Hypervisor-Driven Wait Stats

Paul Randal explains that delays in the hypervisor layer could be responsible for SOS_SCHEDULER_YIELD waits in SQL Server:

Specifically, I was concerned about SOS_SCHEDULER_YIELD waits. This is a special wait type that occurs when a thread is able to run for 4ms of CPU time (called the thread quantum) without needing to get suspended waiting for an unavailable resource. In a nutshell, a thread must call into the SQLOS layer every so often to see whether it has exhausted its thread quantum, and if so it must voluntarily yield the processor. When that happens, a context switch occurs, and so a wait type must be registered: SOS_SCHEDULER_YIELD. A deeper explanation of this wait type is in my waits library here.

My theory was this: if a VM is prevented from running for a few milliseconds or more, that could mean that a thread that’s executing might exhaust its thread quantum without actually getting 4ms of CPU time, and so yield the processor causing an SOS_SCHEDULER_YIELD wait to be registered. If this happened a lot, it could produce a set of wait statistics for a virtualized workload that appears to have lots of SOS_SCHEDULER_YIELDs, when in fact it’s actually a VM performance problem and the SOS_SCHEDULER_YIELD waits are really ‘fake’.

Read on for more details, and definitely check out the link.  It was an eye-opener when I learned that SOS_SCHEDULER_YIELD didn’t mean “need more/more powerful CPUs.”

Related Posts

Welcome, CXCONSUMER

Erik Darling points out that CXCONSUMER is now a wait type in SQL Server: According to Pedro’s slide, but not the ENTIRELY MISSING DOCUMENTATION, this wait is the “safe” type of parallelism wait. It’s a good thing Pedro is a dutiful blogger, so we don’t have to pull our hair out while unfurling these mysteries. Speaking of […]

Read More

Getting Wait Stats From The Query Store

Erin Stellato shows how to retrieve wait stats per query from Query Store: In SQL Server 2016 a new DMV is exposed, sys.dm_exec_session_wait_stats, which provides information about waits for an existing, active session. If you know the session_id, you can track waits for a query when it starts and when it completes (snapshot the information at […]

Read More

Categories

June 2017
MTWTFSS
« May Jul »
 1234
567891011
12131415161718
19202122232425
2627282930