OS Threads DMV

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…

Related Posts

Diving Into Clustered Index Seeks

Hugo Kornelis looks in detail at the clustered index seek: The Clustered Index Seek operator uses the structure of a clustered index to efficiently find either single rows (singleton seek) or specific subsets of rows (range seek). Because a clustered index always contains all columns in a table, a Clustered Index Seek is one of the most […]

Read More

Deep Dive on Index Seeks

Hugo Kornelis gives us a great deal of information on index seeks in SQL Server: Every Seek Keys specification can be either for a “singleton seek”, or for a “range seek”. A singleton seek applies when at most a single row can satisfy the requirement of the Seek Keys specification. A range seek means that (potentially) more than a […]

Read More

Categories

August 2017
MTWTFSS
« Jul Sep »
 123456
78910111213
14151617181920
21222324252627
28293031