Unsung SQLOS: SOS_WaitableAddress

One of the more amusing words in the SQL Server synchronisation lexicon is “lightweight”. Locks bad. Nolocks good. Latches lightweight. The more spinlocks you eat, the more wait you lose!

If only things were that simple… But hey, I love the poetry of compromise. Check out the SOS_WaitableAddress for one of the many competing definitions of “lightweight”.
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Unsung SQLOS: the 2016 SOS_RWLock


Talk about serendipity. I’ve been working on a progression of blog posts that would include dealing with the SOS_RWLock in both 2014 and 2016 versions, and today is a perfect excuse to align it with the 2016-themed T-SQL Tuesday hosted by Michael J Swart.

The 2014 incarnation of the SOS_RWLock looked sensible enough, but since we’ve been told it was improved, it’s a great opportunity to see how one goes about tuning a lock algorithm. So with lock-picking tools in hand, follow me to the launch party of the Spring 2016 SQLOS collection to see what the hype is all about. Is the 2014 implementation truly Derelocte?
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Unsung SQLOS: the classic SOS_RWLock

Moving along with our bestiary of synchronisation classes, the SOS_RWLock, a reader-writer lock, feels like a logical next stop. It has been in the news recently, it has fairly simple semantics, and it is built upon primitives that we have already explored, namely spinlocks, linked lists and the EventInternal class. Its implementation is quite a leap from the simple SOS_Mutex and there is more scope for alternative implementations providing the same functionality. And, would you believe it, as called out by Bob Dorr, the 2012/2014 implementation has now been found wanting and got rewritten for 2016. Today we’re looking at the “classic” version though, because we then get the chance to understand the 2016 rewrite in terms of concrete design decisions. (Update: I examine the 2016 update here).
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Unsung SQLOS: the SOS_Mutex

A mutex, short for “mutual exclusion”, is arguably the simplest waitable synchronisation construct you can imagine. It exposes methods for acquisition and release, and the semantics are straightforward:

  • Initially it isn’t “owned”, and anybody who asks to acquire it is granted ownership
  • While owned, anybody else who comes around to try and acquire it must wait her turn
  • When the owner is done with it, the mutex is released, which then transfers ownership to one waiter (if any) and unpends that waiter

A mutex can also validly be referred to as a critical section, in the sense that it protects a critical section of code, or more accurately, data. When programming libraries expose both a mutex and a critical section, as Windows does, it really just reflects different implementations of synchronisation objects with the same semantics. You could also consider a spinlock to be a flavour of mutex: while the name “spinlock” describes the mechanism by which competing threads jostle for exclusive ownership (it can’t be politely waited upon), the idea of mutual exclusion with at most one concurrent owner still applies.

SOS_Mutex class layout and interface

This class is directly derived from EventInternal<SuspendQSlock>, with three modifications:

  1. The addition of an ExclusiveOwner member.
  2. The override of the Wait() method to implement mutex-specific semantics, although the main act of waiting is still delegated to the base class method.
  3. The addition of an AddAsOwner() method, called by Wait(), which crowns the ambient task as the exclusive owner after a successful wait.

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Unsung SQLOS: the EventInternal

Today we’re taking a step towards scheduler territory by examining the EventInternal class, the granddaddy of SQLOS synchronisation objects. At the outset, let’s get one formality out of the way: although it is a template class taking a spinlock type as template parameter, we only see it instantiated as EventInternal<SuspendQSLock> as of SQL Server 2014. What this means is that spins on its internal spinlock is always going to be showing up as SOS_SUSPEND_QUEUE.

It’s a very simple class (deceptively so even) which can implement a few different event flavours, doing its waiting via SQLOS scheduler methods rather than directly involving the Windows kernel. The desire to keep things simple and – as far as possible – keep control flow out of kernel mode is a very common goal for threading libraries and frameworks. .Net is a good frame of reference here, because it is well documented, but the pattern exists within OS APIs too, where the power and generality of kernel-mode code has to be weighed off against the cost of getting there.
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Unsung SQLOS: the SystemThread

SystemThread, a class within sqldk.dll, can be considered to be at the root of SQLOS’s scheduling capabilities. While it doesn’t expose much obviously exciting functionality, it encapsulates a lot of the state that is necessary to give a thread a sense of self in SQLOS, serving as the beacon for any code to find its way to an associated SQLOS scheduler etc. I won’t go into much of the SQLOS object hierarchy here, but suffice it to say that everything becomes derivable by knowing one’s SystemThread. As such, this class jumps the gap between a Windows thread and the object-oriented SQLOS.
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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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