Microsoft Windows Server can directly address up to 4 gigabytes (GB) of memory address space, regardless of how much physical RAM is installed. From the process perspective, each element of virtual address conceptually refers to a byte of physical memory. It is the responsibility of the Virtual Memory Manager (VMM) in conjunction with processor memory management unit (MMU) to translate or map each virtual address into a corresponding physical address. The VMM performs the mapping by dividing the RAM into fixed-size page frames, creating system PTEs to store information about these page frames, and mapping them. System PTEs are small kernel-mode buffers of memory that are used to communicate with the disk I/O subsystem and the network. Each PTE represents a page frame and contains information necessary for the VMM to locate a page.
On an x86-based system that uses a 4 KB page size, the maximum number of PTEs required to map 2 GB of address space is 524,288 (2 GB/4 KB). On a similar system using the /3GB switch, the number of PTEs requires to map 3 GB of address space is 786,432 (3 GB/4 KB). On a typical system, this space is used as follows:
- A maximum of 50,000 PTEs (approximately 195 MB address space) are reserved for general use.
- The rest is used in mapping system cache, hyperspace, paged pool, non-paged pool, crash dump area, and so on.
This PTE pool size is automatically determined at system startup based on the amount of physical memory in the system. This pool is squeezed in between paged pool and non-paged pool, which also grows with the amount of physical memory in the system as well.
The system PTE pool can become heavily used and heavily fragmented. This can lead to situations where a driver may not load. Also, if the system PTE pool is depleted entirely, other parts of the system will degrade, even resulting in threads not being created, system stalls, and potential system crashes.
Microsoft recommends that this value should be above 12,000, with values below that causing system instability.