Sql-server – odd OS memory consumption by two nearly-identical SQL 2008 R2 instances

Although you limited the memory to 28 GB out of 32 GB, this is not the only memory that SQL Server uses. The running programs and needs within Windows take some additional space.

If you want to closely tune the memory use, I recommend reading this, which works well for my servers. See Jonathan Kehayias at: http://www.sqlskills.com/blogs/jonathan/how-much-memory-does-my-sql-server-actually-need/

It basically says "reserve 1 GB of RAM for the OS, 1 GB for each 4 GB of RAM installed from 4–16 GB, and then 1 GB for every 8 GB RAM installed above 16 GB RAM." This suggests that your Maximum Memory should be tuned to 25 GB.

You can dynamically alter the min/max by setting a new value for those measures. The OS will adjust in a relatively short time without a reboot.

EDIT: Of course, as both Jonathan and Shanky mention, you still need to monitor your performance counters, since any general answer will likely need tuning.

What I am seeing is that in Task Manager, sqlserver.exe is using about 163 Mb of RAM, if I use procexp, the same service shows just under 500 MB of RAM used.

You should never look at task manager to check SQL Server memory utilization. It would never show you correct result. You should use a DMV sys.dm_os_process_memory, if you are using SQL Server 2008 and above, to check SQL Server memory utilization.

select 
(physical_memory_in_use_kb/1024) as PhyMemory_usedby_Sqlserver_MB, 
(locked_page_allocations_kb/1024 ) as Locked_pages_used_Sqlserver_MB, 
(virtual_address_space_committed_kb/1024 ) as Total_MemoryUsed_in_MB, 
process_physical_memory_low, 
process_virtual_memory_low 
from sys. dm_os_process_memory

Note:

PhyMemory_usedby_Sqlserver_MB is the committed memory and is backed by physical RAM.

Total_MemoryUsed_in_MB is the total memory used by SQL Server (RAM and page file)

You are making a mistake, which probably many 'newbies' make when referring to memory used by SQL Server. It would never show you correct memory utilization in task manager if Locked pages in memory privilege(LPIM) is there for the account running SQL Server service. This is because the task manager only shows Process Private Bytes, the memory allocated via Virtual_alloc function which is pageable. Now if the SQL Server service account has LPIM, most part of the memory allocation would be done by AWE API. The memory allocated via AWE API is NOT pageable and thus does not shows up in task manager, which, in the end shows you different (incorrect) values.

Further reading:

Fun with Locked Pages, AWE, Task Manager, and the Working Set… (CSS SQL Server Engineers)