section, while errors associated with
are discussed later in the section
Recovery".
virtual storage paging operations"Virtual Storage Paging Error DMKPAGIO maintains its own subpool of preformatted paging IOBLOKs.
AsI/O operations complete, their IOBLOKs are added to a list of
available blocks; as new blocks are needed, they are taken fro. this
list. If the list is empty,DMKFREE is called to obtain storage for a
new block.DMKPAGIO also periodically calculates system paging
overhead. After200 pages have been moved (read or written), the
elapsed time for the200 page moves is computed, and the paging rate is
calculated in pagemoves per second. The recent paging load, expressed
as the percentage of time that more than one half of thesystem's pages
were idle due to page-wait, is averaged with the previous load and
re-projected as the expected load for the next interval.
PAGINGSUBSYSTEK The paging subsystem has three major components that have resource
optimization algorithms associated with them:I • The page replacement and page selection algorithm that manages the I allocation of real storage frames and selects which virtual page to I replace. I • An algorithm for the allocation of DASD backing store pages. I • An algorithm for ordering the queue of page I/O reqests.
PAGEREPLACEMENT IND PAGE SELECTION ALGORITHM VM/370 is a demand paging system. programs run in virtual storage and
when a storage reference is made to a virtual page not currently in real
storage, a page fault occurs. A page fault is aprogram interruption
that occurs when a pagemarked "not in real storage" is referred to by
anacti!e page. This page fault represents a demand for a real storage frame 1n which to place the virtual page. The page replacement algorithm chooses which real storage frame will be allocated to fulfill
such ademand. If all real frames in real storage are occupied by other
virtual pages, a realframe can only be obtained by replacing one of
those virtual pages. The selection of which virtual page to replace is
carried out by the page selectionalgorithm. The scheduler aids the page selection algorithm by notifying it of
virtualmachines that are no longer eligible for dispatching (either
because they have been dispatched, or because they are being held
suspended in the eligible list). The scheduler calls the page reset
routine when a virtual machine is dropped froa a queue and reset routine
when a virtual aachine is dropped from a queue and does notimmediately reenter the dispatch list. Under heavy paging loads, it is the
responsibility of the page reset routine to group all in-storage virtual
pages belonging to the virtualmachine; it groups them on an available
(or flush) list for easy selection by the page replacementalgorithm. The page reset routine cycles through the virtual machine's segment table looking for valid segment entries. When it finds a valid entry,
it turns on thesegment table entry invalid flag and the page reset
routine begins to process the page table associated with thatsegment table entry_ The page table header is timestaaped, and if it is a
1-114IBM VK/370 system Logic and Problem Determination--Voluae 1
are discussed later in the section
Recovery".
virtual storage paging operations
As
available blocks; as new blocks are needed, they are taken fro. this
list. If the list is empty,
new block.
overhead. After
elapsed time for the
calculated in page
as the percentage of time that more than one half of the
were idle due to page-wait, is averaged with the previous load and
re-projected as the expected load for the next interval.
PAGING
optimization algorithms associated with them:
PAGE
when a storage reference is made to a virtual page not currently in real
storage, a page fault occurs. A page fault is a
that occurs when a page
an
such a
virtual pages, a real
those virtual pages. The selection of which virtual page to replace is
carried out by the page selection
virtual
because they have been dispatched, or because they are being held
suspended in the eligible list). The scheduler calls the page reset
routine when a virtual machine is dropped froa a queue and reset routine
when a virtual aachine is dropped from a queue and does not
responsibility of the page reset routine to group all in-storage virtual
pages belonging to the virtual
(or flush) list for easy selection by the page replacement
it turns on the
routine begins to process the page table associated with that
1-114