Category: Internals

Ewald Cress moves up the internals stack a little further and looks at a DMV:

Broadly speaking, a DMV presents just another iterator that can be plugged into a query plan. The execution engine calls GetRow() repeatedly until it reaches the end, and the iterator emits rows. The only unusual thing is that the ultimate source of the data may have nothing to do with the storage engine.

Now if you asked me to guess where in the world we’d find a list of all threads to iterate over, I would have expected that we’d start with the NodeManager, iterating over all SOS_Nodes, and then for each of them iterating over its collection of associated SystemThreads. After all, we have a guaranteed 1:1 correspondence between threads and SystemThreads, and I figured that all SystemThreads enlist themselves into a parent SOS_Node upon creation. No-brainer, right?

Turns out that this guess would have been completely wrong, and the reason it would have been a broken implementation will become apparent when we look at the started_by_sqlservr column.

Definitely read the whole thing, but you may need to top off your caffeinated beverage of choice first.  Also, I’m pretty sure Ewald promised to hammer one out once per week; that’s how I read it, at least…

Ewald Cress continues his journey to the center of the SQLOS:

The SQLOS scheduler exists in the cracks between user tasks. As we’re well aware, in order for scheduling to happen at all, it is necessary for tasks to run scheduler-friendly code every now and again. In practice this means either calling methods which have the side effect of checking your quantum mileage and yielding if needed, or explicitly yielding yourself when the guilt gets too much.

Now from the viewpoint of the user task, the experience of yielding is no different than the experience of calling any long-running CPU-intensive function: You call a function and it eventually returns. The real difference is that the CPU burned between the call and its return was spent on one or more other threads, while the current thread went lifeless for a bit. But you don’t know that, because you were asleep at the time!

Definitely read the whole thing.

Ewald Cress explains how SQLOS tasks come into being:

The above system tasks wear their purpose on their sleeves, because the function pointer in the SOS_Task::Param is, well, to the point. The tasks that run user queries are more abstract, because the I/O completion port listener can’t be bothered with understanding much beyond the rudiments of reading network packets – it certainly can’t be mucking about with fluent TDS skills, SQL parsing and compilation, or permission checks.

So what it does is to enqueue a task that speaks TDS well, pointing it to a bunch of bytes, and sending it on its way. Here is an example of such a dispatch, which shows the “input” side of the WorkDispatcher:

Read the whole thing.

Ewald Cress has some thoughts on external wait types:

Previously I dug into preemptive waits in SQLOS, and to be honest, I equated “preemptive” with “external”. For the most part the two go hand in hand after all.

To recap, a preemptive wait isn’t necessarily a wait at all. What happens is that a worker needs to run some code that can’t be trusted to play by cooperative scheduling rules. And rather than put the SQLOS scheduler (and all its sibling workers) at the mercy of that code, the worker detaches itself from the scheduler and cedes control to a sibling runnable worker.

Read the whole thing.