. Any help would be most appreciated!

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