Siemens U3585-J-Z125-8-76 1, SM2 monitoring system manual Glossary Fault

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.

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