shared segment, the active segment table entry count is decreased. Por a shared segment, if the active count is still greater than zero, no
further processing is done. If the count has decreased to zero, for a
shared segment, processing continues as if it were a privatesegment. Each page table entry in a segment is then examined for an in-storage
page. If one is found, it has its reference bit reset to zero. In
addition, if the heavy paging condition flag has been set, the page
table entry is aarked invalid, and the real page is placed on the flush
list in last-in-first-out order.
The page replacement/page selection algorithmaust find a real fraBe to
satisfy a demand for a virtual page. It firstattempts to satisfy the demand with a page from the flush list. The flush list contains virtual
pages (if any) that belong to virtual machines that are not eligible for
dispatching, and therefore are not being used.
Note: A virtual machine may reenter the dispatch list after its pages
have been placed on the flush list. If the virtual aachine atteapts to
access any of those pages, theywill be reclaiaed. The pages are placed
on the flush in last-in-first-out order under theassumption that the
longer theyremain on the list, the higher the probability the virtual machine will reenter the dispatch list and reclaia thea.
If no pages are found on the flush list, the selectionalgorithm examines each virtual page in real storage, searching for an available
page that does not have its reference bit on. It begins the search at
the first available virtual page at the high end of real storage and
searchesby descending page address. When it reaches the lowest
available page address, it starts again froa the top of storage.When a
page has been found, that page address minus one is checkpointed. The
nexttime the selection algorithm is invoked, it will start froa the
checkpointed address. As the selection process proceeds, those pages
that were not selected have their reference bits turned off.When the
selection algorithm is operating in this mode, a virtual pageBust be
referenced at least once per resetcycle (loop around real storage) to
avoid selection.I BACKING STORE ALLOCATION ALGORITHM There are two parts to the algoritha for allocation of a DASD page
record. The first is to find the optiaal device on which to allocate a
record. The second is then to optiBize the record allocation on a
particular device.CP maintains the DASD device chain in two parts. The major part is the
ordering of all devices by type and by theTEMP/PAGE classification.
AllPAGE devices are ordered before all TEMP devices. The device type
ordering is:2305, 3350, 3340, 3330, and 2314. All devices of the saBe type are chained together off the priaary chain. CP atteapts to
allocate a page record on the highest-level device until all devices at
that level are full and then it tries the next lower device type.Within a particular device type, CP allocates records in a round-robin
aanner, attempting to evenly distribute the allocated records.CP Introduction 1-115
further processing is done. If the count has decreased to zero, for a
shared segment, processing continues as if it were a private
page. If one is found, it has its reference bit reset to zero. In
addition, if the heavy paging condition flag has been set, the page
table entry is aarked invalid, and the real page is placed on the flush
list in last-in-first-out order.
The page replacement/page selection algorithm
satisfy a demand for a virtual page. It first
pages (if any) that belong to virtual machines that are not eligible for
dispatching, and therefore are not being used.
Note: A virtual machine may reenter the dispatch list after its pages
have been placed on the flush list. If the virtual aachine atteapts to
access any of those pages, they
on the flush in last-in-first-out order under the
longer they
If no pages are found on the flush list, the selection
page that does not have its reference bit on. It begins the search at
the first available virtual page at the high end of real storage and
searches
available page address, it starts again froa the top of storage.
page has been found, that page address minus one is checkpointed. The
next
checkpointed address. As the selection process proceeds, those pages
that were not selected have their reference bits turned off.
selection algorithm is operating in this mode, a virtual page
referenced at least once per reset
avoid selection.
record. The first is to find the optiaal device on which to allocate a
record. The second is then to optiBize the record allocation on a
particular device.
ordering of all devices by type and by the
All
ordering is:
allocate a page record on the highest-level device until all devices at
that level are full and then it tries the next lower device type.
aanner, attempting to evenly distribute the allocated records.