SQL Server 2008 R2 "Ghost Memory"?

Question

We have a dedicated SQL Server 2008 R2 machine that is experiencing some strange memory issues.. The machine itself has plenty of resources including two quad-core processors, 16gb of RAM and 64bit Windows Server 2008 R2 Enterprise (it is a Dell PowerEdge 2950).

The strange problem is that the system is reporting 82% of memory in use but sqlservr.exe is only reporting 155mb in use. The reason that I suspect SQL Server is the issue is because if I restart the sqlservr.exe process the memory consumption returns to normal for a period of time.

Does anyone have any ideas on how I can start to track this issue down?

Thanks, Jason

Answer 1

You won't get a true picture of memory usage from Task Manager if the account the service is running under has the lock pages in memory privilege (edit: as per Mark Rasmussen's comment/link). To determine how much memory is being used you can look at:

• SQLServer:Memory Manager\Total Server Memory perfmon counter
• DMVs

I can't recall if there is a DMV or combination of that will give you the total memory allocation but the following will show the bulk of it.

SELECT TOP(10) [type] AS [Memory Clerk Type], SUM(single_pages_kb) AS [SPA Mem, Kb] 
FROM sys.dm_os_memory_clerks 
GROUP BY [type]  
ORDER BY SUM(single_pages_kb) DESC OPTION (RECOMPILE);

SELECT DB_NAME(database_id) AS [Database Name],
COUNT(*) * 8/1024.0 AS [Cached Size (MB)]
FROM sys.dm_os_buffer_descriptors
WHERE database_id > 4 -- system databases
AND database_id <> 32767 -- ResourceDB
GROUP BY DB_NAME(database_id)
ORDER BY [Cached Size (MB)] DESC OPTION (RECOMPILE);

The second is the most interesting usually, buffer pool allocations by database. This is where the lions share will be used and it can be useful to understand which of your databases are the biggest consumers.

Answer 2

There's a recent article from our own Brent Ozar that treats this case, when Task Manager doesn't show correctly the memory eaten by SQLServer and its additional services. You can find it here: A Sysadmin’s Guide to Microsoft SQL Server Memory.

Quote: "Why Isn’t SQLServer.exe Using Much Memory?

When you remote desktop into a server and look at Task Manager, sqlservr.exe’s Mem Usage always seems wacky. That’s not SQL Server’s fault. Task Manager is a dirty, filthy liar. (I know, it sounds like the SQL guy is shifting the blame, but bear with me for a second.) On 64-bit boxes, this number is somewhat more accurate, but on 32-bit boxes, it’s just completely off-base. To truly get an accurate picture of how much memory SQL Server is using, you need a tool like Process Explorer, and you need to identify all of SQL Server’s processes. In the server I’m showing at right, there’s two SQL Server instances (shown by sqlservr.exe), plus SQL Agent, SQL Browser, and SQL Server backup tools. It’s not unusual to also see SQL Server Analysis Services, Integration Services, and Reporting Services also running on the same server – all of which consume memory.

So how much memory is SQL using? I’ll make this easy for you. SQL Server is using all of the memory. Period."

So I'd advise you to try Mark's query and use a better tool for memory report. Or just trust Perfmon to report memory, not Task Manager.

Answer 3

The amount of memory used by SQL, as shown in the task manager, will mostly be the max-memory setting. This is how the min/max setting works:

When SQL server starts up, it starts taking memory up to the min-memory setting. As your SQL needs increase, SQL will start using more memory up to the max-memory setting. The memory then stays at this (max) point even when the SQL usage goes down. This gives the impression of SQL performing huge tasks and using up that much memory. In reality, this memory is reserved by SQL.

When there is non-SQL memory pressure on the server, SQL will release memory down to the min-memory setting point. This is how the memory settings are used. You can use Mark's scripts to see how SQL is using this memory.