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Category: Wait Stats

HADR_SYNC_COMMIT

Sean Gallardy lays out what HADR_SYNC_COMMIT really tells you:

Initially I thought to myself, “this is the most misunderstood wait type that exists in the HA space for SQL Server”, then I realized maybe this isn’t the case… So, I pondered over this question, “is it truly misunderstood?” and came to the (possibly incorrect) realization that it is quite accurate in the general SQL Server’s users’ space of understanding. I also concluded that, really, it’s the way the wait is used in SQL Server coupled with how waits work in SQL Server, which leads to how it is viewed. Let me explain….

You’ll definitely want to read Sean’s explanation.

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Exchange Spill Wait Stats

Erik Darling looks at exchange spills:

There are quite high waits on PAGEIOLATCH_EX, SLEEP_TASK, and SLEEP_BPOOL_STEAL rounding out the top five. This is quite interesting, because I’ve never explicitly thought of PAGEIOLATCH_EX waits in the context of exchange spills. Normally, I think of them when queries read pages from disk into memory for modification.

Going down the line, SLEEP_TASK is familiar from our time spent with hash spills, but SLEEP_BPOOL_STEAL is so far undocumented anywhere.

Erik also does the math on this query and recommends that you not write a query like this one.

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Hash Spill Wait Stats

Erik Darling continues a series on interesting wait stats:

Hash spills are, as we’ll see, sometimes identified by a different wait than sort spills. In small quantities, spills are often not worth bothering with. But when they pile up, they can really cause some severe performance issues.

In this post, I want to show that both Hash Aggregate and Joins can cause the same wait type to show, along with some evidence that strings make things worse.

Click through for a good explanation of the wait stat you’re liable to see the most.

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Wait Stats on Sort Spills

Erik Darling starts a new series on wait stats, starting with one particular topic:

The point is not that spills are the sole things that cause these waits, it’s just to give you some things to potentially watch out for if you see these waits piling up and can’t pin down where they’re coming from.

In all the queries, I’m going to be using the MAX_GRANT_PERCENT hint to set the memory grant ridiculously low to make the waits I care about stick out.

Click through for the first of several demonstrations.

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Wait Stats Not in Query Store

Erik Darling says wait, wait, don’t tell me:

There are some oddities in the documentation for query store wait stats.

One is that RESOURCE_SEMAPHORE_QUERY_COMPILE is listed as a collected wait, but with an asterisk that says it’s not actually collected. I’ve tested workloads that generate lots of that wait, and just like the docs say, it doesn’t end up there.

Of course, since I added wait stats recently to sp_QuickieStore, I wanted to make sure other waits that I care about actually show up in there.

Read on to see which wait stats you can find in Query Store and which you’ll have to get from someplace else.

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Fixing Those Pesky Wait Stats

David Alcock keeps us from having to think:

CXPACKET

Attempt to pronounce parallellellellellism correctly then set MAXDOP to 1.

PAGELATCH

Mention TempDB and contention in the same sentence. Delete TempDB

BACKUP_

Delete any long running backup jobs. If wait persists then delete all backup jobs.

Click through for plenty of excellent nuggets of advice which definitely won’t land you on the unemployment line.

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Threadpool Waits

Chad Callihan recommends not messing with worker threads:

Our story begins on a test server hosting a couple hundred databases. Over time, the server kept getting slower and slower when trying to navigate in SSMS. Query windows would take too much time to load, logging in would lag a bit, etc. When investigating with sp_Blitz, I found that the server had been experiencing Threadpool waits.

There are some cases where increasing thread count is important, especially when you’re working with database mirroring or availability groups. I worked with a customer with thousands of mirrored databases per server. None of the databases were particularly large or heavily-used, so it was on properly-sized hardware. As a result, to prevent the server from falling over due to threadpool waits, we had to scale thread counts to scary-high levels.

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Collecting Wait Stats Over Time

Michael J. Swart has a repo for us:

I find wait stats so useful that I’ve got Paul Randal’s SQL Server Wait Statistics (or please tell me where it hurts…) bookmarked and I visit it frequently.

But that gives the total amount of waits for each wait type accumulated since the server was started. And that isn’t ideal when I’m troubleshooting trouble that started recently. No worries, Paul also has another fantastic post Capturing wait statistics for a period of time.

You can also get this from various monitoring tools, as Michael mentions, but if you don’t have such a tool in place, here’s how you can roll your own.

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SSMS and Ignoring Certain Waits

Erik Darling has a plea to the SQL Server Management Studio team:

Lock waits are particularly annoying. Imagine (I know, this might be difficult) that you have a friend who is puzzled by why a query is sometimes slow.

They send you an actual plan for when it’s fast, and an actual plan for when it’s slow. You compare them in every which way, and everything except duration is identical.

It’d be a whole lot easier to answer them if LCK waits were collected, but hey. Let’s just make them jump through another hoop to figure out what’s going on.

CXCONSUMER has a similar problem — and here’s the thing — if people are going through the trouble of collecting this information, give’em what they ask for. Don’t just give them what you think is a good idea.

Click through to see the issue and what you can do to work around this limitation.

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Unusual Threadpool Waits

Josh Darnell explains why you might get threadpool waits even when you think you shouldn’t:

I occasionally see (usually brief) THREADPOOL waits on systems that are really not all that heavily loaded. This is my investigation into why. Some might say I have too much time on my hands.

Before getting into these unusual THREADPOOL cases, let’s cover the normal ones.

Read the whole thing. It’s example #9068 of how a particular wait is not always a bad thing.

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