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Virtual memory in UNIX
Oracle Tips by Burleson Consulting
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Virtual memory in UNIX
Virtual memory is an internal “trick” that relies on the
fact that not every executing task is always referencing
it’s RAM memory region. Since all RAM regions are not
constantly in-use, UNIX has developed a paging algorithm
that move RAM memory pages to the swap disk when it appears
that they will not be needed in the immediate future.
(Figure 2-5)
Figure 5: RAM demand paging in UNIX
As memory regions are created, UNIX will not refuse a new
task whose RAM requests exceeds the amount of RAM. Rather,
UNIX will page out the least recently referenced RAM memory
page to the swap disk to make room for the incoming request.
When the physical limit of the RAM is exceeded UNIX can
wipe-out RAM regions because they have already been written
to the swap disk.
When the RAM region is been removed to swap, any subsequent
references by the originating program require UNIX copy page
in the RAM region to make the memory accessible. UNIX page
in operations involve disk I/O and are a source of slow
performance. Hence, avoiding UNIX page in operations is an
important concern for the Oracle Remote DBA. |
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