Computer Systems An Integrated Approach to Architecture and Operating Systems
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Transcript of Computer Systems An Integrated Approach to Architecture and Operating Systems
COMPUTER SYSTEMSAn Integrated Approach to Architecture and Operating Systems
Chapter 8Topics in Page-based Memory
Management
©Copyright 2008 Umakishore Ramachandran and William D. Leahy Jr.
8.1 Demand Paging• Paging as described in Chapter 7 implied whole program was in
memory• But does it have to be? • On average
– 30% of a programs memory footprint is the primary logic of program– 70% is little used error handling
• Therefore, prudent for memory manager not to load entire program into memory on startup.
• Basic idea is to load parts of the program that are not in memory on demand.
• This technique, referred to as demand paging results in better memory utilization.
What would be the main advantage of demand paging?
8.1.1 Hardware for demand paging
0x00230x01240x11110x3F04
0x00000x00440x0068
0x00x10x20x3
0x3E00x3E10x3E2
003E1 234 2340044
v
PTBR
CPU
Memory
8.1.1 Hardware for demand paging
• If I5 page faults…handle
• If I2 page faults
• Let I1 complete and squash I3-I5 before INT.
• INT needs to save PC corresponding to I2 for re-starting I2 after servicing page fault.
• Note that there is no harm in squashing instructions I3-I5 since they have not modified permanent state of program
IF ID/RR EX MEM WBInstruction in
Instruction out
BUFFER
BUFFER
BUFFER
BUFFER
Potential page faults
I1I2I3I4I5
8.1.2 Page fault handler1. Find a free page frame2. Load the faulting virtual page from the disk
into the free page frame3. Update the page table for the faulting
process4. Place the PCB of the process back in the
ready queue of the scheduler
8.1.3 Data structures for Demand-paged Memory Management
• Free-list of page frames• Frame table• Disk map
8.1.3 Data structures for Demand-paged Memory Management
• Free-list of page frames
…Pframe 52 Pframe 20 Pframe 8Free-list
Pframe 200
8.1.3 Data structures for Demand-paged Memory Management
• Frame table <P2, 20> free <P5, 15> <P1, 32> free <P3, 0> <P4, 0> free
<PID, VPN>Pframe
0
1 2 3 4 5 6 7
Note: There are
different ways
of implementing
this.
8.1.3 Data structures for Demand-paged Memory Management
• Disk map
Swap space
P1 …..
P2 Pn
disk address
VPN
0
1 2 3 4 5 6 7
Disk map for P1
disk address disk address disk address disk address disk address disk address disk address
8.1.4 Anatomy of a Page Fault• Find a free page frame• Pick victim page and evict• Load faulting page• Update page table for faulting process and
frame table• Restart faulting process
Eviction
• When evicting a page we must consider its status– Clean: the page has not been written to and thus
matches its counterpart on disk– Dirty: the page has been written to and no longer
matches what is on disk• Clean pages may be simply evicted• Dirty pages must be written back to disk
8.2 Interaction between the Process Scheduler and Memory Manager
CPU
Hardware
CPU scheduler
…PCB1 PCB2
ready_q
Kernel
Process 1 Process 2 Process n ……….
User level
Memory Manager
Timer interrupt Page fault
Process dispatch
…freelist Pframe Pframe
PT1
PT2
.
.
DM1
DM2
.
.FT
(1) (2)
CPU
Hardware
CPU scheduler
…PCB1 PCB2
ready_q
Kernel
Process 1 Process 2 Process n ……….
User level
Memory Manager
Timer interrupt Page fault
Process dispatch
…freelist Pframe Pframe
PT1
PT2
.
.
DM1
DM2
.
.FT
(1) (2)
• CPU scheduler dispatches process, it runs until one of following happens1. HW timer interrupts CPU causing upcall (1) to CPU scheduler that may result in a process switch. CPU scheduler takes
appropriate action to schedule next process on CPU.2. Process incurs a page fault resulting in an upcall (2) to memory manager that results in page fault handling3. Process makes system call resulting in another subsystem (not shown) getting an upcall
8.3 Page Replacement Policies• How to pick victim page to evict from physical memory
when page fault & free-list is empty. • For a given string of page references, policy should result
in least number of page faults. – This attribute ensures that the amount of time spent in OS
dealing with page faults is minimized.• Ideally, once a particular page has been brought into
physical memory, policy should not incur a page fault for same page again. – This attribute ensures that page fault handler attempts to
respect reference pattern of the user programs.
8.3 Page Replacement Policies• May use
– Local victim selection• Simple• Don't need frame table• Poor memory utilization
– Global victim selection• Better memory utilization• The norm
• Ideally there are no page faults and memory manager never runs.
• Goal is to minimize (or eliminate) page faults
8.3.1 Belady’s Min• In 1966 Laszlo Belady proposed an optimal
page replacement algorithm requiring to know in advance the page replacement string
• Obviously this is impossible• But the performance level of Belady's Min
may be used as a reference standard to compare to other policies performance
8.3.2 First In First Out (FIFO)• Affix a timestamp when a
page is brought in to physical memory
• If a page has to be replaced, choose the longest resident page as the victim
• No special hardware needed
• Queue length is number of physical frames
<PID, VPN>
Circular queue
<PID, VPN> …..
<PID, VPN>
Head
Tail free
Full
FIFO
• Maintain queue. As page is read in enqueue. Use head of queue as frame to replace
• Sample 1,2,3,4,1,2,5,1,2,3,4,5
FIFO
1 1 3 3 1 1 5 5 2 2 4 4
2 2 4 4 2 2 1 1 3 3 5
1 1 1 4 4 4 5 5 5 5 5 5
2 2 2 1 1 1 1 1 3 3 3
3 3 3 2 2 2 2 2 4 4
1 1 1 1 1 1 5 5 5 5 4 4
2 2 2 2 2 2 1 1 1 1 5
3 3 3 3 3 3 2 2 2 2
4 4 4 4 4 4 3 3 3
1 2 3 4 1 2 5 1 2 3 4 5 12
12
9
10
Belady’sAnomaly
Time
FIFO1 2 3 4 1 2 5 1 2 3 4 5
5 5 5 5 5 5
1 1 1 1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2 2 2
3 3 3 3 3 3 3 3 3 3
4 4 4 4 4 4 4 4 4
5
Time
8.3.3 Least Recently Used (LRU)• LRU policy makes
assumption that if a page has not been referenced in a long time there is a good chance it will not be referenced in the future as well.
• Thus, victim page in LRU policy is page that has not been used for longest time.
<PID, VPN>
Push down stack
<PID, VPN>
…..
<PID, VPN>
Top
Bottom free
8.3.3 Least Recently Used (LRU)1 2 3 4 1 2 5 1 2 3 4 5
TIME
PhysicalFrames
PushDownStack
8.3.3 Least Recently Used (LRU)
• LRU is appealing but actually not feasible– Stack has as many entries as number of physical
frames. For a physical memory of 64 MB & an 8 KB pagesize, size of stack: 8 KB. Too big in datapath!
– On every access, hardware has to modify stack to place current reference on top of stack. Too slow.
• LRU may be bad choice in certain situations– e.g. Access N+1 pages in a processor with N
frames available
8.3.3.1 Approximate LRU: A Small Hardware Stack
• Add a hardware stack with ~16 entries• Push references onto stack
– If they are already in stack bring to top– Bottom reference falls out of stack
• When free frame needed randomly select one not in stack
• Shown to be successful in some applications• Probably not fast enough for high speed pipelined
processor
8.3.3.2 Approximate LRU:Reference bit per page frame
• Associate a bit with each frame.– hardware sets on reference– software reads and clears
• Have an n-bit counter register for each frame• Periodically (daemon) right shifts all counters
and puts reference bit into high order bit• Highest value counters are recently used
frames; lowest value counters are LRU frames
8.3.4 Second chance page replacement algorithm
• Initially, OS clears reference bits of all frames. As program executes, hardware sets reference bits for pages referenced by program.
• If a page has to be replaced, memory manager chooses replacement candidate in FIFO manner.
• If chosen victim’s reference bit is set, then manager clears reference bit and this page is moved to end of FIFO queue.
• The victim is first candidate in FIFO order whose reference bit is not set.
8.3.5 Review of page replacement algorithms
PAGE REPLACEMENTALGORITHM
HARDWARE ASSIST NEEDED
COMMENTS
FIFO None Could lead to anomalous behavior
Belady’s MIN Oracle Provably optimal performance; not realizable in hardware; useful as a standard for performance comparison
True LRU Push down stack Expected performance close to optimal; infeasible for hardware implementation due to space and time complexity; worst-case performance may be similar or even worse compared to FIFO
Approximate LRU #1 A small hardware stack Expected performance close to optimal; worst-case performance may be similar or even worse compared to FIFO
Approximate LRU #2 Reference bit per page Expected performance close to optimal; moderate hardware complexity; worst-case performance may be similar or even worse compared to FIFO
Second Chance Replacement
Reference bit per page Expected performance better than FIFO; memory manager implementation simplified compared to LRU schemes
8.3.6 Optimizing Memory Management
• Beyond basic techniques presented additiona optimizations are possible
• These optimizations are on top of the already presented techniques
8.3.7 Pool of free page frames• Instead of waiting for free page count to = 0• Periodically run daemon to evict pages
keeping a pool of n free pages
8.3.7.1 Overlapping I/O with Processing
• Upon eviction we add the evicted frame to the free list
• If the frame was dirty it is scheduled for write back
• When a frame is needed only clean frames are selected skipping over dirty frames still awaiting write back
8.3.7.2 Reverse Mapping to Page Tables
• When daemon runs to maintain free list at a certain level it will take pages from processes that might turn around and page fault on those pages
• If we maintain additional info in free list we can know this and give page back to process (since the data is still intact)
freelistPframe 52
Dirty
Pframe 22
Clean
Pframe 200
Clean
….
<PID, VPN> <PID, VPN> <PID, VPN>
8.3.8 Thrashing• Suppose many processes are in memory but
CPU utilization is low. What could cause this?1. Too many I/O bound processes?2. Too many CPU bound processes?
• Should we add more processes into memory?
8.3.8 Thrashing• If some processes don't really have enough
pages to support their current configuration they will constantly be page faulting and trying to grab frames from other processes
• This may lead those processes to also start page faulting and grabbing frames
• Does this sound like the ideal place to introduce more processes into memory?
8.3.8 Thrashing• Controlling thrashing starts with understanding
temporal locality• Temporal locality is the tendency for the same
memory location to be accessed over a short period of time
• During a given time period t certain pages will be accessed others will not
• If during t the pages that need to be accessed are in memory then no page faults will occur
1 2 3 4 51 2 3 4 55 1 2 3 4 5
1
2
3
4
5 5 5 3 3 3
1 1 1 4 4
2 2 2 5
1 1 1 4 4
4 2
5 5
4 2 2 2 5
2 3
5 5
2 2 5 5 5 3 3 3 2 2
3 3 3 1 1 1 4 4 4 3
5
Frames
Pages
MemoryReference
8.3.9 Working set• Working set is the set of pages that defines the
locus of activity of a program• The working set size (WSS) denotes the number
of distinct pages touched by a process in a window of time.
• The total memory pressure (TMP) exerted on the system is the summation of the WSS of all the processes currently competing for resources.
8.3.10 Controlling thrashing1. If TMP > Physical Memory
– Decrease the degree of multiprogramming.– Else: Increase
# physical frames per process
Page fault rate
High water mark
Low water mark
2. Monitor Page fault rate• if pfr>High
• Decrease progs• if pfr<Low
• Increase progs
8.4 Other considerations• Prepaging after swapping out
– Bring entire WS back when swapping back in• Memory manager and I/O system must work
together– Suppose I/O system is writing data into page that is
being swapped out by memory manager!– Solution: Pining– What about pages where memory manager lives?– They are pined
8.5 Speeding up Address Translation
• We will do whatever we can to reduce page fault rate– Context switch time < 100 instructions– Page fault time without disk I/O < 100 instructions– Disk I/O to handle page fault ~1,000,000 instructions
• Nevertheless there remains a big penalty. Each processor memory reference actually requires two memory accesses!– One to page table– One to actual location desired
8.5 Speeding up Address Translation
• Consider spatial locality which is the tendency to access nearby locations over a short period of time
• If we access address 0x12345678 how likely is it that we will access 0x1234567C?
• What frame will we access in each case?• Recall
– 0x12345678– 0x1234567C
8.5.1 Address Translation with TLB
• Solution: Install a hardware device which stores recently accessed page table info
• Divided into two sections one for user entries, the other for kernel entries– On context switch user portion may be flushed
quickly with a special kernel-mode instruction
CPU Page Offset
Page Frame
Page Frame
Page Frame
Page Frame
Page Frame
Page Frame
PageTable
PhysicalMemory
Frame Offset
TLB
CPU Page Offset
Page Frame
Page Frame
Page Frame
Page Frame
Page Frame
Page Frame
PageTable
PhysicalMemory
Frame Offset
TLB
TLB Miss
CPU Page Offset
Page Frame
Page Frame
Page Frame
Page FramePage Frame
Page Frame
PageTable
PhysicalMemory
Frame Offset
TLB
TLB Hit
8.6 Advanced topics in memory management
• How big is a page table?• Assume a 32 bit address and a page size of
4kB• Assume each page table entry is 32 bits long
• And each process has a page table!• How can we make this work?
8.6.1 Multi-level Page Tables• To deal with excessively large page tables we
can page the page table...
0000 0000 0000 0000 0000 00000000 0000
Page 1 Page 2 Offset
Page ofPageTable
PhysicalMemory
OuterPageTable
0
10230
1023
PTBR
8.6.2 Sophisticated use of the page table entry
• Page tables may contain more than just a physical frame number
• They may also contain mode information– RW: Read Write– RO: Read Only– CW: Copy on Write
• Useful for items such as static-read-only areas of memory and process forking
8.6.3 Inverted page tables• Virtual memory is usually much larger than physical memory• Some architectures (e.g. IBM Power processors) use an inverted page
table, essentially a frame table. • The inverted page table alleviates the need for a per-process page table. • Size of table is equal to size of physical memory (in frames) rather than
virtual memory. • Unfortunately, inverted page tables complicate logical to physical
address translation done by hardware. – hardware handles address translations through TLB mechanism. – On TLB miss, hardware hands over control (through a trap) to OS to resolve
translation in software. OS is responsible for updating TLB as well. – Architecture usually provides special instructions for reading, writing, and
purging TLB entries in privileged mode.
8.7 Summary Topics• Demand paging basics including hardware support, and data structures in OS
for demand-paging• Interaction between CPU scheduler and memory manager in dealing with page
faults• Page replacement policies including FIFO, LRU, and second chance replacement• Techniques for reducing penalty for page faults including keeping a pool of page
frames ready for allocation on page faults, performing any necessary writes of replaced pages to disk lazily, and reverse mapping replaced page frames to the displaced pages
• Thrashing and the use of working set of a process for controlling thrashing• Translation look-aside buffer for speeding up address translation to keep the
pipelined processor humming along• Advanced topics in memory management including multi-level page tables, and
inverted page tables
Questions?