SQL Server -g
Posted by Karthick P.K on March 5, 2013
I decide to write this quick blog after seeing a lot of confusion around sqlserver -g switch. sql server -g switch is nop in 64-Bit sqlserver and it is used in 32-bit sqlserver to increase the size of MTL(AKA MemToLeave).
The default value of -g switch is 256 MB. I.e. if you do not specify value for sqlserver -g switch it is defaulted to 256 MB.
Initialization of sqlserver memory during the startup of SQL Server is as follows.
1. Calculate the size of MemtoLeave and reserve it using the algorithm below
MTL (Memory to Leave)= (Stack size * max worker threads) + Additional space (By default 256 MB and can be controlled by -g).
Stack size =512 KB per thread for 32 Bit SQL Server and 904 KB for 32Bit SQL Server running on 64-Bit systems.
I.e = (256 *512 KB) + 256MB =384MB
-g switch is used to increase the additional space from 256 to any desired value.
2. Calculate the size of BPOOL using below algorithm.
SQL Server Buffer Pool is minimum of “Physical RAM “ or “user mode memory(2GB or 3GB) – MTL- BUF structures”
BPool = Minimum (Physical memory, User address space – MTL) – BUF structures
Buf structures are arrays maintained by sqlserver to track the status of each buffer in BPOOL . SQL Server makes maximum of 32 allocation requests to the OS to reserve bpool pages.
SQL Server maintains contiguous array to track status information associated with each buffer (8 KB page) in the BPool. In addition SQL Server maintains a second array to track the committed and reserved bitmap pages.
This bit can be 0 or 1 . 1 indicates buffer is committed and 0 indicated page is reserved.
Size of Buf structure is approximately 16 MB when AWE is not enabled and when AWE is enabled buf structures use additional 8MB per each GB of RAM in the system.
3. Release the MTL region which is reserved initially. We reserve MTL at startup and releases it after BPOOL is reserved to ensure MemToLeave region are contiguous.
More details about the SQL Server memory architecture in https://mssqlwiki.com/sqlwiki/sql-performance/basics-of-sql-server-memory-architecture/
If you are in SQL Server 2012 read https://mssqlwiki.com/2012/10/21/sql-server-2012-memory-2/
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