The Latch Files 4: Superlatch promotion logic

In Part 3 I explained how this beast called the buffer latch cycle-based latch promotion threshold gets calculated and broadly what it means, but I didn’t tackle the obvious question of “who does what with this information?”. This post will tie some global settings together with per-buffer tracking to unravel the mystery of when a buffer latch is deemed hot enough to deserve a promotion. What I describe applies identically to SQL Server 2014 and SQL Server 2016, and it is likely that it wouldn’t have changed much from preceding versions, although I haven’t confirmed this.

Mugatu from Zoolander
Mugatu assesses breferences

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The Latch Files 3: Superlatch promotion threshold

Here’s something odd. If you do an online search for “SQL Server latch promotion”, a number of top hits (e.g. this, this and this) don’t actually concern latch promotion, but rather an obscure informational message that seemed to come out of the woodwork in 2008 R2: Warning: Failure to calculate super-latch promotion threshold.”.

Somewhere among those hits you’ll find one of the very few sources around latch promotion, Bob Dorr’s excellent “How it works” blog post. One thing that bugs me about this picture is that latch promotion clearly isn’t that much talked about until people see unusual messages in their error logs.
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The Latch Files 2: The spinlock that dares not speak its name

Spinlocks live among us. We see them on duty, in uniform, and greet them by name. When we interact, they show a badge and leave a receipt for the time they eroded from our working day. Or so we’d like to think.

A spinlock headbanging orgy
A spinlock headbanging party in full spin

When looking at the 2016 SOS_RWLock, we came across the one-bit spinlock buried within its Count member. Since it protects a very simple wait structure, someone evidently made the decision that it is cheap enough to spin aggressively until acquired, with no backoff logic. This suggests that a low degree of spinlock contention is anticipated, either because few threads are expected to try and acquire the lock simultaneously or because the amount of business to be done while holding the lock is very light and likely to finish quickly.
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The Latch Files: Out for the count

Time to start chipping away at the monster subject of storage engine latches. If you’re anything like me, you were really impressed by the expositions of latches done by James Rowland-Jones (in Professional SQL Server 2008 Internals and Troubleshooting) and Bob Ward (PASS Summit “Inside Latches” session) when this information first started dribbling out. Now we have reached a point in history where latches seem to be used as a swear word. Well, for the record, I am still fascinated by them, and their internals are pretty darn marvellous.

Today I’m going to keep it comparatively focused, looking at nothing other than the Count member of the LatchBase class. Specifically, I’ll only be considering the act of acquiring an uncontended un-promoted latch, based on the SQL Server 2014 and 2016 latch implementation.
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