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Silberschatz, Galvin and Gagne 200210.1Operating System Concepts
Virtual Memory
Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for
execution - some can be on disk Disk address can be stored in place of frame number
A valid–invalid bit is associated with each page table entry (1 in-memory, 0 not-in-memory) During address translation, if valid–invalid bit in page table
entry is 0 page fault bring in to memory
Silberschatz, Galvin and Gagne 200210.2Operating System Concepts
Page Table When Some Pages Are Not in Main Memory
Sum logical address space can therefore be much larger than physical address space. More processes
Silberschatz, Galvin and Gagne 200210.3Operating System Concepts
Page Faults Bringing a Page into Memory
Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction
Silberschatz, Galvin and Gagne 200210.4Operating System Concepts
Need For Page Replacement
Silberschatz, Galvin and Gagne 200210.5Operating System Concepts
What happens if there is no free frame?
Page replacement – find some page in memory, but not really in use, swap it out.
Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.
Silberschatz, Galvin and Gagne 200210.6Operating System Concepts
Basic Page Replacement
1. Find the location of the desired page on disk.
2. Find a free frame:
1. If there is a free frame, use it.
2. If there is no free frame, use a page replacement algorithm to select a victim frame.
3. If victim is dirty, write it out to disk
3. Read the desired page into the (newly) free frame. Update the page and frame tables.
4. Restart the process
Silberschatz, Galvin and Gagne 200210.7Operating System Concepts
Page Replacement Policies
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 - only for crucial OS processes
Local replacement – each process selects from only its own set of allocated frames - the normal case
I/O interlock Pages that are used for copying a file from a device must be
locked
Silberschatz, Galvin and Gagne 200210.8Operating System Concepts
Thrashing
VM works because of locality Process migrates from
one locality to another. Localities may overlap
What happens if size of localities > total memory size
Silberschatz, Galvin and Gagne 200210.9Operating System Concepts
Thrashing
If a process does not have “enough” pages for its locality, the page-fault rate is very high. Thrashing a process is busy swapping pages in and out.
Thrashing may lead to: low CPU utilization. operating system thinks that it needs to increase the degree
of multiprogramming. another process added to the system
It is important to allocate enough frames to each process If that’s not possible, the process must be swapped out
Silberschatz, Galvin and Gagne 200210.10Operating System Concepts
Allocation of Frames
Each process needs minimum number of frames 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
Silberschatz, Galvin and Gagne 200210.11Operating System Concepts
Fixed Allocation
Equal allocation – e.g., if 100 frames and 5 processes, give each 20 frames.
Proportional allocation – Allocate according to the size of process.
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Silberschatz, Galvin and Gagne 200210.12Operating System Concepts
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.
Silberschatz, Galvin and Gagne 200210.13Operating System Concepts
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.
Silberschatz, Galvin and Gagne 200210.14Operating System Concepts
Initial Load
Load pages predicted to be needed (compiler flags)
or Load no pages
Allows for more efficient process creation
Silberschatz, Galvin and Gagne 200210.15Operating System Concepts
Performance of Demand Paging
Page Fault Rate 0 p 1.0 (hopefully low, e.g. 5%) 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)
Values Assume memory access roughly 1 (with TLB hits) Page fault overhead is small Swap is about 10000 Restart is small
Silberschatz, Galvin and Gagne 200210.16Operating System Concepts
Page Replacement Algorithms
Want lowest page-fault rate (disk is slow)
Evaluate algorithm by running it on a particular string of page references (reference string) and computing the number of page faults on that string. A page fault means a disk access (or two), and that’s where
the time goes.
Assume no initial load, local replacement, no I/O (no interlock issues)
Silberschatz, Galvin and Gagne 200210.17Operating System Concepts
Optimal Algorithm
Replace page that will not be used for longest period of time in future.
3 frames example: 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 (9)
4 frames example: 1 2 3 4 1 2 5 1 2 3 4 5 (6) But the future is unknown (like SJF scheduling)
Silberschatz, Galvin and Gagne 200210.18Operating System Concepts
First-In-First-Out (FIFO) Algorithm
3 frames example: 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 (15)
Silberschatz, Galvin and Gagne 200210.19Operating System Concepts
FIFO Illustrating Belady’s Anamoly
4 frames example: 1 2 3 4 1 2 5 1 2 3 4 5 (10) 3 frames example: 1 2 3 4 1 2 5 1 2 3 4 5 (9)
Belady’s Anomaly - more frames does not imply less page faults
Silberschatz, Galvin and Gagne 200210.20Operating System Concepts
Counting Algorithms
Keep a counter of the number of references that have been made to each page. Count is reset each time a page is brought in
LFU Algorithm: replaces page with smallest count. May delay start of counting to avoid bias from an initial rush
MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used.
See example in Geoff’s notes
Silberschatz, Galvin and Gagne 200210.21Operating System Concepts
LRU Page Replacement
3 frames example: 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1 (11)
4 frames example: 1 2 3 4 1 2 5 1 2 3 4 5 (?)
Silberschatz, Galvin and Gagne 200210.22Operating System Concepts
Least Recently Used (LRU) Algorithm
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.
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 Both need HW support, as they are done very very often
Silberschatz, Galvin and Gagne 200210.23Operating System Concepts
Use Of A Stack to Record The Most Recent Page References
Silberschatz, Galvin and Gagne 200210.24Operating System Concepts
LRU Approximation Algorithms
Usually not enough HW support for full LRU, so … Reference bit in the page table
With each page associate a bit, in HW 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. Clear when all 1
Reference bytes in the page table Make top bit the reference bit Shift right reference byte at regular intervals Victim is page with smallest value byte
Second chance bit associated with each frame Looks through frames in order, with wrap-around, starting at frame after last
loaded If frame has bit = 1 then:
set bit 0. leave page in memory. replace next page, subject to same rules.
Silberschatz, Galvin and Gagne 200210.25Operating System Concepts
Second-Chance (clock) Page-Replacement Algorithm
Silberschatz, Galvin and Gagne 200210.26Operating System Concepts
Enhanced Second Chance
Keep a reference bit and a modified bit with each frame 0,0 => not recently used or modified => best to replace 0,1 => not recently used but modified => must be written out 1,0 => recently used but not modified => may be used again soon 1,1 => Recently used and modified => best keep this one in RAM
Victim selection After each load, the current position is set to the frame after the
loaded one. Scan from current position round for a 0,0 frame If not found, scan round for a 0,1 frame, reseting the use bit (ala
2nd chance) If not found, start again (a victim must be found)
Used by Mac OS
Silberschatz, Galvin and Gagne 200210.27Operating System Concepts
Page Buffering
Keep some frames free -a frame pool Immediately swap in on page fault Write out while first process uses CPU
Remember what pages are in the frame pool, in case they are requested again - used by VMS with FIFO to recover from preemptive removal
Write out modified pages whenever the swap disk is idle
Silberschatz, Galvin and Gagne 200210.28Operating System Concepts
Other Techniques
Pre-paging - predicting page needs Windows NT clustering
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.
Silberschatz, Galvin and Gagne 200210.29Operating System Concepts
Other Considerations
Program structure int A[ ][ ] = new int[1024][1024]; Each row is stored in one page Program 1 for (j = 0; j < A.length; j++)
for (i = 0; i < A.length; i++)A[i,j] = 0;
1024 x 1024 page faults
Program 2 for (i = 0; i < A.length; i++)for (j = 0; j < A.length; j++)
A[i,j] = 0;
1024 page faults
Silberschatz, Galvin and Gagne 200210.30Operating System Concepts
Windows NT
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.
On single x86 CPUS, uses a second chance style algorithm to select victims
On Alpha and SMP, uses a modified FIFO
Silberschatz, Galvin and Gagne 200210.31Operating System Concepts
Solaris 2
Maintains a list of free pages to assign faulting processes.
Lotsfree – threshold parameter to begin pageout. Pageout is called more frequently depending upon the
amount of free memory available.
Pageout scans pages using variation on the second chance algorithm (2 handed clock algorithm) Scanrate is the rate at which pages are scanned. This
ranged from slowscan to fastscan. This increases as free memory decreases
Handspread affects time process has to reuse a page before the big-hand send the page out
Pages from shared libraries are kept in memory (recent variation)