Clock-Pro is an approximation of the LIRS page replacement algorithm that is built on the existing CLOCK infrastructure in operating system kernels. It categorizes pages as either cold or hot based on their reuse distances, and uses three clock hands to manage the different page types. The allocation of memory between hot and cold pages is adjusted adaptively. Clock-Pro aims to address some limitations of previous CLOCK-based algorithms by more closely modeling LIRS behavior.

1. Clock-Pro: An Effective
Replacement in OS Kernel
Xiaodong Zhang
College of William and Mary

2. How to Make LIRS Work in OS Kernels?
q Most system kernels use the CLOCK algorithm, an
approximation of LRU.
q We have made efforts to directly implement LIRS in
Linux kernels, but …
q Our experience tells us that we must build on
existing strength.
q CLOCK is the base for LIRS.

3. Basic CLOCK Replacement
 All the resident pages are placed around a circular list, like a clock;
 Each page is associated with a reference bit, indicating if the page has been
accessed.
1 0 0
0
1 0
0 1
0
1 CLOCK hand 0
0 1
On a HIT 0 0
Set Reference bit to 1
(no algorithm operations) 1 0
1 0
0 0
0 1 1
0

4. Basic CLOCK Replacement
Starts from the currently pointed page, and evicts the page if it is”`0”;
Move the clock hand until reach a “0” page;
Give “1” page a second chance, and reset its “1” to “0”
1 0 0
0
1 0
0 0
1
On a sequence of 1 CLOCK hand 0
two MISSes
0 0
0 1
1 0
1 0
0 0
0 1 1
0

5. The CLOCK with a Long History
“In the Multics system a paging algorithm has been developed that
has the implementation ease and low overhead of the FIFO strategy
and is an approximation to the LRU strategy”
“A paging Expreiment with the Multics System” MIT Project MAC
Report MAC-M-384, May 1968, Fernando J. Corbato (1990 Turing Award Laureate)

6. The CLOCK has been Widely Used
Major OS Major OS Textbooks
Multics Tanebaum & Woodhull
UNIX/AIX/Linux/BSD Silberschatz & Galvin
VAX/VMS Stallings (for undergraduate)
DB2
Windows/Oracle/Solaris

7. Prior Work on LRU versus CLOCK
LRU related work CLOCK related work
FBR (1990, SIGMETRICS)
GCLOCK (1978, ACM TDBS)
LRU-2 (1993, SIGMOD)
2Q (1994, VLDB)
SEQ (1997, SIGMETRICS)
LRFU (1999, OSDI)
EELRU (1999, SIGMETRICS)
MQ (2001, USENIX)
1968 2003
LIRS (2002, SIGMETRICS) CAR (2004, FAST)
ARC (2003, FAST) CLOCK-Pro (2005, USENIX)

8. GCLOCK Replacement
 Introduce additional page access information.
 A counter is associated with each page rather than a
single bit;
 The counter is incremented on a page hit;
 The clock hand periodically moves, and decrements
the counter of each block;
 The page with its counter of 0 is replaced.

9. Age-Based CLOCK in Linux and FreeBSD
 An age is associated with each page in addition to a
reference bit;
 When the clock hand sweeps through pages, it
increases its age if the page’s bit is 1, otherwise it
decreases its age.
 The page with its age of 0 is replaced.

10. CAR: CLOCK with Adaptive Replacement
 Two clocks T1 and T2, one is for cold pages touched only once recently (Recency),
another is for hot pages touched at least twice (“Frequency”);
 Queues B1 and B2 are for pages recently replaced from T1 and T2;
 The memory allocations for T1 or T2 depend on the ratio of references
to B2 and B1.

11. Limits of CAR
 A page that is regularly accessed with its reuse distance a little bit
larger memory size has no hits in T1 or T2. (inherited LRU problem).
 A page in T2 can stay in memory without any accesses because
frequency does not reflect ``reuse distance”.
 No system implementations yet

12. Basic Ideas of CLOCK-Pro
 It is an approximation of LIRS based on the CLOCK infrastructure.
 Pages categorized into two groups: cold pages and hot pages
based on their reuse distances (or IRR).
 There are three hands: Hand-hot for hot pages,
Hand-cold for cold pages, and Hand-test for running a reuse
distance test for a block;
 The allocation of memory pages between hot pages (Mhot) and
cold pages (Mcold ) are adaptively adjusted. (M = Mhot + Mcold)
 All hot pages are resident (=Lir blocks), some cold pages are also
resident (= Hir Blocks); keep track of recently replaced pages
(=non-resident Hir blocks)

13. 0 1 Cold resident CLOCK-Pro Structure
0 1 Hot
1 24
Cold non-resident 2 23
Hand-hot: find a hot 0
3 0 0
page to be demoted 0 22
into a cold 0
page. 1
4
0
0 21
5 0 hand-hot
0 20
Two reasons for a resident cold page: All hands move
6 1 19
• A fresh replacement: a first access. hand-cold in the clockwise
• It is demoted from a hotused to find
Hand-cold is page. direction. hand-test
7
a page for replacement. 0 18
8 0 17
Hand-test: (1) to determine if a
0
cold page is promoted to be hot; (2)
9 16
remove non-resident cold pages out 0
of the clock. 10 15
1 0
11 14
12 13

14. Accessing Sequence: 1 24 1 5
….27, 7, 26, 25, 4, 23 2 23 0 0
0 0
3 0 0 22
0 1 0 04
4 3
0 21
0 Resident
5 Cold Pages
0 hand-hot 20
0
6
hand-cold Clock
1 19
7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 0 9
16
0 7 6
22
13 0 1 10
0 15
11 14 8 16
15
0 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

15. Current accessing: 1 24 1 5
Page 23 2 23 0 0
0 0
Hit! 3 0 0
1 22
Set reference bit of 0 1 0 04
Page 23 to 1. 4 3
21
No other operation 0
0 Resident
5 Cold Pages
0 hand-hot 20
0
6
hand-cold Clock
1 19
7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 0
1 9 16
0 7 6
22
13 0 1 10
0
15
11 14 8 16
15
0 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

16. Current accessing: 1 24 1 5
Page 4 2 23 0 0
0 0
Hit! 3 0 1 22
Set reference bit of 0 1 0 04
1
Page 4 to 1. 4 3
21
No other operation 0
1 0 Resident
5 Cold Pages
0 hand-hot 20
0
6
hand-cold Clock
1 19
7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 1 9 16
0 7 6
22
13 0 1 10
0
15
11 14 8 16
15
0 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

17. Current accessing: 1 24 1 5
Page 25 2 23 0 0
0 0
Miss! 3 0 1 22
0 1 0 14
(1) run hand-cold--- 4 3
21
reclaim the first met 1
cold page with ref bit 0 0 Resident
5 Cold Pages
(Page 5) 0 hand-hot 20
0
(2) Reclaim cold page
5 and remove it from 6
resident cold page list hand-cold Clock
1 19
7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 1 9 16
0 7 6
22
13 0 1 10
0
15
11 14 8 16
15
0 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

18. Current accessing: 1 24 1
Page 25 2 23 0
0 0
(3) Leave non-resident 3 0 1 22
cold page 5 in the 0 1 0 14
original position of 4 3
21
clock list 1
0 Resident
5 Cold Pages
(4) Add page 5 into hand-hot 20
non-resident cold 0
page list
6
(5) run hand-test to hand-cold Clock
1 19
find a position for
page 5 7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 1 9 16
0 7 Non-
6
resident
22
13 0 1 10
0
15 colde
page list
11 14 8 is full! 16
15
0 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

19. Current accessing: 1 24 1
Page 25 2 23 0
0 0
(5) Add page 5 to non- 3 0 1 22
resident cold page list 0 1 0 14
4 3
21
Reorganize clock list 1
0 Resident
5 Cold Pages
hand-hot 20
0
6
hand-cold Clock
1 19
7 hand-test 0 18
8 0 17
2 24
9 0 0 23 0
0 1 9
0 7 6
22
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

20. Current accessing: 1 25 1 25
Page 25 2 24 0 0
0 0
Now we already have 3 0 0 23
one empty room for 0 1 0 14
page 25 4 3
22
1 1 Resident
(6) Check page 25 in
hand-hot
5 Cold Pages
non-resident page 21
list---it is not in it 0
(7) Add page 25 into 6
clock and resident hand-cold Clock
0 20
cold page list
7 1 19
hand-test
8 0 18
2 24
9 0 0 23 0 0
0 1 9 17
0 7
page 25
does not
6
22 exit!
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

21. Current accessing: 1 25 1 25
Page 26 2 24 0 0
0 0
Miss! 3 0 0 23
0 1 0 14
(1) run hand-cold to 4 3
22
reach the first met 1 1 Resident
cold resident page hand-hot
5 Cold Pages
with reference bit 0 21
0
6
hand-cold Clock
0 20
7 1 19
hand-test
8 0 18
2 24
9 0 0 23 0 0
0 1 9 17
0 7 6
22
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

22. Current accessing: 1 25 1 25
Page 26 2 24 0 0
0 0
Page 4 has reference 3 0 0 23
bit 1, it should be 0 1 0 14
upgraded to hot list. 4 3
22
1 1 Resident
hand-hot
1. Remove it from 5 Cold Pages
resident cold page list 21
0
6
hand-cold Clock
0 20
7 1 19
hand-test
8 0 18
2 24
9 0 0 23 0 0
0 1 9 17
0 7 6
22
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

23. Current accessing: 1 25 1 25
Page 26 2 24 0 0
0 0
Page 4 has reference 3 0 0 23
bit 1, it should 0 1 0
upgrade to a hot page. 3
22
1. Remove it from
hand-hot 1 Resident
5 Cold Pages
resident cold page list 21
0
2. Reset the reference hand-cold
bit of page 4 and add it 6
to hot page list Clock
0 20
4
7 1 19
0 hand-test
8 0 18
2 24
9 0 0 23 0 0
0 1 9 17
Hot page
0 7 6
22
13 0 1
list is full! 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

24. Current accessing: 1 25 1 25
Page 26 2 24 0 0
0 0
run hand-hot: degrade 3 0 0 23
the first met hot page 0 1 0
with reference bit 0 to 3
22
a cold resident page
hand-hot 1 Resident
5 Cold Pages
21
0
hand-cold
6
Clock
0 20
4
7 1 19
0 hand-test
8 0 18
2 24
9 0 0 23 0 0
0 1 9 17
0 7 6
22
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

25. Current accessing: 1 25 1 25
Page 26 2 0 0
0 0
Run hand-hot: 3 0 23
degrade first met hot 0 1 0
page with ref bit 0 to a 3
22
cold resident page
hand-hot 1 Resident
5 Cold Pages
Reorganize clock list 21
0 24
hand-cold
6
0 20 0
Clock
4
7 1 19
0 hand-test
8 0 18
2
9 0 23 0 0
0 1 9 17
0 7 6
22
13 0 1 10
0
15
5
11 8
0
14 0
15 0 21
12 13
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

26. Current accessing: 1 25 1 25
Page 26 2 4 0 0
0 0
Page 4 is added into 3 0 0 24
hot page list. 0 0 0 0 24
5 3
23
Page 24 is added into
cold page list.
1 Resident
6 Cold Pages
hand-hot 22
1
hand-cold
7
Clock
0 21
8 0 20
9 0 hand-test 1 19
2 4
9 0 0 23 0
0 1 10
0 18
7 6
22
13 0 1 11 0 17
12
0 8
5
0
15 0
15 0 21
13 14
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

27. Current accessing: 1 25 1 25
Page 26 2 4 0 0
0 0
run hand-cold: 3 0 0 24
Page 3 has reference 0 0 0 0 24
bit 0, reclaim it and 5 3
23
move hand-cold to the 1 Resident
next position 6 Cold Pages
hand-hot 22
1
hand-cold
7
Clock
0 21
8 0 20
9 0 hand-test 1 19
2 4
9 0 0 23 0
0 1 10
0 18
7 6
22
13 0 1 11 0 17
12
0 8
5
0
15 0
15 0 21
13 14
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

28. Current accessing: 1 25 1 25
Page 26 2 4 0 0
0 0
Reclaim page 3 and 3 0 0 24
add it into non- 0 0 24
resident cold page list. 5 23
1. Leave non-resident
1 Resident
6 Cold Pages
cold page 3 in the old hand-cold hand-hot 22
position in clock list. 1
2. run hand-test: move 7
14 from non-resident Clock
0 21
cold page list.
8 0 20
9 0 hand-test 1 19 3
2 4 0
9 0 0 23 0
0 1 10
0 18
Non-
resident
7 6
cold page
22
13 0 1 11 0 17 list is full!
12
0 8
5
0
15 0
15 0 21
13 14
0 0 10 14
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

29. Current accessing: 1 25 1 25
Page 26 2 4 0 0
0 0
Add page 3 into non- 3 0 0 24
resident cold page list. 0 0 24
5 23
Reorganize clock list
1 Resident
6 Cold Pages
hand-cold hand-hot 22
1
7
Clock
0 21
8 0 20
9 0 hand-test 1 19
2 4
9 0 0 23 0
0 1 10
0 18
7 6
22
13 0 1 11 0 17
12
0 8
5
0
15 0
15 0 21
13
0 0 10 3
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

30. Current accessing: 1 25 1 25
Page 26 2 4 0 0
0 0
Add page 26 into 3 0 0 24
resident cold page list 0 26
0 0 24
and clock list 5 26
0 Resident
6 Cold Pages
hand-cold 23
hand-hot 1
7
Clock
1 22
8 0 21
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 1 10
0 19
7 6
22
13 0 1 11 0 18
12
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

31. Current accessing: 1 25 1 25
Page 7 2 4 0 0
0 0
Miss! 3 0 0 24
(1) run hand-cold: 0 26
0 0 24
5 26
reclaim the first met
resident cold page 0 Resident
6 Cold Pages
(page 1) hand-cold 23
hand-hot 1
7
Clock
1 22
8 0 21
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 1 10
0 19
7 6
22
13 0 1 11 0 18
12
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

32. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
1. Leave page 1 in the
old position in clock 0 26
0 0 24
list. 5 26
2. run hand-test: to
0 Resident
6 Cold Pages
find a position for hand-cold 23
page 1 hand-hot 1
7
Clock
1 22
8 0 21
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 1 10
0 19
Non-
resident
7 6
cold page
22
13 0 1 11 0 18 list is full!
12
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 12
Hot Pages Non-resident Cold Pages

33. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
3. Add page 1 into
non-resident cold 0 26
0 0 24
page list. 5 26
0 Resident
6 Cold Pages
hand-cold 23
hand-hot 1
7
Clock
1 22
8 0 21
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 1 10
0 19
7 6
22
13 0 1 11 0 18
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages

34. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
Now we have one
empty slot in memory 0 26
0 0 24
5 26
(1) check non-resident
cold page list and find 0 Resident
6 Cold Pages
it---we need to hand-cold 23
upgrade it to hot page
hand-hot 1
(2) run hand-hot: 7
degrade the first met Clock
1 22
hot page with
reference bit 0 to cold 8 0 21
page
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 1 10
0 19
Hot page 7 6
22
13 0 1
list is full! 11 0 18
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages

35. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
Page 23 has reference
bit 1---reset it to 0 0 26
0 0 24
5 26
0 Resident
6 Cold Pages
hand-cold 23
hand-hot 0
1
Clock
1 22
7 8 0 21
0
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 0
1 10
0 19
6
22
13 0 1 11 0 18
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages

36. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
Page 22 has reference
bit 1---reset it to 0 0 26
0 0 24
5 26
0 Resident
6 Cold Pages
hand-cold 23
0
hand-hot
Clock
0
1 22
7 8 0 21
0
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 0 10
0 19
6
22
13 0 0
1 11 0 18
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages

37. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
Page 21 has reference
bit 0---degrade it 0 26
0 0 24
5 26
0 Resident
6 Cold Pages
hand-cold 23
0
Clock hand-hot 0 22
7 8 0 21
0
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 0 10
0 19
6
22
13 0 0 11 0 18
0 0 8
5
0
17 0
15 0 21
13 15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages

38. Current accessing: 1 25 25
Page 7 2 4 0
3 0 0 0 24
(3) Degrade page 21 to
resident cold page 0 26
0 0 24
5 26
Reorganize clock list
0 Resident
6 Cold Pages
hand-cold 23
0
Clock
hand-hot 0 22
7 21
8
0 0
9 0 hand-test 0 20
2 4
9 0 0 23 1
0 0 10
0 19
6
22
13 0 0 11 0 18
0 0 8
5
0 13 15
17 0
15
0 0 10 3
17 0 1 20
18 19 11 1
Hot Pages Non-resident Cold Pages