Unsung SQLOS: the EventInternal

Today we’re taking a step towards scheduler territory by examining the EventInternal class, the granddaddy of SQLOS synchronisation objects. At the outset, let’s get one formality out of the way: although it is a template class taking a spinlock type as template parameter, we only see it instantiated as EventInternal<SuspendQSLock> as of SQL Server 2014. What this means is that spins on its internal spinlock is always going to be showing up as SOS_SUSPEND_QUEUE.

It’s a very simple class (deceptively so even) which can implement a few different event flavours, doing its waiting via SQLOS scheduler methods rather than directly involving the Windows kernel. The desire to keep things simple and – as far as possible – keep control flow out of kernel mode is a very common goal for threading libraries and frameworks. .Net is a good frame of reference here, because it is well documented, but the pattern exists within OS APIs too, where the power and generality of kernel-mode code has to be weighed off against the cost of getting there.
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