advanced operating systems chapter 9 virtual memory

Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Chapter 9: Virtual Memory

9.2 Silberschatz, Galvin and Gagne ©2013
Chapter 9: Virtual Memory
 Background
 Demand Paging
 Copy-on-Write
 Page Replacement
 Allocation of Frames
 Thrashing
 Memory-Mapped Files
 Allocating Kernel Memory
 Other Considerations
 Operating-System Examples

Objectives
 To describe the benefits of a virtual memory system
 To explain the concepts of demand paging, page-replacement
algorithms, and allocation of page frames
 To discuss the principle of the working-set model
 To examine the relationship between shared memory and
memory-mapped files
 To explore how kernel memory is managed

Background
 Code needs to be in memory to execute, but entire program rarely
used
 Error code, unusual routines, large data structures
 Entire program code not needed at same time
 Consider ability to execute partially-loaded program
 Program no longer constrained by limits of physical memory
 Each program takes less memory while running -> more
programs run at the same time
 Increased CPU utilization and throughput with no increase
in response time or turnaround time
 Less I/O needed to load or swap programs into memory ->
each user program runs faster

Background (Cont.)
 Virtual memory – separation of user logical memory from
physical memory
 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
 More programs running concurrently
 Less I/O needed to load or swap processes

Background (Cont.)
 Virtual address space – logical view of how process is
stored in memory
 Usually start at address 0, contiguous addresses until end of
space
 Meanwhile, physical memory organized in page frames
 MMU must map logical to physical
 Virtual memory can be implemented via:
 Demand paging
 Demand segmentation

Virtual Memory That is Larger Than Physical Memory

Virtual-address Space
 Usually design logical address space for
stack to start at Max logical address and
grow “down” while heap grows “up”
 Maximizes address space use
 Unused address space between
the two is hole
 No physical memory needed
until heap or stack grows to a
given new page
 Enables sparse address spaces with
holes left for growth, dynamically linked
libraries, etc
 System libraries shared via mapping into
virtual address space

Shared Library Using Virtual Memory

Demand Paging
 Could bring entire process into memory
at load time
 Or bring a page into memory only
when it is needed
 Less I/O needed, no unnecessary
I/O
 Less memory needed
 Faster response
 More users
 Similar to paging system with swapping
(diagram on right)
 Page is needed  reference to it
 invalid reference  abort
 not-in-memory  bring to memory
 Lazy swapper – never swaps a page
into memory unless page will be needed
 Swapper that deals with pages is a
pager

Basic Concepts
 With a process is to be swapped in, pager guesses which pages will
be used before the process is swapped out again
 Instead of swapping in a whole process, the pager brings only those
pages into memory
 How to determine that set of pages?
 Need new MMU functionality to implement demand paging
 The valid–invalid bit scheme can be used for this purpose
 when this bit is set to “valid,” the associated page is both legal
and in memory
 If the bit is set to “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.

Basic Concepts (Cont.,)
 If pages needed are already memory resident
 No difference from non demand-paging
 If page needed and not memory resident
 Access to a page marked invalid causes a page fault
 The paging hardware will notice that the invalid bit is set, causing
a trap to the operating system

Basic Concepts (Cont.,)
 The procedure for handling this page fault :
 Check an internal table (usually kept with the process control block) for this process to determine
whether the reference was a valid or an invalid memory access.
 If the reference was invalid, we terminate the process. If it was valid but we have not yet brought in
that page, we now page it in.
 Find a free frame (by taking one from the free-frame list, for example).
 Schedule a disk operation to read the desired page into the newly allocated frame.
 When the disk read is complete, we modify the internal table kept with the process and the page
table to indicate that the page is now in memory.
 We restart the instruction that was interrupted by the trap. The process can now access the page
as though it had always been in memory.

Valid-Invalid Bit
 With each page table entry a valid–invalid bit is associated
(v  in-memory – memory resident, i  not-in-memory)
 Initially valid–invalid bit is set to i on all entries
 Example of a page table snapshot:

Page Table When Some Pages Are Not in Main Memory

Steps in Handling a Page Fault

Aspects of Demand Paging
 Extreme case – start process with no pages in memory
 OS sets instruction pointer to first instruction of process, non-
memory-resident -> page fault
 And for every other process pages on first access
 Pure demand paging
 Actually, a given instruction could access multiple pages (one page for
the instruction and many for data) -> may cause multiple page faults
 Consider fetch and decode of instruction which adds 2 numbers
from memory and stores result back to memory
 Hardware support needed for demand paging
 Page table with valid / invalid bit
 Secondary memory (swap device with swap space)
 Instruction restart (next slide)

Aspects of Demand Paging (Cont.,)
 Instruction Restart
 A crucial requirement for demand paging is the ability to restart any
instruction after a page fault
 Because we save the state (registers, condition code, instruction
counter) of the interrupted process when the page fault occurs
 we must be able to restart the process in exactly the same place and
state, except that the desired page is now in memory and is accessible
 A page fault may occur at any memory reference.
 If the page fault occurs on the instruction fetch, we can restart by
fetching the instruction again
 If a page fault occurs while we are fetching an operand, we must
fetch and decode the instruction again and then fetch the operand.

Aspects of Demand Paging (Cont.,)
 As a worst-case example, consider a three-address instruction such as ADD the content of A to B,
placing the result in C. These are the steps to execute this instruction:
 Fetch and decode the instruction (ADD).
 Fetch A.
 Fetch B.
 Add A and B.
 Store the sum in C
 If we fault when we try to store in C (because C is in a page not currently in memory), we will have to
get the desired page, bring it in, correct the page table, and restart the instruction
 The restart will require fetching the instruction again, decoding it again, fetching the two operands
again, and then adding again.

Performance of Demand Paging
 Stages in Demand Paging (worse case)
1. Trap to the operating system
2. Save the user registers and process state
3. Determine that the interrupt was a page fault
4. Check that the page reference was legal and determine the location of the page on the disk
5. Issue a read from the disk to a free frame:
1. Wait in a queue for this device until the read request is serviced
2. Wait for the device seek and/or latency time
3. Begin the transfer of the page to a free frame
6. While waiting, allocate the CPU to some other user
7. Receive an interrupt from the disk I/O subsystem (I/O completed)
8. Save the registers and process state for the other user
9. Determine that the interrupt was from the disk
10. Correct the page table and other tables to show page is now in memory
11. Wait for the CPU to be allocated to this process again
12. Restore the user registers, process state, and new page table, and then resume the
interrupted instruction

Performance of Demand Paging (Cont.)
 Three major activities in Demand Paging
 Service the interrupt – careful coding means just several hundred
instructions needed
 Read the page – lots of time
 Restart the process – again just a small amount of time
 Page Fault Rate 0  p  1
 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 )

Demand Paging Example
 Memory access time = 200 nanoseconds (1s=109ns)
 Average page-fault service time = 8 milliseconds (1s=103ms)
 EAT = (1 – p) x 200 + p (8 milliseconds)
= (1 – p ) x 200 + p x 8,000,000
= 200 + p x 7,999,800
 If no page fault (p=0), then EAT = 200 ns
 If one access out of 1,000 causes a page fault (p=0.001), then
 EAT = 8.2 microseconds (8200ns).(1s=106ms)
 This is a slowdown by a factor of 40!!
 If want performance degradation < 10 percent
 220 > 200 + 7,999,800 x p
20 > 7,999,800 x p
 p < .0000025
 < one page fault in every 400,000 memory accesses

advanced operating systems chapter 9 virtual memory

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