Emery Berger

1
Operating SystemsCMPSCI 377Lecture 14 VM Meets
the Real World

• Emery Berger
• University of Massachusetts Amherst

2
Last Time Demand-Paged VM

• Reading pages
• Swap space
• Page eviction
• Cost of paging
• Page replacement algorithms
• Evaluation

3
Virtual Memory in the Real World

• Implementing exact LRU
• Approximating LRU
• Hardware Support
• Clock
• Segmented queue
• Multiprogramming
• Global LRU
• Working Set

4
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A, B, C, B, C, C, D
5
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
A, B, C, B, C, C, D
6
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 2
A, B, C, B, C, C, D
7
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 2
C 3
A, B, C, B, C, C, D
8
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 4
C 3
A, B, C, B, C, C, D
9
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 4
C 5
A, B, C, B, C, C, D
10
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 4
C 6
A, B, C, B, C, C, D
11
Implementing Exact LRU

• On each reference, time stamp page
• When we need to evict select oldest page
  least-recently used

A 1
B 4
C 6
D 7
A, B, C, B, C, C, D

• How should we implement this?

12
Implementing Exact LRUData Structures

• Could keep pages in order optimizes eviction
• Priority queueupdate O(log n), eviction
  O(log n)
• Optimize for common case!
• Common case hits, not misses
• Hash tableupdate O(1), eviction O(n)

13
Cost of Maintaining Exact LRU

• Hash tables too expensive
• On every reference
• Compute hash of page address
• Update time stamp
• Unfortunately 10x 100x more expensive!

14
Cost of Maintaining Exact LRU

• Alternative doubly-linked list
• Move items to front when referenced
• LRU items at end of list
• Still too expensive
• 4-6 pointer updates per reference
• Can we do better?

15
Virtual Memory in the Real World

• Implementing exact LRU
• Approximating LRU
• Hardware Support
• Clock
• Segmented queue
• Multiprogramming
• Global LRU
• Working Set

16
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits

A 1
B 1
C 1
A, B, C, B, C, C, D
17
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits

A 0
B 0
C 0
A, B, C, B, C, C, D
reset reference bits
18
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits

A 0
B 1
C 0
A, B, C, B, C, C, D
19
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits

A 0
B 1
C 1
A, B, C, B, C, C, D
20
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits

A 0
B 1
C 1
A, B, C, B, C, C, D
21
Hardware Support

• Maintain reference bits for every page
• On each access, set reference bit to 1
• Page replacement algorithm periodically resets
  reference bits
• Evict page with reference bit 0
• Cost per miss O(n)

A 0
B 1
C 1
D 1
A, B, C, B, C, C, D
22
Virtual Memory in the Real World

• Implementing exact LRU
• Approximating LRU
• Hardware Support
• Clock
• Segmented queue
• Multiprogramming
• Global LRU
• Working Set

23
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 1
C 1
A 1
D 1
A, B, C, D, B, C, E, F, C, G
24
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 1
C 1
A 1
D 1
A, B, C, D, B, C, E, F, C, G
25
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 1
C 1
A 1
D 1
A, B, C, D, B, C, E, F, C, G
26
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 1
C 1
A 0
D 1
A, B, C, D, B, C, E, F, C, G
27
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
C 1
A 0
D 1
A, B, C, D, B, C, E, F, C, G
28
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
C 0
A 0
D 1
A, B, C, D, B, C, E, F, C, G
29
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
C 0
A 0
D 0
A, B, C, D, B, C, E, F, C, G
30
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
C 0
A 0
E 1
D 0
A, B, C, D, B, C, E, F, C, G
31
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
C 0
A 0
E 0
D 0
A, B, C, D, B, C, E, F, C, G
32
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
F 1
C 0
A 0
E 0
D 0
A, B, C, D, B, C, E, F, C, G
33
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
F 1
C 0
A 0
E 0
C 1
D 0
A, B, C, D, B, C, E, F, C, G
34
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
F 0
C 0
A 0
E 0
C 1
D 0
A, B, C, D, B, C, E, F, C, G
35
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
F 0
C 0
A 0
E 0
C 1
C 0
D 0
A, B, C, D, B, C, E, F, C, G
36
The Clock Algorithm

• Variant of FIFO LRU
• Keep frames in circle
• On page fault, OS
• Checks reference bit of next frame
• If reference bit 0, replace page, set bit to 1
• If reference bit 1, set bit to 0, advance
  pointer to next frame

B 0
F 1
C 0
A 0
E 0
C 1
C 0
D 0
G 1
A, B, C, D, B, C, E, F, C, G
37
Enhancing Clock

• Recall we dont write back unmodified pages
• Idea favor eviction of unmodified pages
• Extend hardware to keep another bitmodified bit
• Total order of tuples (ref bit, mod bit)
• (0,0), (0,1), (1,0), (1,1)
• Evict page from lowest nonempty class

38
Page Replacementin Enhanced Clock

• OS scans at most three times
• Page (0,0) replace that page
• Page (0,1) write out page, clear mod bit
• Page (1,0), (1,1) clear reference bit
• Passes
• all pages (0,0) or (0,1)
• all pages (0,1) - write out pages
• all pages (0,0) replace any page
• Fast, but still coarse approximation of LRU

39
Segmented Queue

• Real systems segment queue into two parts
• approximate for frequently-referenced pages
• e.g., first 1/3 page frames fast
• exact LRU for infrequently-referenced pages
• last 2/3 page frames doubly-linked list
  precise
• How do we move between two segments?

clock
exact LRU
40
Virtual Memory in the Real World

• Implementing exact LRU
• Approximating LRU
• Hardware Support
• Clock
• Segmented queue
• Multiprogramming
• Global LRU
• Working Set

41
Multiprogramming VM

• Multiple programs compete for main memory
• Processes move memory from and to disk
• Pages needed by one process may get squeezed out
  by another process
• thrashing - effective cost of memory access
  cost of disk access really really bad
• Must balance memory across processes to avoid
  thrashing

42
Global LRU

• Put all pages from all processes in one pool
• Manage with LRU (Segmented Queue)
• Used by Linux, BSD, etc.
• Advantages
• Easy
• Disadvantages
• Many

43
Global LRU Disadvantages

• No isolation between processes
• One process touching many pages can force another
  process pages to be evicted
• Priority ignored, or inverted
• All processes treated equally
• Greedy (or wasteful) processes rewarded
• Programs with poor locality squeeze out those
  with good locality
• Result more page faults

44
Global LRU Disadvantages, Cont.

• Sleepyhead problem
• Intermittent, important process
• Every time it wakes up no pages! back to
  sleep...
• Susceptible to denial of service
• Non-paying guest, lowest priority, marches over
  lots of pages gets all available memory
• Alternatives?

45
Working Set

• Denning Only run processes whose working set
  fits in RAM
• Other processes deactivate (suspend)
• Working set pages touched in last ? references
• Provides isolation
• A processs reference behavior only affects itself

46
Working Set Problems

• Algorithm relies on key parameter, ?
• How do we set ??
• Is there one correct ??
• Different processes have different timescales
  over which they touch pages
• Not acceptable (or necessarily possible) to
  suspend processes altogether
• Not really used
• Very rough variant used in Windows

47
Alternative Approach

• Just buy more RAM!
• Simplifies memory management
• Workload fits in RAM no more swapping!
• Sounds great

48
Memory Prices Over Time
Soon it will be free
49
Memory Prices Inflection Point!
50
Memory Is Actually Expensive

• Desktops
• Most ship with 256MB
• 1GB 50 more
• Laptops 70, if possible
• Limited capacity
• Servers
• Buy 4GB, get 1 CPU free!
• Sun Enterprise 10000 8GB extra 150,000!
• Fast RAM new technologies
• Cosmic rays

8GB Sun RAM 1 Ferrari Modena
51
SummaryVirtual Memory in the Real World

• Implementing exact LRU
• Approximating LRU
• Hardware Support
• Clock
• Segmented queue
• Multiprogramming
• Global LRU
• Working Set

52
SummaryIf You Have to Spend ...
more Ferraris good
more memory bad