Network Hardware User Manual

U3585-J-Z125-8-76 451
Glossary
page fault
BS2000 is an operating system with virtual addressing, i.e. it supports several address
spaces simultaneously. The virtual address spaces and the real main memory are
managed in units of 4-Kb pages. The DEMAND PAGING method is used for mapping
virtual pages onto real memory page frames:
When an attempt is made to access a page that is not in main memory, the hardware
detects this condition and uses a page fault interrupt to notify the operating system. If
the addressed page is on disk (paging device), the page is read in (page transfer).
If free page frames are required, the system tries to return modified main memory
pages to disk.
SM2 counts the number of paging I/O operations (number of I/O requests to the central
I/O control module of the system, which corresponds to the EXCP calls). This number
is supplied in the ACTIVITY and DEVICE reports, in the I/O record and in SM2R1
reports 3 and 4.
For each I/O request, SM2 increments this number by 1 when
– a page is read in,
– one or more pages are written (the system tries to group up to 8 pages for one I/O
operation).
The number of pages written is also given in the MEMORY report and in report 55, and
the number of pages read is given in report 56.
SM2 supplies the following additional data:
1. Total number of page fault interrupts. "Real" page faults are not included in this
number.
2. Number of page fault interrupts for which the addressed page was still in main
memory (PAGE RECLAIMS).
3. Number of page fault interrupts for which a page transfer is required. This count is
also incremented when 2 or more tasks try simultaneously to access the same
virtual page and the page is not in main memory.
4. Number of page fault interrupts for the first access to a new page.