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HP OpenVMS Systemsask the wizard |
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The Question is: What causes page faulting, how does it affect run time and how can it be minimized? Thanks for your time. The Answer is :
A virtual address space is implemented using a combination of physical
memory, backing storage, and memory management data structures known
as page tables.
The operation known as paging is a transition, and refers to the
relocation of the contents of memory between physical memory and
backing storage, or from the memory allocated to a process to one
of the various system-defined pools of pages of physical memory, or
from one of these pools to a process, or from one of these pools to
backing storage.
Examples of events that trigger page faults include references to any
portion of virtual address space that is not currently valid, meaning
that there is currently no physical memory associated with the virtual
memory. Depending on tracking information in the page tables, resolving
this page fault may require acquiring and initializing a page of physical
memory from a list of free pages, acquiring the previous contents from
a cache of modified pages, or acquiring a free physical page and reloading
the previous contents of the virtual page from the backing storage area.
The page tables track the translation of virtual address space into the
associated physical memory, or to the contents of the memory page in the
backing storage area.
A page fault is an event that is triggered when a page is to be moved.
("Traps" and "faults" are basic reporting mechanisms underlying OpenVMS.
A "trap" occurs at the end of the processing of an instruction, while
a "fault" occurs during the processing of an instruction. An AST is
based on a trap. A page fault occurs during the processing of an
instruction, and means that the instruction must be restarted once
the paging completes and the virtual memory is valid.)
The memory management system also provides the core of system security,
as well as providing constructs such as process-private memory, system
memory, and shared memory.
For information on tuning OpenVMS systems and particularly on a systematic
approach at locating and removing system performance bottlenecks, please
see the OpenVMS Performance Management Manual. And remember that all
virtual memory systems will always have some level of paging activity.
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