Latch Promotion

Ewald Cress discusses latch promotion threshold calculations:

Now I wish I could use the phrase “cycle-based promotion threshold” in a tone that suggests we were all born knowing the context, but to be honest, I don’t yet have all the pieces. Here is the low-down as it stands in SQL Server 2014:

  • Everything I’m describing applies only to page latches.

  • A cycle-based promotion simply means one that is triggered by the observation that the average acquire time for a given page latch (i.e. the latch for a given page) has exceeded a threshold.

  • Because the times involved are so short, they are measured not in time units but in CPU ticks.

  • There exists a global flag that can enable cycle-based promotions, although I do not know what controls that flag.

  • If cycle-based promotion is disabled, there is another path to promotion; this will be be discussed in Part 4.

I don’t think I’d ever seen the informational message Ewald mentions, so this was a brand new topic to me.

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