Savings and Loans
Whomever decided to give “memory bank” its moniker was wise beyond their years, or maybe they just made a very apt observation: all memory is on loan.
Even in the context we’ll be talking about, when SQL Server has lock pages in memory enabled, the pages that are locked in memory may not have permanent residency.
If your SQL Server doesn’t have enough memory, or if various workload elements are untuned, you may hit one of these scenarios:
- Query Memory Grant contention (RESOURCE_SEMAPHORE)
- Buffer Cache contention (PAGEIOLATCH_XX)
- A mix of the two, where both are fighting over finite resources
It’s probably fair to note that not all query memory grant contention will result in RESOURCE_SEMAPHORE. There are times when you’ll have just enough queries asking for memory grants to knock a significant pages out of the plan cache to cause an over-reliance on disk without ever hitting the point where you’ve exhausted the amount of memory that SQL Server will loan out to queries.
To help you track down any of these scenarios, you can use my stored procedure sp_PressureDetector to see what’s going on with things.
Most servers I see have a mix of the two issues. Everyone complains about SQL Server being a memory hog without really understanding why. Likewise, many people are very proud about how fast their storage is without really understanding how much faster memory is. It’s quite common to hear someone say they they recently got a whole bunch of brand new shiny flashy storage but performance is still terrible on their server with 64GB of RAM and 1TB of data.
I recently had a client migrate some infrastructure to the cloud, and they were complaining about how queries got 3x slower. As it turned out, the queries were accruing 3x more PAGEIOLATCH waits with the same amount of memory assigned to SQL Server. Go figure.
If you’d like to see those waits in action, and how sp_PressureDetector can help you figure out which queries are causing problems, check out this video.
The primary driver of how much memory you need is how much control you have over the database. The less control you have, the more memory you need.
Here’s an example: One thing that steals control from you is using an ORM. When you let one translate code into queries, Really Bad Things™ can happen. Even with Perfect Indexes™ available, you can get some very strange queries and subsequently very strange query plans.
One of the best ways to take some control back isn’t even available in Standard Edition.
there are a lot of bad defaults in sql server. most of them you know about and change. here’s another bad one: pic.twitter.com/555xPmgwRW
— Erik Darling Data (@erikdarlingdata) May 8, 2021
If you do have control, the primary drivers of how much memory you need are how effective your indexes are, and how well your queries are written to take advantage of them. You can get away with less memory in general because your data footprint in the buffer pool will be a lot smaller.
You can watch a video I recorded about that here:
Thanks for watching!