CSC 246 Final

Question 1

Deadlock Avoidance - simplest technique

Answer 1

Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need

The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition

Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes

Question 2

Deadlock avoidance - Safe state (and basic facts)

Answer 2

System is in safe state if there exists a sequence of ALL the processes in the systems such that for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j < I

That is:
If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished
When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate
When Pi terminates, Pi +1 can obtain its needed resources, and so on

Basic facts:
If a system is in safe state ⇒ no deadlocks

If a system is in unsafe state ⇒ possibility of deadlock

Avoidance ⇒ ensure that a system will never enter an unsafe state.

Question 3

Resource-Allocation Graph Scheme

Answer 3

If Single instance of a resource type, Use a resource-allocation graph

• Claim edge Pi → Rj indicated that process Pj may request resource Rj; represented by a dashed line
• Claim edge converts to request edge when a process requests a resource
• Request edge converted to an assignment edge when the resource is allocated to the process
• When a resource is released by a process, assignment edge reconverts to a claim edge
• Resources must be claimed a priori in the system

Suppose that process Pi requests a resource Rj:
The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph

Question 4

Banker’s Algorithm

Answer 4

Used Multiple instances of resources, Each process must a priori claim maximum use

Let n = number of processes, and m = number of resources types.

Available

Max

Allocation

Need [i,j] = Max[i,j] – Allocation [i,j]

Request algorithm:
Must check that Request <= need (cannot exceed maximum claim)

Must check that request <= allocation, if not, must wait for other processes to go

Give safety sequence

Question 5

Deadlock Detection - single instance of resources

Answer 5

Maintain wait-for graph
Nodes are processes
Pi → Pj if Pi is waiting for Pj

Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock

An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph

Question 6

Deadlock Detection algorithm usage issues

Answer 6

When, and how often, to invoke depends on:
How often a deadlock is likely to occur?
How many processes will need to be rolled back?
one for each disjoint cycle

If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes “caused” the deadlock.

Question 7

Recovery from Deadlock: Process Termination (order considerations)

Answer 7

Abort all deadlocked processes

Abort one process at a time until the deadlock cycle is eliminated

In which order should we choose to abort?

• Priority of the process
• How long process has computed, and how much longer to completion
• Resources the process has used
• Resources process needs to complete
• How many processes will need to be terminated
• Is process interactive or batch?

Question 8

Recovery from Deadlock: Resource Preemption

Answer 8

Selecting a victim – minimize cost

Rollback – return to some safe state, restart process for that state

Starvation – same process may always be picked as victim, include number of rollback in cost factor

Question 9

Base and Limit Registers (basic definition)

Answer 9

A pair of base and limit registers define the logical address space
CPU must check every memory access generated in user mode to be sure it is between base and limit for that user

Question 10

Address Binding

Answer 10

Programs on disk, ready to be brought into memory to execute form an input queue
Without support, must be loaded into address 0000

Further, addresses represented in different ways at different stages of a program’s life

• Source code addresses usually symbolic
• Compiled code addresses bind to relocatable addresses
• i.e. “14 bytes from beginning of this module”
• Linker or loader will bind relocatable addresses to absolute addresses
• i.e. 74014
• Each binding maps one address space to another

Question 11

Address binding of instructions and data to memory addresses can happen at three different stages

Answer 11

-Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes
-Load time: Must generate relocatable code if memory location is not known at compile time
-Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another
Need hardware support for address maps (e.g., base and limit registers)

Question 12

Logical vs. Physical Address Space (Concepts)

Answer 12

-The concept of a logical address space that is bound to a separate physical address space is central to proper memory management
Logical address – generated by the CPU; also referred to as virtual address
Physical address – address seen by the memory unit
-Logical and physical addresses are the same in compile-time and load-time address-binding schemes; logical (virtual) and physical addresses differ in execution-time address-binding scheme

• Logical address space is the set of all logical addresses generated by a program
• Physical address space is the set of all physical addresses generated by a program

Question 13

Memory Management Unit (MMU)

Answer 13

Hardware device that at run time maps virtual to physical address

To start, consider simple scheme where the value in the relocation register is added to every address generated by a user process at the time it is sent to memory

• Base register now called relocation register
• MS-DOS on Intel 80x86 used 4 relocation registers

The user program deals with logical addresses; it never sees the real physical addresses

• Execution-time binding occurs when reference is made to location in memory
• Logical address bound to physical addresses

Question 14

Dynamic relocation using relocation register (benefits)

Answer 14

Routine is not loaded until it is called
Better memory-space utilization; unused routine is never loaded
All routines kept on disk in relocatable load format
Useful when large amounts of code are needed to handle infrequently occurring cases
No special support from the operating system is required
Implemented through program design
OS can help by providing libraries to implement dynamic loading

Question 15

Static vs. Dynamic linking

Answer 15

• Static linking – system libraries and program code combined by the loader into the binary program image
• Dynamic linking –linking postponed until execution time

Small piece of code, stub, used to locate the appropriate memory-resident library routine
Stub replaces itself with the address of the routine, and executes the routine
Operating system checks if routine is in processes’ memory address
-If not in address space, add to address space
Dynamic linking is particularly useful for libraries
-Also known as shared libraries

Question 16

Swapping

Answer 16

• A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution
  
  • Total physical memory space of processes can exceed physical memory
• Backing store and roll in, roll out
• Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped
• System maintains a ready queue of ready-to-run processes which have memory images on disk

Does the swapped out process need to swap back in to same physical addresses?

• Depends on address binding method
• Plus consider pending I/O to / from process memory space

Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows)
Swapping normally disabled
Started if more than threshold amount of memory allocated
Disabled again once memory demand reduced below threshold

Question 17

Backing store (swapping)

Answer 17

fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images

Question 18

Roll out, roll in (swapping)

Answer 18

swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed

Question 19

Context Switch Time including Swapping

Answer 19

If next processes to be put on CPU is not in memory, need to swap out a process and swap in target process
Context switch time can then be very high

Can reduce if reduced size of memory swapped – by knowing how much memory is really being used
System calls to inform OS of memory use via request_memory() and release_memory()

Other constraints as well on swapping

• Pending I/O – can’t swap out as I/O would occur to wrong process
• Or always transfer I/O to kernel space, then to I/O device
  
  • Known as double buffering, adds overhead
• Standard swapping not used in modern operating systems
  
  • But modified version common
  • Swap only when free memory extremely low

Question 20

Swapping on Mobile Systems

Answer 20

Not typically supported

Instead use other methods to free memory if low

• iOS asks apps to voluntarily relinquish allocated memory
• Read-only data thrown out and reloaded from flash if needed
• Failure to free can result in termination
• Android terminates apps if low free memory, but first writes application state to flash for fast restart
• Both OSes support paging as discussed below

Question 21

Contiguous Memory Allocation

Answer 21

Contiguous allocation is one early method to allocate resources efficiently

Main memory usually into two partitions:
Resident operating system, usually held in low memory with the interrupt vector
User processes then held in high memory
Each process contained in single contiguous section of memory

Relocation registers used to protect user processes from each other, and from changing operating-system code and data

 - Base register contains value of smallest physical address
 - Limit register contains range of logical addresses
 - MMU maps logical address dynamically
 - Can then allow actions such as kernel code being transient and kernel changing size

Question 22

Multiple-partition allocation

Answer 22

Multiple-partition allocation:
Degree of multiprogramming limited by number of partitions
Variable-partition sizes for efficiency (sized to a given process’ needs)
Hole – block of available memory; holes of various size are scattered throughout memory
When a process arrives, it is allocated memory from a hole large enough to accommodate it
Process exiting frees its partition, adjacent free partitions combined
Operating system maintains information about:
a) allocated partitions b) free partitions (hole)

Dynamic Storage-Allocation Problem:
How to satisfy a request of size n from a list of free holes?

First-fit: Allocate the first hole that is big enough

Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size
Produces the smallest leftover hole

Worst-fit: Allocate the largest hole; must also search entire list
Produces the largest leftover hole (worst choice for speed and storage)

Question 23

Fragmentation

Answer 23

External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous

Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used

First fit analysis reveals that given N blocks allocated, 0.5 N blocks lost to fragmentation
1/3 may be unusable -> 50-percent rule

Question 24

compaction

Answer 24

Reduce external fragmentation by compaction

• Shuffle memory contents to place all free memory together in one large block
• Compaction is possible only if relocation is dynamic, and is done at execution time
• I/O problem
  
  • Latch job in memory while it is involved in I/O
  • Do I/O only into OS buffers

Now consider that backing store has same fragmentation problems

Question 25

Paging

Answer 25

Physical address space of a process can be noncontiguous; process is allocated physical memory whenever the latter is available
-Avoids external fragmentation
-Avoids problem of varying sized memory chunks
Divide physical memory into fixed-sized blocks called frames
-Size is power of 2, between 512 bytes and 16 Mbytes

Divide logical memory into blocks of same size called pages
Keep track of all free frames
To run a program of size N pages, need to find N free frames and load program
Set up a page table to translate logical to physical addresses
Backing store likewise split into pages
Still have Internal fragmentation

Question 26

Address Translation Scheme (for Paging)

Answer 26

Address generated by CPU is divided into:
Page number (p) – used as an index into a page table which contains base address of each page in physical memory
Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit

For given logical address space 2^m and page size 2^n

Question 27

Swapping vs. Paging

Answer 27

In general:
swapping refers to copying an entire process to disk
Paging is a memory mgmt. technique

However, the concept of paging also includes copying data to disk.
Windows has a page file and a swap file (pagefile.sys, swapfile.sys)

To complicate things, a lot of times these terms are used interchangeably!

Question 28

Implementation of Page Table

Answer 28

Page table is kept in main memory
Page-table base register (PTBR) points to the page table
Page-table length register (PTLR) indicates size of the page table

In this scheme every data/instruction access requires two memory accesses
-One for the page table and one for the data / instruction

The two memory access problem can be solved by the use of a special fast-lookup hardware cache called associative memory or translation look-aside buffers (TLBs)

Question 29

translation look-aside buffers (TLBs)

Answer 29

special fast-lookup hardware cache called associative memory or translation look-aside buffers (TLBs) used in page tables to solve two memory access problem

Question 30

Effective Access Time (EAT - Paging)

Answer 30

Hit ratio = α
Hit ratio – percentage of times that a page number is found in the associative registers; ratio related to number of associative registers

Consider α = 80%, 20ns for TLB search, 100ns for memory access

Effective Access Time (EAT) = 0.80 x 100 + 0.20 x 200 = 120ns (THE 200ns is double for 2 memory accesses)

Consider more realistic hit ratio -> α = 99%, 100ns for memory access
EAT = 0.99 x 100 + 0.01 x 200 = 101ns

Question 31

Valid-invalid (Paging)

Answer 31

Valid-invalid bit attached to each entry in the page table:

• “valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page
• “invalid” indicates that the page is not in the process’ logical address space
• Or use page-table length register (PTLR)

Any violations result in a trap to the kernel

Memory protection implemented by associating protection bit with each frame to indicate if read-only or read-write access is allowed
-Can also add more bits to indicate page execute-only, and so on

Question 32

Structure of the Page Table (3 different types)

Answer 32

Memory structures for paging can get huge using straight-forward methods
Consider a 32-bit logical address space as on modern computers
Page size of 4 KB (212)
Page table would have 1 million entries (232 / 212)
If each entry is 4 bytes -> 4 MB of physical address space / memory for page table alone
-That amount of memory used to cost a lot
-Don’t want to allocate that contiguously in main memory

Hierarchical Paging*
Hashed Page Tables
Inverted Page Tables*

Question 33

Hierarchical Page Tables

Answer 33

Break up the logical address space into multiple page tables
A simple technique is a two-level page table
We then page the page table

Forward mapped page table
-where p1 is an index into the outer page table, and p2 is the displacement within the page of the inner page table

Question 34

Inverted Page Table

Answer 34

Rather than each process having a page table and keeping track of all possible logical pages, track all physical pages

One entry for each real page of memory

Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page

Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs

Use hash table to limit the search to one — or at most a few — page-table entries
-TLB can accelerate access
But how to implement shared memory?
-One mapping of a virtual address to the shared physical address

Question 35

Shared Pages Vs private code

Answer 35

Shared code

• One copy of read-only (reentrant)
• code shared among processes (e.g., text editors, compilers)
• Similar to multiple threads sharing the same process space
• Also useful for interprocess communication if sharing of read-write pages is allowed

Private code and data
-Each process keeps a separate copy
of the code and data
-The pages for the private code and data can appear anywhere in the logical address space

Question 36

Segmentation

Answer 36

Memory-management scheme that supports user view of memory 
A program is a collection of segments
A segment is a logical unit such as:
		main program
		procedure 
		function
		method
		object
		local variables, global variables
		common block
		stack
		symbol table
		arrays

Question 37

Segmentation architecture

Answer 37

Logical address consists of a two tuple:
,

Segment table – maps two-dimensional physical addresses; each table entry has:

 - base – contains the starting physical address where the segments reside in memory
 - limit – specifies the length of the segment

Segment-table base register (STBR) points to the segment table’s location in memory

Segment-table length register (STLR) indicates number of segments used by a program;
segment number s is legal if s < STLR

Protection:
With each entry in segment table associate:
-validation bit = 0 ⇒ illegal segment
-read/write/execute privileges
Protection bits associated with segments; code sharing occurs at segment level
Since segments vary in length, memory allocation is a dynamic storage-allocation problem
No internal fragmentation since segments can be different sizes
A segmentation example is shown in the following diagram

Question 38

Virtual Memory (Pros and Cons/background)

Answer 38

Code needs to be in memory to execute, but entire program rarely used
Error code, unusual routines, large data structures
Entire program code not needed at same time

Pros
Consider ability to execute partially-loaded program
Program no longer constrained by limits of physical memory
Each program takes less memory while running -> more programs run at the same time
Increased CPU utilization and throughput with no increase in response time or turnaround time
Less I/O needed to load or swap programs into memory -> each user program runs faster

Question 39

Virtual Memory (definition)

Answer 39

Virtual memory – separation of user logical memory from physical memory

Only part of the program needs to be in memory for execution
Logical address space can therefore be much larger than physical address space
Allows address spaces to be shared by several processes
Allows for more efficient process creation
More programs running concurrently
Less I/O needed to load or swap processes

Question 40

Virtual Address Space (definition)

Answer 40

Virtual address space – logical view of how process is stored in memory

Usually start at address 0, contiguous addresses until end of space
Meanwhile, physical memory organized in page frames
MMU (memory management unit) must map logical to physical
Virtual memory can be implemented via:
Demand paging
Demand segmentation

Question 41

Demand Paging

Answer 41

Could bring entire process into memory at load time

Or bring a page into memory only when it is needed:
     Less I/O needed, no unnecessary I/O
     Less memory needed 
     Faster response
     More users

Similar to paging system with swapping (diagram on right, but processes reside in secondary memory like a disk)

Page is needed ⇒ reference to it
invalid reference ⇒ abort
not-in-memory ⇒ bring to memory

Lazy swapper – never swaps a page into memory unless page will be needed
Swapper that deals with pages is a pager

Extreme case – start process with no pages in memory
OS sets instruction pointer to first instruction of process, non-memory-resident -> page fault
And for every other process pages on first access
Pure demand paging

Actually, a given instruction could access multiple pages -> multiple page faults
Consider fetch and decode of instruction which adds 2 numbers from memory and stores result back to memory
Pain decreased because of locality of reference

Hardware support needed for demand paging
Page table with valid / invalid bit
Secondary memory (swap device with swap space)
Instruction restart

Question 42

Page Fault

Answer 42

If there is a reference to a page, first reference to that page will trap to operating system:
page fault
1) Operating system looks at another table to decide:
Invalid reference ⇒ abort
Just not in memory
2) Find free frame
3) Swap page into frame via scheduled disk operation
4) Reset tables to indicate page now in memory
Set validation bit = v
5) Restart the instruction that caused the page fault

Question 43

Demand Paging Optimizations

Answer 43

Swap space I/O faster than file system I/O even if on the same device
Swap allocated in larger chunks, less management needed than file system

Copy entire process image to swap space at process load time
Then page in and out of swap space
Used in older BSD Unix

Demand page in from program binary on disk, but discard rather than paging out when freeing frame
Used in Solaris and current BSD
Still need to write to swap space
Pages not associated with a file (like stack and heap) – anonymous memory
Pages modified in memory but not yet written back to the file system

Mobile systems
Typically don’t support swapping
Instead, demand page from file system and reclaim read-only pages (such as code)

Question 44

Copy-on-Write (COW)

Answer 44

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

In general, free pages are allocated from a pool of zero-fill-on-demand pages
Pool should always have free frames for fast demand page execution
Don’t want to have to free a frame as well as other processing on page fault

vfork() variation on fork() system call has parent suspend and child using copy-on-write address space of parent
Designed to have child call exec()
Very efficient

Question 45

Page Replacement (definition and basic steps)

Answer 45

Prevent over-allocation of memory by modifying page-fault service routine to include page replacement

Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk

Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

Basic Steps:

1) Find the location of the desired page on disk
2) Find a free frame:
- If there is a free frame, use it
- If there is no free frame, use a page replacement algorithm to select a victim frame
- Write victim frame to disk if dirty
3) Bring the desired page into the (newly) free frame; update the page and frame tables
4) Continue the process by restarting the instruction that caused the trap

Note now potentially 2 page transfers for page fault – increasing EAT

Question 46

Page and Frame Replacement Algorithms

Answer 46

Frame-allocation algorithm determines
How many frames to give each process
Which frames to replace

Page-replacement algorithm
Want lowest page-fault rate on both first access and re-access

Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string
Results depend on number of frames available

FIFO, Optimal, LRU

Question 47

First-In-First-Out (FIFO) Algorithm (page replacement)

Answer 47

Adding more frames can cause more page faults!
Belady’s Anomaly
How to track ages of pages?
Just use a FIFO queue

Question 48

Optimal Algorithm (page replacement)

Answer 48

Replace page that will not be used for longest period of time
9 is optimal for the example
How do you know this?
Can’t read the future
Used for measuring how well your algorithm performs

No Belady’s anamoly

Question 49

Least Recently Used (LRU) Algorithm (page replacement)

Answer 49

Use past knowledge rather than future
Replace page that has not been used in the most amount of time
Associate time of last use with each page

12 faults – better than FIFO but worse than OPT
Generally good algorithm and frequently used
But how to implement?

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 find smallest value
Search through table needed

Stack implementation (better)
Keep a stack of page numbers in a double link form:
Page referenced:
move it to the top
requires 6 pointers to be changed
easier to get from top, But each update more expensive
No search for replacement

No Belady’s anamoly

LRU needs special hardware and is still slow so need approximations

Question 50

LRU approximation algorithms

Answer 50

LRU needs special hardware and is still slow

Reference bit
With each page associate a bit, initially = 0
When page is referenced bit set to 1
Replace any with reference bit = 0 (if one exists)
We do not know the order, however

Second-chance algorithm
Generally FIFO, plus hardware-provided reference bit
Clock replacement
If page to be replaced has
Reference bit = 0 -> replace it
reference bit = 1 then:
set reference bit 0, leave page in memory
replace next page, subject to same rules

Question 51

Counting Algorithms (page replacement)

Answer 51

Keep a counter of the number of references that have been made to each page
Not common

Least Frequently Used (LFU) Algorithm: replaces page with smallest count

Most Frequently Used (MFU) Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used

Question 52

Fixed frame allocation

Answer 52

Each process needs minimum number of frames

Equal allocation – For example, if there are 100 frames (after allocating frames for the OS) and 5 processes, give each process 20 frames
Keep some as free frame buffer pool

Proportional allocation – Allocate according to the size of process
Dynamic as degree of multiprogramming, process sizes change

Question 53

Priority frame allocation

Answer 53

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

Question 54

Global vs local allocation (replacement)

Answer 54

Global replacement – process selects a replacement frame from the set of all frames on the system; one process can take a frame from another
But then process execution time can vary greatly
But greater throughput so more common

Local replacement – each process selects from only its own set of allocated frames
More consistent per-process performance
But possibly underutilized memory

Question 55

Thrashing

Answer 55

If a process does not have “enough” pages, the page-fault rate is very high
Page fault to get page
Replace existing frame
But quickly need replaced frame back
This leads to:
Low CPU utilization
Operating system thinking that it needs to increase the degree of multiprogramming
Another process added to the system

Thrashing ≡ a process is busy swapping pages in and out

Why does demand paging work?
Locality model
Process migrates from one locality to another
Localities may overlap

Why does thrashing occur?
Σ size of locality > total memory size
Limit effects by using local or priority page replacement

Question 56

Working-Set Model

Answer 56

Δ ≡ working-set window ≡ a fixed number of page references

Example: 10,000 instructions
WSSi (working set of Process Pi) =
total number of pages referenced in the most recent Δ (varies in time)
if Δ too small will not encompass entire locality
if Δ too large will encompass several localities
if Δ = ∞ ⇒ will encompass entire program
D = Σ WSSi ≡ total demand frames
Approximation of locality
if D > m ⇒ Thrashing
Policy if D > m, then suspend or swap out one of the processes

Approximate with interval timer + a reference bit
Example: Δ = 10,000
Timer interrupts after every 5000 time units
Keep in memory 2 bits for each page
Whenever a timer interrupts copy and sets the values of all reference bits to 0
If one of the bits in memory = 1 ⇒ page in working set
Why is this not completely accurate?
Improvement = 10 bits and interrupt every 1000 time units

Direct relationship between working set of a process and its page-fault rate
Working set changes over time
Peaks and valleys over time

Question 57

page fault frequency (establish as fix)

Answer 57

More direct approach than WSS
Establish “acceptable” page-fault frequency (PFF) rate and use local replacement policy
If actual rate too low, process loses frame
If actual rate too high, process gains frame

Question 58

Memory mapped files

Answer 58

Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory

A file is initially read using demand paging
A page-sized portion of the file is read from the file system into a physical page
Subsequent reads/writes to/from the file are treated as ordinary memory accesses

Simplifies and speeds file access by driving file I/O through memory rather than read() and write() system calls

Also allows several processes to map the same file allowing the pages in memory to be shared

But when does written data make it to disk?
Periodically and/or at file close() time
For example, when the pager scans for dirty pages

Question 59

Allocating Kernel Memory

Answer 59

Treated differently from user memory
Often allocated from a free-memory pool
Kernel requests memory for structures of varying sizes
Some kernel memory needs to be contiguous
I.e. for device I/O

Question 60

Buddy System (kernel memory strategy)

Answer 60

Allocates memory from fixed-size segment consisting of physically-contiguous pages

Memory allocated using power-of-2 allocator
Satisfies requests in units sized as power of 2
Request rounded up to next highest power of 2
When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of 2
Continue until appropriate sized chunk available

For example, assume 256KB chunk available, kernel requests 21KB
Split into AL and AR of 128KB each
One further divided into BL and BR of 64KB
One further into CL and CR of 32KB each – one used to satisfy request

Advantage – quickly coalesce unused chunks into larger chunk
Disadvantage - fragmentation

Question 61

Slab Allocator (Kernel memory strategy)

Answer 61

Alternate strategy
Slab is one or more physically contiguous pages
Cache consists of one or more slabs
Single cache for each unique kernel data structure
Each cache filled with objects – instantiations of the data structure
When cache created, filled with objects marked as free
When structures stored, objects marked as used
If slab is full of used objects, next object allocated from empty slab
If no empty slabs, new slab allocated
Benefits include no fragmentation, fast memory request satisfaction

Question 62

Prepaging

Answer 62

Prepaging
To reduce the large number of page faults that occurs at process startup
Prepage all or some of the pages a process will need, before they are referenced
But if prepaged pages are unused, I/O and memory was wasted
Assume s pages are prepaged and α of the pages are used
Is cost of s * α saved pages faults > or < than the cost of prepaging
s * (1- α) unnecessary pages?
α near zero ⇒ prepaging loses

Question 63

Page size selection considerations

Answer 63

Page size selection must take into consideration:
Fragmentation
Page table size 
Resolution
I/O overhead
Number of page faults
Locality
TLB size and effectiveness
Always power of 2, usually in the range 212 (4,096 bytes) to 222 (4,194,304 bytes)
On average, growing over time

Question 64

TLB Reach

Answer 64

TLB Reach - The amount of memory accessible from the TLB

TLB Reach = (TLB Size) X (Page Size)

Ideally, the working set of each process is stored in the TLB
Otherwise there is a high degree of page faults

Increase the Page Size
This may lead to an increase in fragmentation as not all applications require a large page size

Provide Multiple Page Sizes
This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation

Question 65

I/O Interlock

Answer 65

I/O Interlock – Pages must sometimes be locked into memory
Consider I/O - Pages that are used for copying a file from a device must be locked from being selected for eviction by a page replacement algorithm
Pinning of pages to lock into memory

Question 66

Virtual Machine Overview

Answer 66

Fundamental idea – abstract hardware of a single computer into several different execution environments
Similar to layered approach
But layer creates virtual system (virtual machine, or VM) on which operation systems or applications can run

Formal def
A VMM provides an environment for programs that is essentially identical to the original machine
Programs running within that environment show only minor performance decreases
The VMM is in complete control of system resources

Several components
Host – underlying hardware system that runs VMs
Virtual machine manager (VMM) or hypervisor – creates and runs virtual machines by providing interface that is identical to the host
(Except in the case of paravirtualization)
Guest Process – process provided with virtual copy of the host
Usually an operating system
Provided with a virtual copy of the host
Single physical machine can run multiple operating systems concurrently, each in its own virtual machine

Question 67

Virtual Machine Types

Answer 67

Type 0 hypervisors - (old idea “partitions”) Hardware-based solutions that provide support for virtual machine creation and management via firmware
IBM LPARs and Oracle LDOMs are examples

Type 1 hypervisors - (company data center OSs) Operating-system-like software built to provide virtualization
Including VMware ESX, Joyent SmartOS, and Citrix XenServer

Type 1 hypervisors – Also includes general-purpose operating systems that provide standard functions as well as VMM functions
Including Microsoft Windows Server with HyperV and RedHat Linux with KVM

Type 2 hypervisors - (use at home) Applications that run on standard operating systems but provide VMM features to guest operating systems
Including VMware Workstation and Fusion, Parallels Desktop, and Oracle VirtualBox (www.virtualbox.org)

Question 68

Variations of VMMs

Answer 68

Paravirtualization - Technique in which the guest operating system is modified to work in cooperation with the VMM to optimize performance

- VMM provides services that guest must be modified to use
 - Leads to increased performance
 - Less needed as hardware support for VMs grows

Programming-environment virtualization - VMMs do not virtualize real hardware but instead create an optimized virtual system
Used by Oracle Java (JVM) and Microsoft.Net
-similar to interpreted languages
-Programs written in Java run in the JVM no matter the underlying system

Emulators – Allow applications written for one hardware environment to run on a very different hardware environment, such as a different type of CPU

• performance challenge
• translate all guest instructions from guest CPU to native CPU
• Useful when host system has one architecture, guest compiled for other architecture

Application containment - Not virtualization at all but rather provides virtualization-like features by segregating applications from the operating system, making them more secure, manageable
-ex. Oracle containers / zones for example create virtual layer between OS and apps

Question 69

Benefits and features of VMMs

Answer 69

Host system protected from VMs, VMs protected from each other
I.e. A virus less likely to spread
Sharing is provided though via shared file system volume, network communication
Freeze, suspend, running VM, Snapshot of a given state, able to restore back to that state
Clone by creating copy and running both original and copy
Great for OS research, better system development efficiency
Run multiple, different OSes on a single machine
Consolidation, app dev, …

Templating – create an OS + application VM, provide it to customers, use it to create multiple instances of that combination

Live migration – move a running VM from one host to another!
No interruption of user access
All those features taken together -> cloud computing

Question 70

VMM implementation

Answer 70

Most VMMs implement a virtual CPU (VCPU) to represent state of CPU per guest as guest believes it to be
When guest context switched onto CPU by VMM, information from VCPU loaded and stored

Whatever the type, a VM has a lifecycle

• Created by VMM
• Resources assigned to it (number of cores, amount of memory, networking details, storage details)
• In type 0 hypervisor, resources usually dedicated
• Other types dedicate or share resources, or a mix
• When no longer needed, VM can be deleted, freeing resources

Steps simpler, faster than with a physical machine install
Can lead to virtual machine sprawl with lots of VMs, history and state difficult to track

CPU scheduling
Even single-CPU systems act like multiprocessor ones when virtualized
One or more virtual CPUs per guest
-overcommitment of CPU

Question 71

Solid State disks (SSDs)

Answer 71

Nonvolatile memory used like a hard drive
Many technology variations
Can be more reliable than HDDs
More expensive per MB
Maybe have shorter life span
Less capacity
But much faster
Busses can be too slow -> connect directly to PCI for example
No moving parts, so no seek time or rotational latency

Question 72

Network-attached storage (NAS)

Answer 72

Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus)
Remotely attaching to file systems

NFS and CIFS are common protocols
Implemented via remote procedure calls (RPCs) between host and storage over typically TCP or UDP on IP network

Question 73

File Concept (types and attributes)

Answer 73

Contiguous logical address space
Types: 
Data
    numeric
    character
    binary
Program
Contents defined by file’s creator
Many types
Consider text file, source file, executable file

Attributes
Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different types
Location – pointer to file location on device
Size – current file size
Protection – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring

Information about files are kept in the directory structure, which is maintained on the disk
Many variations, including extended file attributes such as file checksum
Information kept in the directory structure

Question 74

Disk structure

Answer 74

Disk can be subdivided into partitions
Disks or partitions can be RAID protected against failure
Disk or partition can be used raw – without a file system, or formatted with a file system

Entity containing file system known as a volume
Each volume containing file system also tracks that file system’s info in a device directory or volume table of contents

Question 75

Tree structure directories (and linux commands)

Answer 75

Absolute or relative path name
.. (one up)
. (current directory)
/ (one down)
touch filename (create file in this directory)
mkdir
rm (delete file)
pwd (print working directory)
cd (where you are or go to)

mount
A file system must be mounted before it can be accessed
An unmounted file system (i.e., Fig. 11-11(b)) is mounted at a mount point

Question 76

file sharing

Answer 76

Sharing of files on multi-user systems is desirable
Sharing may be done through a protection scheme
On distributed systems, files may be shared across a network
Network File System (NFS) is a common distributed file-sharing method
If multi-user system
User IDs identify users, allowing permissions and protections to be per-user
Group IDs allow users to be in groups, permitting group access rights
Owner of a file / directory
Group of a file / directory

Mode of access:  read, write, execute
Three classes of users on Unix / Linux
					RWX
		a) owner access 	7	⇒	1 1 1
				RWX
		b) group access 	6	 ⇒	1 1 0
					RWX
		c) public access	1	 ⇒	0 0 1

Ask manager to create a group (unique name), say G, and add some users to the group.
For a particular file (say game) or subdirectory, define an appropriate access.

Question 77

logical file system

Answer 77

Logical file system manages metadata information
Translates file name into file number, file handle, location by maintaining file control blocks (inodes in UNIX)
Inodes contain info about files – ownership, permissions, location
Directory management
Protection

Question 78

Boot control block

Answer 78

Boot control block contains info needed by system to boot OS from that volume
Needed if volume contains OS, usually first block of volume

Question 79

Volume control block (superblock, master file table)

Answer 79

Volume control block (superblock, master file table) contains volume details
Total # of blocks, # of free blocks, block size, free block pointers or array
Directory structure organizes the files
Names and inode numbers, master file table

Question 80

Partitions and Mounting

Answer 80

Partition can be a volume containing a file system (“cooked”) or raw – just a sequence of blocks with no file system

Boot block can point to boot volume or boot loader which is a set of blocks that contain enough code to know how to load the kernel from the file system
Or a boot management program for multi-os booting

Root partition contains the OS, other partitions can hold other Oses, other file systems, or be raw
Mounted at boot time

Other partitions can mount automatically or manually
At mount time, file system consistency checked
Is all metadata correct?
If not, fix it, try again
If yes, add to mount table, allow access

Question 81

Interrupts

Answer 81

CPU Interrupt-request line triggered by I/O device
Checked by processor after each instruction
Interrupt handler receives interrupts
Maskable to ignore or delay some interrupts
Interrupt vector to dispatch interrupt to correct handler
Context switch at start and end
Based on priority
Some nonmaskable
Interrupt chaining if more than one device at same interrupt number

Question 82

Block and Character Devices

Answer 82

Block devices include disk drives
Commands include read, write, seek
Memory-mapped file access possible

Character devices include keyboards, mice, serial ports
Commands include get(), put()
Libraries layered on top allow line editing