Scheduler stories: The joy of fiber mode

Probably the funniest thing I had ever seen on stage was a two-hander called “Frank ‘n Stein”. It’s a telling of the classic Frankenstein story, with the physical comedy of two actors having to rotate continuously between a large number of roles, including a whole crowd chasing the monster. This was all made possible by them never leaving the stage, but instead changing characters in front of the audience, using only rudimentary props to help differentiate the characters.

If this is the only thing you remember about fiber mode scheduling, it should see you through.
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Scheduler Stories: When does your scheduler run?

I am planning to burn a fair number of cycles on SQLOS scheduling internals for the foreseeable future, and with some luck, this turns into an interesting series. OS scheduling is already a subject that belongs “on the other side of the looking glass”, and this only gets more interesting when we look at user-mode SOS_Scheduler scheduling built on top of it.

If I don’t specifically mention a version, my frame of reference is SQL Server 2014. Yes, things changed since then, but the 2012-2014 scheduler is a good starting point, and the fundamental mechanisms I’ll initially cover have changed very little since the User Mode Scheduler (UMS) of SQL Server 7.0.
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A spanking new ReaderWriterSpinlock

Well, I never. There is life in the old spinlock yet!

Being a synchronisation fetishist, I took great interest in last week’s great blog post by Sanjay Mishra and Arvind Shyamsundar about the ReaderWriterSpinlock added in 2016 CU2. Great story, happy ending, room for a sequel, all good news.

We have already seen recent improvement in the reader-writer lock algorithm, so clearly people are finding more corners to squeeze. Now what, pray tell, is a reader-writer spinlock?
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The Thread in the Head: Dr Sqlos explains context switching

The thread in the head

Oh, the things you can schedule,
the games you can play!
If you keep a clear head, you’ll
be well on your way.

You start with a blank sheet,
three nuts and a bolt,
a strong sense of fairness,
a large can of Jolt.
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DBCC PAGE and buffer pool disfavoring

A page on a table, with "DBCC" in music notation written on it

While preparing material for my post on latch promotion rules, I found this very interesting Stack Exchange question by Jeremiah Peschka about SQL Server’s LRU-K algorithm and the metrics that support it. It turned out that SQL Server doesn’t expose those in a useful way, but I was really impressed by some experimental evidence provided by Martin Smith, and his excellent deductions. Martin has clearly been on the case for a while and has highlighted that LRU-K (or for that matter Time of Last Access) isn’t well documented at all.

I’m not going to look at the general case of aging out buffers today; instead I’m just confirming and extending Martin’s observations about how DBCC PAGE interacts with buffers.

The return of bUse1

Quick refresher: the lazywriter maintains an internal 16-bit “clock hand” that counts seconds and thus rolls over every eighteen hours or so. Its current value is used as a cheap and easy way to measure the progress of time in places where we don’t care about absolute time, but only need the ability to note how many seconds have passed since a logged event.
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King Arthur, Energizer bunnies, and the search for the SQLOS scheduler

ARTHUR: Who lives in that castle?
WOMAN: No one lives there.
ARTHUR: Then who is your lord?
WOMAN: We don’t have a lord.
DENNIS: I told you. We’re an anarcho-syndicalist commune. We take it in turns to act as a sort of executive officer for the week.

(from Monty Python and the Holy Grail)

Yeah, whatever. I want to hear about bunnies

Picture a world consisting of forty Energizer bunnies, grouped into four teams of ten. Each team has one battery between them, and the main rule of the game is that each bunny may only use the battery for a little while before transferring it to a teammate. There is no way for a sleeping player to be woken up except by being passed the battery, and each battery strictly stays within one team.
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How SQL Server counts page refs without counting

While researching my previous post on latch promotion, I came across an odd piece of magic that made me do a double take. And sleep on it. And tear out my hair. I took it on faith that this is more likely a clever algorithm than a brain fart, but I could not stop asking myself repeatedly…

What were they smoking?

The piece of code in question is really simple, involving the maintenance of the BUF structure’s breferences member, which notionally is a count of how many times the page has been touched since the page’s counters were last reset.
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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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