![]() ![]() Yes, the good ol' blocking chains with a head blocker that you are so familiar with, is the most common example. Most Common Causes of Deadlocked Schedulers:Įven though many conditions can cause all schedulers to be stuck at the same time, in 95% of the time that I have looked at memory dumps, the reason has been that the majority of worker threads were tied up waiting for a lock resource. So if you are thinking of "traditional" lock deadlocks, you may steer away from such thoughts. ![]() The term applies to SOS Schedulers and the fact that none of them are processing queries, logins, etc. The term does NOT imply a classic Lock-Manager deadlock where multiple sessions are trying to access locks in the opposite order and permanently block each other. The phraase "deadlocked schedulers" has confused many and rightly so. When the Scheduler monitor detectes a "deadlock schedulers" condition, it reports an error in the Errorlog and triggers a memory dump to be generated by the SQLDumper.exe against SQL Server process. If any of these conditions is met (not all), the Scheduler Monitor declares that schedulers are stuck - deadlocked.Ī note on NUMA: SQL Server can report Deadlocked Schedulers if all the SOS schedulers on a single NUMA node are stuck, even if other NUMA nodes are processing tasks just fine. have any new tasks been assigned to workers)? Has any work been processed since last check (i.e. ![]() Actually, a white paper has been written already - SOS Scheduler white paper Īn entire white paper could be written about how SOS scheduler works but for this discussion I will limit the information to some summary points. Typically this Scheduler Monitor thread will report problems every 60 seconds, though it monitors the IOCP thread that accepts connections every 15 seconds. Examples of irregularities include a thread not yielding voluntarily (a non-yielding scheduler) or all schedulers are "stuck" not processing any requests (deadlocked schedulers). Because of that, the SOS scheduler has a dedicated thread -Scheduler Monitor - that periodically checks the state of each scheduler and reports any "irregularities". DEADLOCK SQL CODEBut even the best-intentioned developer could make mistakes (and introduce a bug in his code), or a SQL thread could be at the mercy of some external component – like calling into code outside of SQL Server. Since SQL Server uses a cooperative scheduler, it relies on the good heart of each developer who writes code in SQL Server to call a Yield() function of some kind that prevents the thread from monopolizing the CPU. The ultimate goal is to reduce the very expensive kernel-mode context switching from one thread to another. The goal of SOS is for SQL Server to expose only one thread at a time per CPU and thus minimize competition among SQL threads exposed to the OS. DEADLOCK SQL WINDOWSOf course, once a thread "leaves" SQL Server SOS kingdom, that thread is still handed off to the Windows preemptive scheduler (ruled by another Master so to speak). That means that there are locations in the SQL Server code where the Microsoft developer built in yield points, causing execution to “pause” and gracefully let another citizen in the SQL kingdom to exercise its right to execute. UMS/SOS is a cooperative (non-preemptive) scheduler which means that it relies on threads to voluntarily give up CPU usage - yield - to the next thread waiting in line. and later renamed to SOS (SQL on OS scheduler). Since SQL Server 7.0, SQL Server has used its own scheduling mechanism, called UMS (User-mode scheduler) in 7. Just thought I would summarize the concept of a SOS Scheduler and a Deadlocked Scheduler since many have inquired over the years for a brief summary of these concepts ![]()
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