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Saturday, June 18, 2011

Understanding how SQL Server behaves in a non-preemptive mode while still running on the OS which is preemptive

Today , I was asked by someone how SQL Server behaves in a non-preemptive way while still running on the OS which is preemptive .Even though I explained this theoritically ,I was requested if there is a way we can see it practically . It wasn't difficult but it really proved to what I explained .I alos felt that if we add practicals to theory it will have much deeper impact ...

NOTE : I am doing all the testing on SQL Server 2008 RTM EVAL as what I am trying to explain can be done in 2008 onwards.On 2005 you will not see what is written here.So I would request to use SQL 2008 and above .

Windows Scheduling (Preemptive) :
Starting from Windows NT 3.1 (XP ,2000,2003 etc) Windows scheduling was priority driven i.e preemptive scheduling .So every thread will have a priority associated with it .Based on this priority the threads will get the time slice (Quantum) to run on the CPU .So , even if thread of lower priority is running and all of a sudden another thread of a higher
priority comes up , the low priority thread will be preempted interrupted) and the higher priority thread will be scheduled to run on the CPU.However , the scheduler is smart enough .It will keep the preempted thread on the top of the waiting threads by adjusting its priority (Lets not get too deep in to this at this point).

Prior to this OS scheduling was non-preemptive i.e. cooperative scheduling .Remember the days when Windows 98 use to hang and we use to reboot the server quite often ,to get rid of it.Cooperative scheduling is good if all the threads leave the CPU after some time and give chance to other threads (including kernal mode threads which are more
Important and get a chance to run whenever required) after a fixed interval of time . But that normally does not happen .some nasty application threads don't yield and hence blocks other threads.

SQL Server Scheduling (non-preemptive) :
SQL Server has its own scheduling mechanism and it does not follow OS scheduling (looks strange as it runs on the preemptive OS) .Its called as UMS (User Mode Scheduling) in 2000 and SOS (SQL OS) in 2005 and above .BUT :
1) Why SQL Server does not hang just like windows 98 use to ?
Answer: SQL Server does not hang because its threads yield every voluntarily.In case a thread does not yield in 60 seconds (unlike the faulty application where the threads does not yield)SQL Server throws non-yielding scheduler hung error and throws a mini dump with the stack information of all the threads in it .

2) How SQL Server manages to schedule in the non-preemptive way ?
Answer: Windows OS will not schedule any thread which is running in infinite wait loop and simply ignores it. SQL Server (actually UMS) takes advantage of this and cleverly puts all the threads which it does not want to schedule to infinitely sleep by calling WaitforsingleObject function in an infinite loop .When SQL Server wants the thread to run it simply signal the thread and it comes out of the sleep modes .Its the Windows which then schedules the thread .Its important to know that UMS schedules only ONE SQl Server thread per CPU .However , there is an exception to this . There are moments where to complete a task the thread leaves the SQL Server scheduler and goes to preemptive mode scheduling . For example using xp_cmdshell to open notepad or running an extended stored procedure that deals with filesystem (like reading a file) or a linked server query
etc.In that situation , you will see more than one thread on a single CPU in runnable status .That is because one thread is scheduled via UMS\SOS and the other one directly via OS scheduling.

Let me show you a Demo since my Laptop has only one dual core processor (Its SQL Server 2008 RTM):
Lets first run a simple query and find runnable and sleeping threads Select Status ,* from sysprocesses where status not in ('background')

you will notice that all the SPIDS will show you the status of sleeping and there is only one SPID that will be showing you the status of runnable .Its waittype will be PREEMPTIVE_os_WAITFORSINGLEOBJECT .Notice that only runnable SPID has a KPID associated with it .This KPID is nothing but the worker thread associated with the SPID.You can run it a few times but the output will not change except the KPID which means that one thread is yielding to another after the context switching .The reason why we see runnable state and not running ,because by the time we get the query output the thread again goes to runnable state.You might also see other runnable or suspended SPIDs but its because they are running in preemptive mode .

Now lets open another Query window and execute the same command there .
Select Status ,* from sysprocesses where status not in ('background'). This time its SPID 53 (current SPID on my machine) which is showing us the Runnable state while SPID 52 (the previous SPID)is now sleeping .

Lets do one more experiment.Open a new query window (SPID 51 in my case) and run select @@servicename around 1000 times . Come back to SPID 53 window and notice if the runnable state is showing for SPID 53 or SPID 51.You will notice that we have the SPID 51 doing its task .But why is it showing as sleeping while CPU value is still increasing ? The reason is that when we run the query via SPID 53 , during (and only) that time 51 shows as sleeping because 53 needs to run . So thread related to 51 yields voluntarily. when this query finishes , SPID 51 again picks up , but we cant see that since we have only one processor :) ...

Anyway , let me show you the small test when the SQL Server thread goes preemptive .We have 2 Query windows.One with sysprocesses query (SPID 53) and one with calling xp_readerrorlog 100 times (SPID 51).I further modified my sysprocesses query by filtering sleeping SPIDs.

Select Status ,* from sysprocesses where status not in ('background','sleeping') .Lets run the query through SPID 51 and then by SPID 53.Notice that you have 2 runnable SPIDs now . Thats because SPID 51 is scheduled by OS and not SQL OS \UMS .

Have a nice day and Happy Learning !!!

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