14-10-2016, 02:57 PM
[attachment=73030]
Background
Virtual memory is a technique that allows execution of processes that may not be completely in the physical memory.
Virtual Memory gives the illusion of more physical memory than there really is (via demand paging)
Virtual memory provides many benefits:
Only part of the program needs to be in memory for execution.
Logical address space can therefore be much larger than physical address space.
Allows address spaces to be shared by several processes.
Allows for more efficient process creation.
Use of virtual memory is also justified for many reasons such as:
There may be some code (e.g. error code) that may never be executed.
Arrays, list and tables are often allocated more memory than they actually use.
Certain options and features of a program may be used rarely.
Virtual memory can be implemented via:
Demand paging
Demand segmentation
Program Execution in Virtual memory
Operating system brings into main memory a few pieces of the program
Resident set - portion of process that is in main memory
An interrupt is generated when an address is needed that is not in main memory
Operating system places the process in a blocking state
Piece of process that contains the logical address is brought into main memory
Operating system issues a disk I/O Read request
Another process is dispatched to run while the disk I/O takes place
An interrupt is issued when disk I/O complete which causes the operating system to place the affected process in the Ready state
Demand Paging
When we want to execute a process, it is swapped into the memory. However, a pager (swapper) does not bring the whole process into the memory. Only those pages, which are needed, are brought into the memory. That is, bring a page into memory only when it is needed.
Often page 0 is loaded initially when a job is scheduled
In Demand Paging a program’s “working set” is kept in memory, reference outside WS causes corresponding code to be retrieved from disk (“page fault”)
Provides the illusion of virtual memory
Demand paging has the many benefits such as
Less I/O needed
Less memory needed
Faster response
More users
Valid-Invalid Bit
For the above-mentioned scheme, we need some hardware support.
With each page table entry a valid–invalid bit is associated (1 in-memory, 0 not-in-memory)
Initially valid–invalid but is set to 0 on all entries.
During address translation, if valid–invalid bit in page table entry is 0 page fault.
Page Fault - Interrupt that arises upon a reference to a page that is not in main memory.
Page Fault Handling
If there is ever a reference to a page, first reference will trap to
OS page fault
OS looks at an internal table, usually kept in process’s PCB, to decide:
Invalid reference abort.
Just not in memory.
If the page is not in the memory, then
Get empty frame.
Swap page into frame.
Reset tables, validation bit = 1.
Restart instruction
Note that when a page fault happens, the hardware traps to the OS. The OS reads the desire page and restart the process from the point where it was interrupted.
What Happens If There is no Free Frame?
Page replacement – find some page in memory, but not really in use, swap it out.
Algorithm
Performance – want an algorithm which will result in minimum number of page faults
Same page may be brought into memory several times
Performance of Demand Paging
Page Fault Rate 0 p 1.0
if p = 0 no page faults
if p = 1, every reference is a fault
Effective Access Time (EAT)
EAT = (1 – p) x memory access
+ p ( page fault overhead
+ [swap page out ] + swap page in
+ restart overhead )
Example
Memory access time = 1 microsecond
50% of the time the page that is being replaced has been modified and therefore needs to be swapped out.
Swap Page Time = 10 msec = 10,000 msec
EAT = (1 – p) x 1 + p (15000)
1 + 15000P (in msec)
Page Replacement
Prevent over-allocation of memory by modifying page-fault service routine to include page replacement.
Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.
Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.
Basic Page Replacement Algorithm
Find the location of the desired page on disk.
Find a free frame:
- If there is a free frame, use it.
- If there is no free frame, use a page replacement algorithm to select a victim frame.
Read the desired page into the (newly) free frame. Update the page and frame tables.
Restart the process.
Page Replacement Algorithms
Want lowest page-fault rate.
Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string.
FIFO Algorithm
When a page must be replaced, the oldest page is chosen.
FIFO page replacement algorithm associates with each page the time when the page was brought into the memory.
A FIFO queue (instead of time) may also be used to implement this algorithm.
Optimal Algorithm
According to optimal (OPT or MIN), the page that is to be replaced is the one that will not be used for the longest period of time.
Difficult to implement because it requires future knowledge of the reference string.
LRU Approximation Algorithms
Use a reference bit
With each page associate a bit, initially = 0
When page is referenced bit set to 1.
Replace the one which is 0 (if one exists). We do not know the order, however.
Addition reference-bits algorithm
Reference bits are recorded in regular intervals. For example, we can keep an 8-bit for each page in a table in the memory.
After regular intervals, a timer interrupt transfers the control to the OS which shifts the reference bit for each page into the high-order bit of 8-bit shift registers containing the history of page.
The page with the lowest number is the LRU page.
Counting Algorithms
Keep a counter of the number of references that have been made to each page. The following two schemes can be used:
LFU Algorithm: Replaces page with smallest count is to be replaced.
MFU Algorithm: replace page with the largest value of count. It is based on the argument that the page with the smallest count was probably just brought in and has yet to be used.
Not very commonly used algorithm.
Allocation of Frames
Each process needs minimum number of pages. Minimum number of frames is defined by the computer architecture.
Two major allocation schemes:
Fixed allocation
Priority allocation
Fixed Allocation
Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.
Proportional allocation – Allocate according to the size of process.
Priority Allocation
Use a proportional allocation scheme using priorities rather than size.
If process Pi generates a page fault,
Select for replacement one of its frames.
Select for replacement a frame from a process with lower priority number.
Global vs. Local Allocation
Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another.
Local replacement – each process selects from only its own set of allocated frames.
Process Creation
Virtual memory allows other benefits during process creation:
Copy-on-Write
Memory-Mapped Files
Copy on Write
Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory.
If either process modifies a shared page, only then is the page copied.
COW allows more efficient process creation as only modified pages are copied.
Memory Mapped Files
Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory.
A file is initially read using demand paging. A page-sized portion of the file is read from the file system into a physical page. Subsequent reads/writes to/from the file are treated as ordinary memory accesses.
Simplifies file access by treating file I/O through memory rather than read() write() system calls.
Also allows several processes to map the same file allowing the pages in memory to be shared
Pros/Cons of Demand Paging
Advantages:
Can run program larger than physical memory
Allows higher multiprogramming level than pure paging
Efficient memory usage
No compaction is required
Portions of process that are never called are never loaded
Simple partition management due to discontinuous loading and fixed partition size
Easy to share pages
Disadvantages:
Internal fragmentation
Program turnaround time increases each time a page is replaced, then reloaded
Need special address translation hardware
Background
Virtual memory is a technique that allows execution of processes that may not be completely in the physical memory.
Virtual Memory gives the illusion of more physical memory than there really is (via demand paging)
Virtual memory provides many benefits:
Only part of the program needs to be in memory for execution.
Logical address space can therefore be much larger than physical address space.
Allows address spaces to be shared by several processes.
Allows for more efficient process creation.
Use of virtual memory is also justified for many reasons such as:
There may be some code (e.g. error code) that may never be executed.
Arrays, list and tables are often allocated more memory than they actually use.
Certain options and features of a program may be used rarely.
Virtual memory can be implemented via:
Demand paging
Demand segmentation
Program Execution in Virtual memory
Operating system brings into main memory a few pieces of the program
Resident set - portion of process that is in main memory
An interrupt is generated when an address is needed that is not in main memory
Operating system places the process in a blocking state
Piece of process that contains the logical address is brought into main memory
Operating system issues a disk I/O Read request
Another process is dispatched to run while the disk I/O takes place
An interrupt is issued when disk I/O complete which causes the operating system to place the affected process in the Ready state
Demand Paging
When we want to execute a process, it is swapped into the memory. However, a pager (swapper) does not bring the whole process into the memory. Only those pages, which are needed, are brought into the memory. That is, bring a page into memory only when it is needed.
Often page 0 is loaded initially when a job is scheduled
In Demand Paging a program’s “working set” is kept in memory, reference outside WS causes corresponding code to be retrieved from disk (“page fault”)
Provides the illusion of virtual memory
Demand paging has the many benefits such as
Less I/O needed
Less memory needed
Faster response
More users
Valid-Invalid Bit
For the above-mentioned scheme, we need some hardware support.
With each page table entry a valid–invalid bit is associated (1 in-memory, 0 not-in-memory)
Initially valid–invalid but is set to 0 on all entries.
During address translation, if valid–invalid bit in page table entry is 0 page fault.
Page Fault - Interrupt that arises upon a reference to a page that is not in main memory.
Page Fault Handling
If there is ever a reference to a page, first reference will trap to
OS page fault
OS looks at an internal table, usually kept in process’s PCB, to decide:
Invalid reference abort.
Just not in memory.
If the page is not in the memory, then
Get empty frame.
Swap page into frame.
Reset tables, validation bit = 1.
Restart instruction
Note that when a page fault happens, the hardware traps to the OS. The OS reads the desire page and restart the process from the point where it was interrupted.
What Happens If There is no Free Frame?
Page replacement – find some page in memory, but not really in use, swap it out.
Algorithm
Performance – want an algorithm which will result in minimum number of page faults
Same page may be brought into memory several times
Performance of Demand Paging
Page Fault Rate 0 p 1.0
if p = 0 no page faults
if p = 1, every reference is a fault
Effective Access Time (EAT)
EAT = (1 – p) x memory access
+ p ( page fault overhead
+ [swap page out ] + swap page in
+ restart overhead )
Example
Memory access time = 1 microsecond
50% of the time the page that is being replaced has been modified and therefore needs to be swapped out.
Swap Page Time = 10 msec = 10,000 msec
EAT = (1 – p) x 1 + p (15000)
1 + 15000P (in msec)
Page Replacement
Prevent over-allocation of memory by modifying page-fault service routine to include page replacement.
Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.
Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.
Basic Page Replacement Algorithm
Find the location of the desired page on disk.
Find a free frame:
- If there is a free frame, use it.
- If there is no free frame, use a page replacement algorithm to select a victim frame.
Read the desired page into the (newly) free frame. Update the page and frame tables.
Restart the process.
Page Replacement Algorithms
Want lowest page-fault rate.
Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string.
FIFO Algorithm
When a page must be replaced, the oldest page is chosen.
FIFO page replacement algorithm associates with each page the time when the page was brought into the memory.
A FIFO queue (instead of time) may also be used to implement this algorithm.
Optimal Algorithm
According to optimal (OPT or MIN), the page that is to be replaced is the one that will not be used for the longest period of time.
Difficult to implement because it requires future knowledge of the reference string.
LRU Approximation Algorithms
Use a reference bit
With each page associate a bit, initially = 0
When page is referenced bit set to 1.
Replace the one which is 0 (if one exists). We do not know the order, however.
Addition reference-bits algorithm
Reference bits are recorded in regular intervals. For example, we can keep an 8-bit for each page in a table in the memory.
After regular intervals, a timer interrupt transfers the control to the OS which shifts the reference bit for each page into the high-order bit of 8-bit shift registers containing the history of page.
The page with the lowest number is the LRU page.
Counting Algorithms
Keep a counter of the number of references that have been made to each page. The following two schemes can be used:
LFU Algorithm: Replaces page with smallest count is to be replaced.
MFU Algorithm: replace page with the largest value of count. It is based on the argument that the page with the smallest count was probably just brought in and has yet to be used.
Not very commonly used algorithm.
Allocation of Frames
Each process needs minimum number of pages. Minimum number of frames is defined by the computer architecture.
Two major allocation schemes:
Fixed allocation
Priority allocation
Fixed Allocation
Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.
Proportional allocation – Allocate according to the size of process.
Priority Allocation
Use a proportional allocation scheme using priorities rather than size.
If process Pi generates a page fault,
Select for replacement one of its frames.
Select for replacement a frame from a process with lower priority number.
Global vs. Local Allocation
Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another.
Local replacement – each process selects from only its own set of allocated frames.
Process Creation
Virtual memory allows other benefits during process creation:
Copy-on-Write
Memory-Mapped Files
Copy on Write
Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory.
If either process modifies a shared page, only then is the page copied.
COW allows more efficient process creation as only modified pages are copied.
Memory Mapped Files
Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory.
A file is initially read using demand paging. A page-sized portion of the file is read from the file system into a physical page. Subsequent reads/writes to/from the file are treated as ordinary memory accesses.
Simplifies file access by treating file I/O through memory rather than read() write() system calls.
Also allows several processes to map the same file allowing the pages in memory to be shared
Pros/Cons of Demand Paging
Advantages:
Can run program larger than physical memory
Allows higher multiprogramming level than pure paging
Efficient memory usage
No compaction is required
Portions of process that are never called are never loaded
Simple partition management due to discontinuous loading and fixed partition size
Easy to share pages
Disadvantages:
Internal fragmentation
Program turnaround time increases each time a page is replaced, then reloaded
Need special address translation hardware