[SOLVED] CS Carnegie Mellon

Carnegie Mellon
Memory Mapping
VM areas initialized by associating them with disk objects.
Process is known as memory mapping.
Area can be backed by (i.e., get its initial values from) : Regular file on disk (e.g., an executable object file)
Initial page bytes come from a section of a file Anonymous file (e.g., nothing)
First fault will allocate a physical page full of 0s (demand-zero page) Once the page is written to (dirtied), it is like any other page
Dirty pages are copied back and forth between memory and a special swap file.
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Carnegie Mellon
Demand paging
Key point: no virtual pages are copied into physical memory until they are referenced!
Known as demand paging
Crucial for time and space efficiency
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Carnegie Mellon
Sharing Revisited: Shared Objects
Process 1 virtual memory
Physical memory
Process 2 virtual memory
Process 1 maps the shared
object.
Shared object
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Carnegie Mellon
Sharing Revisited: Shared Objects
Process 1 virtual memory
Physical memory
Process 2 virtual memory
Process 2 maps the shared
object.
Notice how the virtual
addresses can be different.
Shared object
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Carnegie Mellon
Sharing Revisited:
Private Copy-on-write (COW) Objects
Process 1 virtual memory
Physical memory
Process 2 virtual memory
Private copy-on-write area
Two processes mapping a private copy-on-write (COW) object.
Private copy-on-write object
Area flagged as private copy-on- write
PTEs in private areas are flagged as read-only
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Carnegie Mellon
Sharing Revisited:
Private Copy-on-write (COW) Objects
Process 1 virtual memory
Physical Process 2 memory virtual memory
Instruction writing to private page triggers protection fault.
Handler creates new R/W page.
Instruction restarts upon handler return.
Copying deferred as long as
possible!
Copy-on-write
Write to private copy-on-write page
Private copy-on-write object
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