12/18/2023 0 Comments Sos online backup slow uploadYielding while performing a sort (in this case, as part of running a nested-loop join).Yielding while scanning a table (in this case, as part of an ordered parallel table scan). Known occurrences in SQL Server (list number matches call stack list): This can elevate the number of SOS_SCHEDULER_YIELD waits, as I describe in this post.įinally, some workloads can suffer from SOS_SCHEDULER_YIELD waits when automatic soft-NUMA is enabled – see this post for details. One more thing to consider is whether your workload is running on a VM that’s experiencing delays because of the host being oversubscribed. SOS_SCHEDULER_YIELD waits always have a 0 resource wait component, because no resource wait occurs (which is why the thread doesn’t go onto the waiter list). If the context switch does not occur (because the thread fails to check whether the quantum has expired), that’s a non-yielding scheduler and you’ll see message 17883 in the error log. It’s the context switch that forces the registration of a wait type within SQLOS. The thread cannot decide to just not yield. It has no knowledge of other threads on that scheduler and there is *always* a context switch when the thread goes to the bottom of the runnable queue, even if it’s the only thread on the scheduler. When the thread quantum expires, the thread *must* yield the processor. Note that queries incurring SOS_SCHEDULER_YIELD waits don’t show up in sys.dm_os_waiting_tasks so you need a script that looks at sys.dm_exec_requests instead – and I have one in this blog post. is there a missing nonclustered index causing an in-memory table scan?). I wrote a long article on understanding and troubleshooting SOS_SCHEDULER_YIELD waits on that explains more about thread scheduling and quantum exhaustion, plus troubleshooting. Basically this involves identifying the query that’s producing the SOS_SCHEDULER_YIELD waits and making sure the query plan looks correct (e.g. Just like CXPACKET waits, don’t jump to the conclusion that SOS_SCHEDULER_YIELD waits are bad. This could be because a query plan is erroneously doing a table scan, or it could be a normal part of your workload. The most common cause of SOS_SCHEDULER_YIELD waits that I see is queries doing scans of pages that are in memory and aren’t changing, hence there’s no contention for page access and the scanning thread can run until it exhausts its thread quantum. “The query needs more CPU” – no, see #2.“There must be CPU pressure” – no, CPU pressure is indicated by increasing signal-wait times and long Runnable Queues, not by the prevalence of SOS_SCHEDULER_YIELD waits.“It must be spinlocks that are the problem” – no, spinlocks are not tracked by wait types.There are various knee-jerk reactions to this wait type: After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds. The map_key value in sys.dm_xe_map_values is 120 in 20 R2, and 124 in 20 RTM. Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic. During this wait the task is waiting for its quantum to be renewed.”) ( Books Online description: “Occurs when a task voluntarily yields the scheduler for other tasks to execute. Even though the thread doesn’t need to wait, it must register a wait type when it context switches off the processor, and that wait type is SOS_SCHEDULER_YIELD. Although the thread is immediately in the RUNNABLE state, it does not go onto the Waiter List because it doesn’t have to wait for a resource. This wait type is when a thread was able to execute for its full thread quantum (4 milliseconds in all versions of SQL Server, unchangeable), and so voluntarily yielded the scheduler, moving to the bottom of the Runnable Queue in its scheduler. (Republishing, or using this info in a commercial product/website, is prohibited without permission.
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