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.
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. Continue reading “The Latch Files 2: The spinlock that dares not speak its name”
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.
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