(See the MEM_LARGE_PAGE flag on the VirtualAlloc, VirtualAllocEx, and VirtualAllocExNuma functions.) You can also specify other device drivers to be mapped with large pages by adding a multistring registry value to HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\LargePageDrivers and specifying the names of the drivers as separately null-terminated strings.Īttempts to allocate large pages may fail after the operating system has been running for an extended period, because the physical memory for each large page must occupy a significant number (see Table 10-1) of physically contiguous small pages, and this extent of physical pages must furthermore begin on a large page boundary. In addition, Windows allows applications to map their images, private memory, and page-file-backed sections with large pages. Windows also automatically maps I/O space requests (calls by device drivers to MmMapIoSpace) with large pages if the request is of satisfactory large page length and alignment. To take advantage of large pages on systems with more than 2 GB of RAM, Windows maps with large pages the core operating system images (Ntoskrnl.exe and Hal.dll) as well as core operating system data (such as the initial part of nonpaged pool and the data structures that describe the state of each physical memory page).
The TLB is a very small cache, and thus large pages make better use of this limited resource. This, in turn, means having to go back to the page table structures when references are made to virtual addresses outside the scope of a small page whose translation has been cached. If small pages are used, more TLB entries are needed for the same range of virtual addresses, thus increasing recycling of entries as new virtual addresses require translation. This advantage exists because the first reference to any byte within a large page will cause the hardware’s translation look-aside buffer (TLB, described in a later section) to have in its cache the information necessary to translate references to any other byte within the large page. The primary advantage of large pages is speed of address translation for references to other data within the large page. Additionally, recent 圆4 processors support a size of 1 GB for large pages, but Windows does not use this feature. Windows on Itanium uses 8 KB and 16 MB for small and large pages, respectively, as a result of performance tests that confirmed these values as optimal. IA64 | 8 KB | 16 MB | 2,048 Note IA64 processors support a variety of dynamically configurable page sizes, from 4 KB up to 256 MB. Architecture | Small Page Size | Large Page Size | Small Pages per Large Page
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