Performance Tuning : Tiny Queries Help in Identifying Memory Pressure

Often, when given a database server experiencing severe performance problem, our attention is focused more on what is the obvious. And typically, we consider large, complex and long running queries as immediate suspects. We often consider simple, small, and short running queries the least of our worries. In this blog, I’ll show you how these tiny queries help in determining if your SQL Server box is under memory pressure.

However, you have to understand first how SQL Server utilizes memory. In a nutshell, queries that need memory allocation ‘requests’ a certain amount of memory from SQL Server’s memory broker. SQL Server then computes how much memory it can give to the query. Initially, 25% of the query memory is computed and goes through some more belt-tightening schemes to determine the final X amount of memory to grant to the query. When memory is available, the query is immediately granted an X amount of memory and the query executes and returns back the memory right after it has done its work. When memory isn’t available on the other hand, the query is put on hold in a queue until its next turn to do the cycle of ‘requesting’ memory or until it has timed out (few seconds up to 24 hours).

On a server experiencing severe memory pressure where simple, small and short running queries are put on hold for execution due to lack of memory to use, a few lines of query that needs 20KB of memory and used to execute for under a second can take minutes, hours of waiting before it gets its requested memory. Then it continous to execute and finishes its work. It can even timeout if memory isn’t ever granted. Thus, this gives everyone the impression that something is very wrong with the query and/or the server.

If you have these tiny queries put on hold very often by the memory broker, your SQL Server badly needs additional memory (RAM) even if you have 256GB of memory in your machine. If you have these tiny queries timed out after 24 hours of waiting, your server is funny. Though you may have 256GB of RAM in your server, it is simply not enough.

For a server experiencing severe memory pressure, you would get a handful of these tiny queries, which I would personally define as queries needing 2 Megabytes or less of memory to execute, put on hold every 5 seconds. Your circumstances might be different from mine so you can vary this number. For a server with plenty of memory to spare, you would not see these tiny queries put on hold for days or weeks or months or ever.

SQL Server provides a way though to determine if tiny queries are put on hold often. You may capture the results of the following query in a regular interval for days to monitor. I’d usually do 7-day monitoring runs:

SELECT
    COUNT(*) AS small_queries_on_hold
FROM sys.dm_exec_query_memory_grants  
WHERE
    requested_memory_kb <= 2048  — 2 MB
    AND grant_time IS NULL

Do this for every 15 seconds or less depending on how your server is experiencing the pressure. For a quick simple run, you may execute this query in SSMS when you are experiencing severe slowdown and have F5 pressed is regular intervals for a few minutes.

If the result for small_queries_on_hold  is always zero, you may vary/increase requested_memory_kb for up to 10MB. If you get zero result still, you may be experiencing problems other than having memory pressure.

A non-zero result for small_queries_on_hold suggest you may be experiencing severe memory pressure. It means, even small queries have a hard time getting memory grants from SQL Server. This should prompt you right away to request for a purchase of  additional RAM for SQL Server.

You may also want to run the following query to see if there are queries, regardless of memory size requirements, that are put on hold.

SELECT
    COUNT(*) AS small_queries_on_hold
FROM sys.dm_exec_query_memory_grants  
WHERE
   grant_time IS NULL

Once you have upgraded your server’s memory and give more memory to SQL Server, you may want to run the above queries once again to see if there are small queries that are put on hold by SQL Server’s memory broker. Chances are, you would see less occurrences of small_queries_on_hold or you have completely eliminated the occurrence of small queries having put on hold by SQL Server.

Increase your RAM until you don’t see the occurrences or if you can tolerate the number of small_queries_on_hold on a certain time window.

This is all folks. Hope this helps!

**************************************************
Toto Gamboa is a consultant specializing on databases, Microsoft SQL Server and software development operating in the Philippines. He is currently a member and one of the leaders of Philippine SQL Server Users Group, a Professional Association for SQL Server (PASS) chapter and is one of Microsoft’s MVP for SQL Server in the Philippines. You may reach him by sending an email to totogamboa@gmail.com

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