Category: Internals

Ewald Cress has finally snapped:

You start with a blank sheet,
three nuts and a bolt,
a strong sense of fairness,
a large can of Jolt.

And you try to imagine,
as best as you’re able
a rulerless kingdom
that won’t grow unstable.

That’s what happens when you dig into internals for too long.

Ewald Cress builds an extended metaphor for the SQLOS scheduler:

When the time comes for a bunny to pass the battery, it may be out of free choice, or it might be because its script went down a path where passing it along is The Thing To Do. At this juncture, the team’s collective memory and playbook comes to the fore, and agreed rules dictate who the battery goes to. It doesn’t really matter what those rules are for the moment. The important point is that control is transferred by the players themselves using shared rules and a team whiteboard tracking who is ready to go, which team member might be most deserving, who has been waiting the longest etc. This code of conduct and state, this bushido of bunny bonhomie, is what we call a scheduler.

This is building up to something big…

Ewald Cress discusses the breferences member:

I’ll spare you my false starts, but I think I finally have it. The first observation is that, on the occasions breferences increments, it does not increment linearly, but instead has an exponential growth pattern. These increments take it through the sequence 0, 1, 3, 7, 15, 31, 127, 255 etc. Or in binary: 0, 1, 11, 111, 1111, 11111, 111111, 1111111, 11111111…

Those numbers can be seen as off-by-one variations of powers of two. Forget the offset, and think of the number as simply doubling on each increment if it keeps your head clearer – instead of accuracy, we have a order-of-magnitude reference count.

I’d never heard of an algorithm like this, although that could be due to my having dealt with relatively little low-level structural code.  I’m glad Ewald sussed out the mechanics driving breferences.

Ewald Cress discusses superlatch promotion:

There are quite a few pieces of machinery that are involved in our little drama. First, I’ll introduce some instance-global settings:

• A flag that controls whether latch promotion is enabled at all. Although I don’t have any information about this, let’s assume that it will be enabled on any system that “warrants it”.
• A flag that controls whether cycle-based promotion is enabled. Again, I can’t currently tell you what determines this setting.
• sm_promotionThreshold, the current calculated cycle-based promotion threshold described in Part 3.
• sm_promotionUpperBoundCpuTicks, used as a ceiling value to prevent outliers from skewing stats. As described in Part 3, this is simply sm_promotionThreshold * 5.
• Trace flag 844, which lowers the threshold for non-cycle-based promotions.
• Trace flag 827, which causes each latch promotion to be noted in the SQL Server log (“Latch promotion, page %u:%u in database %u, objid %u.”)

Assume that the first flag is set on our system of interest, otherwise promotions won’t happen and we have nothing to talk about.

Read the whole thing.

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.

Mike Ruthruff discusses changes in checkpoint behavior in SQL Server 2016:

The following are the primary changes which will impact behavior of checkpoint in SQL Server 2016.

1. Indirect checkpoint is the default behavior for new databases created in SQL Server 2016. Databases which were upgraded in place or restored from a previous version of SQL Server will use the previous automatic checkpoint behavior unless explicitly altered to use indirect checkpoint.

2. When performing a checkpoint SQL Server considers the response time of the I/O’s and adjusts the amount of outstanding I/O in response to response times exceeding a certain threshold. In versions prior to SQL Server 2016 this threshold was 20ms. In SQL Server 2016 the threshold is now 50ms. This means that SQL Server 2016 will wait longer before backing off the amount of outstanding I/O it is issuing.

3. The SQL Server engine will consolidate modified pages into a single physical transfer if the data pages are contiguous at the physical level. In prior versions, the max size for a transfer was 256KB. Starting with SQL Server 2016 the max size of a physical transfer has been increased to 1MB potentially making the physical transfers more efficient. Keep in mind these are based on continuity of the pages and hence workload dependent.

Definitely read the whole thing.

Ewald Cress discusses a “secret” spinlock within latches:

No matter how bad contention gets for normal spinlocks, at least we account for cycles spent spinning: this stuff gets exposed in sys.dm_os_spinlock_stats and can allow us to diagnose and quantify contention. However, spinning done on a latch’s spinlock gets no obviously visible accounting. As such, if we did somehow manage to accrue a large number of spins on a very hot latch, it wouldn’t be obvious that the time went into spinning. This is not to say of course that it is necessarily a common problem, just that it would be hard to prove one way or the other.

If I appear to be painting a bleak picture, I apologise. Given the importance of latches, especially buffer latches, one would assume that the SQL Server development teams would be on the constant lookout for opportunities to identify issues and mitigate against them. And this is exactly the kind of scenario where some bright spark comes up with the idea of superlatches.

Read the whole thing.