SQL SERVER 2008

Virtual Memory Manager
Physical and Virtual Memory ❘ 57
NOTE x64 is the predominant 64-bit architecture in use today, but Intel
developed an alternative architecture known as IA-64 that is implemented in its
Itanium processors. It was designed as a high-end alternative to mainframes, but
the platform didn’t have very many production implementations of SQL Server
and has subsequently been dropped from SQL Server 2012. Windows has also
dropped support for IA-64 with the release of Windows Server 2012, which runs
only on x64.
NOTE The virtual address space for a 32-bit system is only 4GB, which when
broken down into 2GB for kernel mode and 2GB for user mode, doesn’t provide
much space at all. It is this memory addressing limitation that is the key driver
for the adoption of 64-bit.
Chapter 2 of the previous edition of this book, Professional SQL Server 2008
Internals and Troubleshooting, provides extensive coverage of 32-bit Windows
and SQL Server environments, including all of the tuning options.
The Virtual Memory Manager (VMM) is the part of Windows that links together physical
memory and virtual address space. When a process needs to read from or write something
into memory, it references an address in its VAS; and the VMM will map it to an address in RAM.
It isn’t guaranteed, however, to still be mapped to an address in RAM the next time you access it
because the VMM may determine that it needs to move your data to the page fi le temporarily to
allow another process to use the physical memory address. As part of this process, the
VMM updates the VAS address and makes it invalid (it doesn’t point to an address in RAM
anymore). The next time you access this address, it has to be loaded from the page fi le on disk,
so the request is slower — this is known as a page fault and it happens automatically without you
knowing.
The portion of a process’s VAS that currently maps to physical RAM is known as the working set.
If a process requests data that isn’t currently in the working set, then it needs to be reloaded back
into memory before use. This is called a hard page fault (a soft page fault is when the page is still
on the standby list in physical memory); and to fi x it, the VMM retrieves the data from the page
fi le, fi nds a free page of memory, either from its list of free pages or from another process, writes
the data from the page fi le into memory, and then maps the new page back into the process’s virtual
address space.