EventInternal

Ewald Cress continues to dig into scheduling, this time looking at EventInternal:

signalMode adds a twist. The behaviour described for the traffic light corresponds to a signal mode of 0, also known as a manual reset event. Here the event stays signalled irrespective of how many consumers pass through it (=successfully wait on it).

A signal mode of 1, however, turns it into an auto-reset event, where the act of successfully waiting on the event resets it to unsignalled. This is now more akin to a turnstile that only lets one person through after being signalled, e.g. by a scan of a valid transport pass or a button press by a security guard.

Interestingly, a event object is also sometimes known as a latch – that’s something to chew on for SQL Server folks. Don’t get hung up about who or what signals it; that is a separate issue altogether. Just keep in mind that the signal mode is a permanent attribute of the event – you construct it as manual reset or auto-reset. Full disclosure: there seems to be at least one more SignalMode (2, used by the related SOS_WaitableAddress), but let’s ignore it today.

This is part of a great series, and I hope Ewald keeps it up.  I’d probably drop a few bucks on a cleaned up and edited version of his discussion of internals in an 80-page or so e-book.

Related Posts

Understanding Hash Match Aggregates

Itzik Ben-Gan continues his series on grouping and aggregating data by looking at the hash match aggregation process: The estimated CPU cost for the Hash Aggregate in the plan for Query 8 is 0.166344, and in Query 9 is 0.16903. It could be an interesting exercise to try and figure out exactly in what way […]

Read More

Row Width And Snapshot Isolation

Kendra Little shows us the impact that row width has on snapshot isolation: So I went to work to demonstrate row width impact on the version store — when only a tiny bit column is changed in the row. Here’s how I did the test: I created two tables, dbo.Narrow and dbo.Wide. They each each have a […]

Read More

Categories

June 2016
MTWTFSS
« May Jul »
 12345
6789101112
13141516171819
20212223242526
27282930