Unsung SQLOS: linked lists

I’m slowly working towards some more juicy subjects again, but for the moment it is sensible to get fundamentals out of the way. The use of linked lists by SQL Server and SQLOS is a great place to start, because so much is built upon them. Grok this and you’re set for countless hours of fun.

From general exposure to SQLOS scheduling, many of us are familiar with concepts like “the worker is put on the runnable list”. At a high level this is simple to grasp, and the good news is that not much is hidden from us in the implementation detail. Nonetheless, it seems that it’s only systems programmers who deal with these details nowadays, but it’s still useful for the rest of us to get a chance to get comfortable with such internals.

Linked list implementation in SQLOS

A SQLOS doubly linked list follows a common Windows pattern based on a ListEntry structure. This is remarkably simple, containing only two pointer-sized members, i.e. taking up 16 bytes on x64:

  • flink (forward link) – a pointer to the next entry in the list
  • blink (backward link) – a pointer to the previous entry in the list

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Anatomy and psychology of a SQLOS spinlock

SQL Server spinlocks are famously elusive little beasties, tending to stay in the shadows except when they come out to bother you in swarms. I’m not going to add to the documentation of where specific spinlock types show up, or how to respond to contention on different types; the interested reader likely already knows where to look. Hint: Chris Adkin is quite the spinlock exterminator of the day.

In preparation for Bob Ward’s PASS Summit session, I figured it would make sense to familiarise myself a bit more with spinlock internals, since I have in the past found it frustrating to try and get a grip on it. Fact is, these are actually simple structures that are easy to understand, and as usual, a few public symbols go a long way. Being undocumented stuff, the usual caveats apply that one should not get too attached to implementation details.

Spinlock structure

It doesn’t get any simpler. A SQLOS spinlock is just a four-byte integer, embedded as a member variable in various classes, with two states:

  • Not acquired – the value is zero
  • Acquired – the value is the Windows thread ID of the owner

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The road to UTC is paved with DatetimeOffset

Here is a dirty little secret: most of us who live in single-time-zone countries are not comfortable with time zones. Hardly surprising given that we tend to treat wall clock time as a law of nature that only get adjusted twice a year. Now relating one’s day-to-day life to local time is all very well, but is this a good way to store timestamps in your data?

The simple solution is to store all times in UTC, and many systems are set up like that. The beauty of UTC is that it can be unambiguously translated to anything else you’d want as display time. However, once you are in the habit of storing local time – and more importantly, thinking in it – making the move could be a tricky culture shift.

We have a peculiar pitfall here in the UK, where local time happens to correspond to UTC during the winter months. This means that code written during the winter has a chance of showing up with a time zone bug when summer rolls around, either through straightforward confusion about what the code is persisting or as an integration problem where you thought both systems were speaking local time but it turns out that that one was UTC and one was local. Make no mistake, consistent use of UTC is a solid, simple solution, but that is easier said than done outside of greenfield development.
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Milliseconds 10, ticks 3

Since the year dot, SQL Server’s native Datetime type has given us a resolution of 300 ticks per second. With the introduction of Datetime2 and DatetimeOffset in 2008, we now get up to 100ns resolution. For those who like the finer things in life, this appears to be cause for rejoicing.

Image by frankieleon CC Attribution 2.0

Fun observation: if 100ns corresponded to 1 inch, 3.3ms translates to just over half a mile. That’s a pretty big difference in accuracy. Or should that be precision? Hmmmm, here’s a can of worms.
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