[IOS] Ch 9 Virtual-Memory Management

Ch 9 Virtual-Memory Management

• 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
  

Ch 9.1 Background

• 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
• Virtual memory can be implemented via:
  
  • Demand paging
  • Demand segmentation

Virtual-address Space:

Shared Library Using Virtual Memory

Ch 9.2 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
• Lazy swapper – never swaps a page into memory unless page will be needed
  
  • Swapper that deals with pages is a pager

Transfer of a Paged Memory to Contiguous Disk Space:

9.2.1 Basic Concept

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
• Example of a page table snapshot:

Frame #	valid-invalid bit
	V
	V
	V
	V
	i
…
	i
	i

• During address translation, if valid–invalid bit in page table entry is i –> page fault

Page Fault:

• If there is a reference to a page, first reference to that page will trap to operating system: page fault
  
  1. Operating system looks at another table to decide:
     
     • Invalid reference –> abort
     • Just not in memory
  2. Get empty frame
  3. Swap page into frame
  4. Reset tables
  5. Set validation bit = v
  6. Restart the instruction that caused the page fault

Steps in Handling a Page Fault:

9.2.2 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 = 200 nanoseconds
• Average page-fault service time = 8 milliseconds
• EAT = (1 – p) x 200 + p (8 milliseconds)
  = (1 – p x 200 + p x 8,000,000
  = 200 + 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!!

舊名稱	法制度量衡	英文	縮寫
公里*	千米 / 公里	kilometer	km
公引		hectometer	hm
公丈		decameter	dam
公尺*	米 / 公尺	meter	m
公寸		decimeter	dm
公分*	厘米 / 厘公尺 / 公分	centimeter	cm
公釐*	毫米 / 毫公尺	millimeter	mm
	微米 / 微公尺	micrometer	μm
	奈米 / 奈公尺	nanometer	nm

Ch 9.3 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
• Free pages are allocated from a pool of zeroed-out pages
• If 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

Ch 9.4 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

Need for Page Replacement

9.4.1 Basic Page Replacement

1. Find the location of the desired page on disk
2. 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
3. Bring the desired page into the (newly) free frame; update the page and frame tables
4. 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
• In all our examples, the reference string is

        1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

9.4.2 First-In-First-Out (FIFO) Algorithm

• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• 3 frames (3 pages can be in memory at a time per process)

input	1	2	3	4	1	2	5	1	2	3	4	5
Page1	1	1	1	4	4	4	5	5	5	5	5	5
Page2	-	2	2	2	1	1	1	1	1	3	3	3
Page3	-	-	3	3	3	2	2	2	2	2	4	4
Page fault	v	v	v	v	v	v	v	x	x	v	v	x

• 9 page faults
• 4 frames

input	1	2	3	4	1	2	5	1	2	3	4	5
Page1	1	1	1	1	1	1	5	5	5	5	4	4
Page2	-	2	2	2	2	2	2	1	1	1	1	5
Page3	-	-	3	3	3	3	3	3	2	2	2	2
Page3	-	-	-	4	4	4	4	4	4	3	3	3
Page fault	v	v	v	v	x	x	v	v	v	v	v	v

• 10 page faults
• With FIFO, contents can be completely different after adding memory
  10 page faults
• Belady’s Anomaly: more frames –> more page faults

9.4.3 Optimal Algorithm

• Replace page that will not be used for longest period of time
• 4 frames example

input	1	2	3	4	1	2	5	1	2	3	4	5
Page1	1	1	1	1	1	1	1	1	1	1	4	4
Page2	-	2	2	2	2	2	2	2	2	2	2	2
Page3	-	-	3	3	3	3	3	3	3	3	3	3
Page3	-	-	-	4	4	4	5	5	5	5	5	5
Page fault	v	v	v	v	x	x	v	x	x	x	v	x

• 6 Page fault
• Used for measuring how well your algorithm performs
• With OPTIMAL, contents of memory with X pages are a subset of contents with X+1 pages after adding memory

9.4.4 Least Recently Used (LRU) Algorithm

• Assumption: A page that has not been referenced for the longest time would wait for the longest time to be accessed again
  
  • Use of known history to predict unknown future
  • Does the intuition really work?
• 4 frames example

input	1	2	3	4	1	2	5	1	2	3	4	5
Page1	1	1	1	1	1	1	1	1	1	1	1	5
Page2	-	2	2	2	2	2	2	2	2	2	2	2
Page3	-	-	3	3	3	3	5	5	5	5	4	4
Page3	-	-	-	4	4	4	4	4	4	3	3	3
Page fault	v	v	v	v	x	x	v	x	x	v	x	v

• 7 Page fault
• Counter implementation
  
  • Every page entry has a counter; every time page is referenced through this entry, copy the clock into the counter
  • When a page needs to be changed, look at the counters to determine which are to change
• With LRU, contents of memory with X pages are a subset of contents with X+1 pages after adding memory
• Stack implementation – keep a stack of page numbers in a double link form:
  
  • Page referenced:
    
    • move it to the top
    • requires 6 pointers to be changed
  • No search for replacement

Reference	4	7	0	7	1	0	1	2	1	2	7	1	2
s	-	-	-	-	-	-	-	2	1	2	7	1	2
t	-	-	-	-	1	0	1	1	2	1	2	7	1
a	-	-	0	7	7	1	0	0	0	0	1	2	7
c	-	7	7	0	0	7	7	7	7	7	0	0	0
k	4	4	4	4	4	4	4	4	4	4	4	4	4

9.4.5 LRU-Approximation Page Replacement

• Both timestamp and stack implementations are too expensive to implement in practice
• 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
• Second chance
  
  • Need reference bit
  • Clock replacement
  • If page to be replaced (in clock order) has reference bit = 1 then:
    
    • set reference bit 0
    • leave page in memory
    • replace next page (in clock order), subject to same rules

Second-Chance (clock) Page-Replacement Algorithm

• What if all reference bits are set?
  
  • The original victim will be selected after looping around
  • The clock algorithm degenerates into FIFO
• The clock algorithm replaces an old page, not the oldest page
• What if hand moves slowly?
  
  • Not many page faults
  • Or, victim page can be found quickly
• What if hand moves quickly?
  
  • Lots of page faults
  • Or, lots of reference bits set
• One way to view clock algorithm
  
  • Crude partitioning of pages into two groups: young and old
  • Why not partition into more than 2 groups?

N^th Chance version of Clock Algorithm:

• N^th chance algorithm: Give page N chances
  
  • OS keeps counter per page: # sweeps
  • On page fault, OS checks reference bit:
    
    • 1 –> clear reference bit and also clear counter (used in last sweep)
    • 0 –> increment counter; if count=N, replace page
  • Means that clock hand has to sweep by N times without page being used before page is replaced
• How do we pick N?
  
  • Why pick large N? Better approximation to LRU
    
    • If N ~ 1K, really good approximation
  • Why pick small N? More efficient
    
    • Otherwise might have to look a long way to find free page
• What about dirty pages?
  
  • Takes extra overhead to replace a dirty page, so give dirty pages an extra chance before replacing.
  • Common approach:
    
    • Clean pages, use N=1
    • Dirty pages, use N=2

9.4.6 Counting-Based Page Replacement

• Keep a counter of the number of references that have been made to each page
• LFU Algorithm: replaces page with smallest count
• MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used.

Ch 9.5 Allocation of Frames

9.5.1 Minimum Number of Frames

• Each process needs minimum number of pages
• Example: IBM 370 – 6 pages to handle SS MOVE instruction:
  
  • instruction is 6 bytes, might span 2 pages
  • 2 pages to handle from
  • 2 pages to handle to
• Two major allocation schemes
  
  • fixed allocation
  • priority allocation

9.5.2 Allocation Algorithms

Fixed Allocation:
- Equal allocation – For example, if there are 100 frames and 5 processes, give each process 20 frames.

• Proportional allocation – Allocate according to the size of process
  
  • s_i = size of process p_i
  • S = sum of all s_i
  • m = total number of frames
  • a_i = allocation for p_i = s_i / S * m

    m = 64
    s1 = 10
    s2 = 127
    a1 = 10/137 * 64 = 5
    a2 = 127/137 * 64 = 59

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

Ch 9.6 Thrashing

• If a process does not have enough pages, the page-fault rate is very high. This leads to:
  
  • low CPU utilization
  • operating system thinks that it needs to increase the degree of multiprogramming
  • another process added to the system
• Thrashing: a process is busy swapping pages in and out

Demand Paging and Thrashing:

• Pages should only be brought into memory if the executing process demands them
  
  • As opposed to anticipatory paging
  • Many page faults will occur until most of a process’s working set of pages is located in physical memory
• Why does demand paging work? Locality model
  
  • Process migrates from one locality to another
  • Localities may overlap
• Why does thrashing occur? size of locality > total memory size
  
  • Could be a process is too large for memory
    
    • There is nothing the OS can do
  • Could also be the sum of several processes is too large
    
    • Figure out how much memory each process needs and schedule them accordingly
• Thrashing occurs because the system doesn’t know when it has taken on more work than it can handle
• LRU-type mechanism, such as clock, order pages in terms of last access, but don’t give absolute numbers indicating pages that must not be thrown out

Working-Set Model:
- A conceptual model by Peter Denning to prevent thrashing

• Informal definition of working set
  
  • The collection of pages that a process is working with, and which must thus be resident if the process is to avoid thrashing
• X = working-set window = a fixed number of page references
  (Example: 10,000 instruction)
• WSS i (working set of Process Pi ) =
  total number of pages referenced in the most recent X (varies in time)
  
  • if X too small will not encompass entire locality
  • if X too large will encompass several localities
  • if X = inf –> will encompass entire program
• Use the recent needs of a process to predict its future needs
  
  • At any given time, all pages referenced by a process in the last working-set window are considered to comprise its working set
• A process will never be executed unless its working set is resident in memory
• Pages outside the working set may be swapped out at any time
• D = SUM(WSSi) = total demand frames
• if D > m –> Thrashing
• Policy if D > m, then suspend one of the processes (swap it out if needed)
• How to determine when a page was last accessed?
• Approximate with interval timer + a reference bit
• Example: X = 10,000
  
  • Timer interrupts after every 5000 time units
  • Keep in memory 2 bits for each page
  • Whenever a timer interrupts copy and sets the values of all reference bits to 0
  • If one of the bits in memory = 1 –> page in working set
• Why is this not completely accurate?
• Improvement = 10 bits and interrupt every 1000 time units

Page-Fault Frequency Scheme

• Establish acceptable page-fault rate
  
  • If actual rate too low, process loses frame
  • If actual rate too high, process gains frame

Ch 9.7 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

Shared memory vs. Memory mapped (mmap):
memory mapped file 本質上是 share memory，but memory mapped include dist files
And if share file size is large, mmap can have hard disk, but share memory does not have
But if it is really large, swap in & out 's cost is really big

• Also allows several processes to map the same file allowing the pages in memory to be shared

Ch 9.8 Allocating Kernel Memory

• Treated differently from user memory
• Often allocated from a free-memory pool
  
  • Kernel requests memory for structures of varying sizes
  • Some kernel memory needs to be contiguous
    
    • Memory buffer will be accessed by a DMA device on a physically addressed bus (like PCI)
    • Base kernel is placed on a continuous block that can fit into one page
      
      • Reduce chance of TLB(Transaction Lookaside Buffer) miss
    • Contiguous page frame allocation leaves kernel page tables unchanged, preserving TLB and reducing effective access time
    • kmalloc

9.8.1 Buddy System

• Allocates memory from fixed-size segment consisting of physically-contiguous pages
• Memory allocated using power-of-2 allocator
  
  • Satisfies requests in units sized as power of 2
  • Request rounded up to next highest power of 2
  • When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of 2
    
    • Continue until appropriate sized chunk available

• Buddy system addresses external fragmentation
• How about internal fragmentation?
  -i.e. when allocating a 65 bytes structure, buddy system returns a 128 bytes block
  - 128-65 = 63 bytes are wasted

9.8.2 Slab Allocation

• Introduced in Linux 2.2 kernel to deal with internal fragmentation
• Kernel functions often request small objects of the same type repeatedly
  
  • Process descriptors, file descriptors, etc.
• Slab is one or more physically contiguous pages
• Cache consists of one or more slabs
• Single cache for each unique kernel data structure
  
  • Each cache filled with objects – instantiations of the data structure
• When cache created, filled with objects marked as free
• When structures stored, objects marked as used
• If slab is full of used objects, next object allocated from empty slab
  
  • If no empty slabs, new slab allocated
• Benefits include no fragmentation, fast memory request satisfaction
• In the Linux kernel, we try to avoid allocating noncontiguous memory areas
• But there are occasions when we want to create a large buffer in the kernel that can’t fit into a contiguous kernel memory area
  
  • We can use paging for the allocation
  • Linux uses most of the reserved addresses above PAGE_OFFSET to map non-contiguous memory area
  • vmalloc

Ch 9.9 Other Issues

9.9.1 Prepaging

• Prepaging
  
  • To reduce the large number of page faults that occurs at process startup
  • Prepage all or some of the pages a process will need, before they are referenced
  • But if prepaged pages are unused, I/O and memory was wasted
  • Assume s pages are prepaged and α of the pages is used
    
    • Is cost of s * α save pages faults > or < than the cost of prepaging s * (1- α) unnecessary pages?
    • α near zero –> prepaging loses

9.9.2 Page Size

• Page size selection must take into consideration:
  
  • fragmentation
  • table size
  • I/O overhead
  • locality

9.9.3 TLB Reach

• TLB Reach - The amount of memory accessible from the TLB
• TLB Reach = (TLB Size) X (Page Size)
• Ideally, the working set of each process is stored in the TLB
  
  • Otherwise there is a high degree of page faults
• Increase the Page Size
  
  • This may lead to an increase in fragmentation as not all
    applications require a large page size
• Provide Multiple Page Sizes
  
  • This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation

9.9.5 Program Structure

int data[128;

• Each row is stored in one page

Program 1

for (j = 0; j <128; j++)
    for (i = 0; i < 128; i++)
        data[i,j] = 0;

• 128 x 128 = 16,384 page faults

Program 2

for (i = 0; i < 128; i++)
    for (j = 0; j < 128; j++)
        data[i,j] = 0;

• 128 page faults

9.9.6 I/O Interlock

• I/O Interlock – Pages must sometimes be locked into memory
• Consider I/O - Pages that are used for copying a file from a device must be locked from being selected for eviction by a page replacement algorithm.

Ch 9.10 Operating System Examples

9.10.1 Windows XP

-Uses demand paging with clustering. Clustering brings in pages surrounding the faulting page

• Processes are assigned working set minimum and working set
  maximum
• Working set minimum is the minimum number of pages the process is guaranteed to have in memory
• A process may be assigned as many pages up to its working set maximum
• When the amount of free memory in the system falls below a threshold, automatic working set trimming is performed to restore the amount of free memory
• Working set trimming removes pages from processes that have pages in excess of their working set minimum

9.10.2

• Maintains a list of free pages to assign faulting processes
• Lotsfree – threshold parameter (amount of free memory) to begin paging
• Desfree – threshold parameter to increasing paging
• Minfree – threshold parameter to being swapping
• Paging is performed by pageout process
• Pageout scans pages using modified clock algorithm
• Scanrate is the rate at which pages are scanned. This ranges from slowscan to fastscan
• Pageout is called more frequently depending upon the amount of free memory available

Reference: Operating System Concepts 8th, by Silberschatz, Galvin, Gagne