1. Chapter 9: Virtual Memory

2. Objectives To describe the benefits of a virtual memory system
To explain the concepts of demand paging

3. Background

4. Background In chapter of memory management ,
various memory-management strategies used in computer systems were discussed and all of these strategies have the same goal: to keep many processes in memory simultaneously to allow multiprogramming.
However, they tend to require that an entire process be in memory before it can execute.
Virtual memory is a technique that allows the execution of processes that are not completely in memory.

5. Virtual Memory That is Larger Than Physical Memory
Background
Advantages
programs can be larger than physical memory.
Map main memory into an extremely large, uniform array of storage,
separating logical memory as viewed by the user from physical memory.
This technique frees programmers from the concerns of memory-storage limitations.
Virtual Memory That is Larger Than Physical Memory

6. Shared Library Using Virtual Memory
Background
Advantages (cont)
Virtual memory also allows processes to share files easily and to implement shared memory.
Shared Library Using Virtual Memory

7. Background disadvantage
Virtual memory is not easy to implement, however, and may substantially decrease performance if it is used carelessly

8. Background Virtual memory can be implemented via: Demand paging
Demand segmentation

9. 1-Demand Paging
Consider how an executable program might be loaded from disk into memory.
One option is to load the entire program in physical memory at program execution time. However, a problem with this approach
is that we may not initially need the entire program in memory.
Ex: Suppose a program starts with a list of available options from which the user is to select. Loading the entire program into memory results in loading the executable code for all options, regardless of whether an option is ultimately selected by the user or not.
An alternative strategy is to load pages only as they are needed during program excution. This technique is known as demand paging and is commonly used in virtual memory systems.

10. 1-Demand Paging
A demand-paging system is similar to a paging system with swapping
But rather than swapping the entire process into memory, we use a lazy swapper.
lazy swapper never swaps a page into memory unless that page will be needed.
Since we are now viewing a process as a sequence of pages, rather than as one
large contiguous address space, use of the term swapper is technically incorrect.
A swapper manipulates entire processes,
whereas a pager is concerned with the individual pages of a process. We thus use pager, rather than swapper, in connection with demand paging.

11. 1-Demand Paging Bring a page into memory only when it is needed
Less I/O needed
Less memory needed
Faster response
More users
Page is needed  reference to it
invalid reference  abort
not-in-memory  bring to memory

12. Transfer of a Paged Memory to Contiguous Disk Space

13. 1-Demand Paging (cont..)
With this scheme, we need some form of hardware support to distinguish between
the pages that are in memory
the pages that are on the disk.
The valid–invalid bit scheme can be used for this purpose.
“valid,” :
the associated page is both legal and in memory.
“invalid,”
the page either is not valid (that is, not in the logical address space of the process) or is valid but is currently on the disk.

14. 1-Demand Paging(cont..) Valid-Invalid Bit
With each page table entry a valid–invalid bit is associated
(v  in-memory, i  not-in-memory)
Initially valid–invalid bit is set to i on all entries
page table

15. Page Table When Some Pages Are Not in Main Memory

16. Page Fault
If there is a reference to a page, first reference to that page will trap to operating system:
page fault
Operating system looks at another table to decide:
Invalid reference  abort
Just not in memory
Get empty frame
Swap page into frame
Reset tables
Set validation bit = v
Restart the instruction that caused the page fault

17. Steps in Handling a Page Fault

18. Performance of Demand Paging
Demand paging can significantly affect the performance of a computer system. why ?
if no page faults, the effective access time is equal to the memory access time.
If, however, a page fault occurs, we must first read the relevant page from disk and then access the desired word.
Let p be the probability of a page fault (0 ≤ p ≤ 1).
expect p to be close to only a few page faults.
effective access time = (1 − p) × ma + p × page fault time.

19. Demand Paging Example Memory access time = 200 nanoseconds
Average page-fault service time = 8 milliseconds
EAT = (1 – p) x p (8 milliseconds)
= (1 – p x p x 8,000,000
= p x 7,999,800
If one access out of 1,000 causes a page fault, then
EAT = 8.2 microseconds.
This is a slowdown by a factor of 40!!

20. End of Chapter 9