Post on 31-Mar-2015
OS Fall’02
Virtual Memory: Page Replacement
Operating Systems Fall 2002
OS Fall’02
Realizing Virtual Memory Hardware support
Memory Management Unit (MMU): address translation, bits, interrupts
Operating system supportPage replacement policyResident set managementLoad control degree of multiprogramming
OS Fall’02
Page Replacement Policy Resident set maintenance
Fixed or variable allocation Per-process or global replacement
Page replacement problemA fixed number of frames, M, is used to map the process virtual memory pagesWhich page should be replaced when a page fault occurs and all M frames are occupied?
OS Fall’02
Requirements and Metrics Workload: a sequence of virtual
memory references (page numbers) Page fault rate =
#page faults/#memory references Minimize the page fault rate for
workloads obeying the principle of locality
Keep hardware/software overhead as small as possible
OS Fall’02
Algorithms Optimal (OPT) Least Recently Used (LRU) First-In-First-Out (FIFO) Clock
OS Fall’02
Optimal Policy (OPT) Replace the page which will be
referenced again in the most remote future
Impossible to implementWhy?
Serves as a baseline for other algorithms
OS Fall’02
Least Recently Used (LRU) Replace the page that has not been
referenced for the longest time The best approximation of OPT for
the locality constrained workloads Possible to implement Infeasible as the overhead is high
Why?
OS Fall’02
First-In-First-Out (FIFO) Page frames are organized in a
circular buffer with a roving pointer Pages are replaced in round-robin
styleWhen page fault occur, replace the page to which the pointer points to
Simple to implement, low overhead High page fault rate, prone to
anomalous behavior
OS Fall’02
Clock (second chance) Similar to FIFO but takes page usage
into accountCircular buffer + page use bitWhen a page is referenced: set use_bit=1When a page fault occur: For each page:
if use_bit==1: give page a second chance: use_bit=0; continue scan;
if use_bit==0: replace the page
OS Fall’02
Example: Page 727 is needed
0
1
2
3
4
56
7
8
n
.
.
.
Page 9use = 1
Page 19use = 1
Page 1use = 0
Page 45use = 1
Page 191use = 1
Page 556use = 0
Page 13use = 0
Page 67use = 1
Page 33use = 1
Page 222use = 0
next frame pointer
OS Fall’02
After replacement0
1
2
3
4
56
7
8
n
.
.
.
Page 9use = 1
Page 19use = 1
Page 1use = 0
Page 45use = 0
Page 191use = 0
Page 727use = 0
Page 13use = 0
Page 67use = 1
Page 33use = 1
Page 222use = 0
next frame pointer
OS Fall’02
Example of all algorithms
OS Fall’02
LRU and non-local workloads Workload: 1 2 3 4 5 1 2 3 4 5…
Typical for array based applications
What is the page fault rate for M=1,…,5?
A possible alternative is to use a Most Recently Use (MRU) replacement policy
OS Fall’02
Belady’s Anomaly It is reasonable to expect that
regardless of a workload, the number of page faults should not increase if we add more frames: not true for the FIFO policy:
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
2
3
1
2
3
4
1
2
5
3
4
1
2
3
1
2
3
5
1
2
4
5
44 3
OS Fall’02
Algorithm comparison
OS Fall’02
Clock algorithm with 2 bits Use “modified” bit to evict
unmodified (clean) pages in preference over modified (dirty) pages
Four classes:u=0; m=0: not recently used, cleanu=0; m=1: not recently used, dirtyu=1; m=0: recently used, cleanu=1; m=1: recently used, dirty
OS Fall’02
First scan: look for (0,0) frame, do not change the use bit
If (0,0) frame is found, replace it
Second scan: look for (0,1) frame, set use bit to 0 in each frame bypassed
If (0,1) frame is found, replace it
If all failed, repeat the above procedure
this time we will certainly find something
Clock algorithm with 2 bits
OS Fall’02
Page buffering Evicted pages are kept on two lists:
free and modified page lists
Pages are read into the frames on the free page list
Pages are written to disk in large chunks from the modified page list
If an evicted page is referenced, and it is still on one of the lists, it is made valid at a very low cost
OS Fall’02
Resident set management With multiprogramming, a fixed
number of memory frames are shared among multiple processes
How should the frames be partitioned among the active processes?
Resident set is the set of process pages currently allocated to the memory frames
OS Fall’02
The working set model [Denning’68]
Working set is the set of pages in the most recent page references
Working set is an approximation of the program locality
OS Fall’02
The working set strategy Monitor the working set for each
currently active process Adjust the number of pages
assigned to each process according to its working set size
Monitoring working set is impractical The optimal value of is unknown
and would vary
OS Fall’02
Approximating the WS Global page replacement
All memory frames are candidates for page eviction a faulting process may evict a page of other
process
Processes with larger WS are expanding whereas those with smaller WS are shrinking
Problem: may unjustly reduce the WS of some processes
Combine with page buffering
OS Fall’02
Approximating the WS Local page replacement
Only the memory frames of a faulting process are candidates for replacement
Dynamically adjust the process allocation
Page-Fault Frequency (PFF) algorithm
OS Fall’02
Page-Fault Frequency (PFF) Approximate the page-fault frequency:
Count all memory references for each active processWhen a page fault occurs, compare the current counter value with the previous page fault counter value for the faulting processIf < F, expand the WS; Otherwise, shrink the WS by discarding pages with use_bit==0
OS Fall’02
Multiprogramming level Too many processes in memory
Thrashing, inability to run new processes
The solution is swapping: save all the resident set of a process to the disk (swapping out)load the pages of another process instead (swapping in)
Long-term and medium term scheduling decides which processes to swap in/out
OS Fall’02
Long (medium) term scheduling
Decision of which processes to swap out/in is based on
The CPU usageCreating a balanced job mix with respect to I/O vs. CPU bound processes
Two new process states: Ready swappedBlocked swapped
OS Fall’02
UNIX process statesrunning
user
runningkernel
readyuser
readykernel
blocked
zombie
sys. callinterrupt
schedule
created
return
terminated
wait for event
event done
schedule
preempt
interrupt
readyswapped
blockedswapped
Swap out
event done
Swap outSwap in
OS Fall’02
Segmentation with paging Segmentation
simplifies protection and sharing, enforce modularity, but prone to external fragmentation
Paging transparent, eliminates ext. fragmentation, allows for sophisticated memory management
Segmentation and paging can be combined
OS Fall’02
Address translation
Main Memory
PageFrame
Offset
Paging
Page Table
P#
+
Frame # Offset
Seg Table Ptr
+S #
SegmentationProgram
SegmentTable
Seg # Page # Offset
OS Fall’02
Page size considerations Small page size
better approximates localitylarge page tablesinefficient disk transfer
Large page sizeinternal fragmentation
Most modern architectures support a number of different page sizes
a configurable system parameter
OS Fall’02
Next: File system, disks, etc