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Chapter 8 - Processes
• Scheduling models:
– FCFS - First Come First Serve; aka FIFO First In First Out; considered fair scheduling.
– Figure 8.1: supermarket scheduling
– SJF - Shortest Job First - fair if you consider
the average wait time decreases: favors short
jobs and minimizes the average wait time over
all processes.
– Figure 8.2: short job line.
• Scheduling:
– Highest-Response-Ratio-Next: discriminates less
against long jobs (copy room analogy)
– HRRN:
• response ratio = (wait time + job time) / job time
• Pick job with highest response ratio to go next.
• The longer you wait, the better chance you get to go next.
– Priority scheduling: Do the task with the highest
priority first (or next). Difficult part is determining
the priority values.
– Deadline scheduling: Run the one that has to finish
the soonest.
• Scheduling:
– Round Robin - Take turns with the resource; 10minute video game analogy (Figure 8.3).
– Round Robin lends itself well to preempted
resources.
– Round Robin scheduling of processes in an operating
system is preferred, since preemption of the CPU is
relatively cheap.
• Preemptive Scheduling Methods
– All follow the classic process flow diagram (Figure
8.4) with a timer enforcing preemption.
– The Dispatcher() selects a new candidate from the
ready queue; we are interested in the selection
method.
• Preemptive Round-Robin Scheduling
– Pick first process in ready queue.
– Set the timer to the time quantum or time slice value
and let the process run.
– When timer expires, put this process at the end of the
list and pick the next process in the ready queue,
assigning it a time quantum, etc.
– Must select a reasonable size for the time slice.
– One model:
• Include context switch overhead in time slice
• Suppose context switch time was 200 microsecs
• Can measure the time slice in units of context switch times;
10 milliseconds = 50 context switches = 10,000 microsecs
• Preemptive Round-Robin Scheduling
– One model (continued):
• If time slice = 1 context switch time, overhead for context
switching is 100 percent.
• If time slice = 100 context switch times, overhead for
context switch is 1 percent.
• In general, this is 1/N percent overhead; Figure 8.5 shows
the curve.
• End result: bad to have a very small time slice; bad to have
a very long time slice (if interactive response is important).
• Could pick a target interactive response of, say, 1/4 of a
second and set the time slice to 250
milliseconds/NumberOfProcesses.
• Preemptive Round-Robin Scheduling
– Under large ready queue sizes, round robin degrades
interactive response time, since the wait time is a
function of the size of the queue.
– Can adjust the time quantum based on the size of the
ready queue (adaptive RR).
• Multi-level queues
– To alleviate this, the process scheduling algorithm
can create a multi-tiered queuing system.
– 2 queue example: any jobs (processes) in the top
queue get scheduled before any in the bottom queue.
A process that uses up it’s entire time slice is moved
into the bottom queue (Figure 8.6).
• Multi-level queues
– Can extend to 3 queues that have variable parameters
determining the job flow between the queues (Figure
8.7).
– Dispatcher will select a job in Queue 1, else Queue 2
else finally Queue 3.
– Let q1, q2 and q3 represent the time quantum per
queue.
– Let n1 = number of times a process can go through
Queue 1 before being demoted to Queue 2. Let n2 do
the same between Queue 2 and 3.
– If n1 and q1 are infinite, FCFS.
– If n1 is infinite and q1 is finite, Round Robin.
– If n1 & q1 are finite, have two-queue system (etc.)
• Multi-level queues
– The parameterization of the multilevel queues is an
excellent example of the separation between policy
and mechanism.
– The mechanism (the use of a three-queued system
with parameters to control each queue’s behavior) is
flexible enough to support a broad range of policies.
– A good operating system will provide enough
flexibility in such things as dispatcher parameters to
permit a particular site the ability to support different
policies.
• Scheduling Problems
– The table on page 288 gives four jobs and their CPU
time, arrival time and priority.
– Figure 8.8 shows the job scheduling for four
algorithms (FCFS, SJF, Priority, RR w/ quantum=2
and RR with continuous zero-time context switches)
– Note SJF gives best average job turnaround.
– FCFS is horrible, but subject to “convoy effect”
(large processes up front create large waits).
– The first RR I respectfully disagree with -- it’s RR
with SJF selection (note how Job 2 runs first, Job 4
second, etc.)!
– The RRs, though, aren’t the best or worst, but give
fair response time.
• Scheduling Problems
– Non-SJF RR is more realistic, since all the other
algorithms rely on knowledge about how much CPU
the process is going to need. This is usually not
known!
– Can predict the next “CPU burst” based on past
behavior, with a running average.
• Scheduling in Real Operating Systems
–
–
–
–
Almost all use priority scheme.
Preemptive.
Equal priority jobs are scheduled Round Robin.
Jobs migrate depending on how greedy they are,
except for real-time processes.
• Scheduling in Real Operating Systems
– UNIX SVR4
• 160 Priority levels
– 0-59: timesharing/interactive
– 60-99: system processes
– 100-159: processes running in kernel mode
• Highest priority runs first; ties are “Round Robined”.
• Timesharing priorities goes down if it chews up entire time
quantum; can go back up if it blocks on an event or waits in
the ready queue for a long time.
• Time quanta range from 100 milliseconds (priority 0) to 10
milliseconds (priority 159?)
• Scheduling in Real Operating Systems
– Solaris
• Similar to SVR4
• Adds 10 more priority levels (160-169 for interrupt
handlers).
• Shorter time quanta: from 4 to 20 milliseconds.
• Priority inheritance - a process with a lock will inherit the
priority of a process blocked on the lock. Prevents priority
inversion -- the starvation of a chain of processes that have
different priorities.
– OS/2 version 2.0
• 128 priority levels in 4 classes (time critical, server, regular
and idle classes).
• Time quanta: 32 to 248 milliseconds.
• Interactive, freshly-unblocked & waiting priorities increase
• Scheduling in Real Operating Systems
– Windows NT 3.51
• Priority thread scheduler with 32 levels in two classes
(real-time & time-sharing threads).
• Priorities can increase due to I/O (disk, keyboard, mouse,
etc.) and decrease due to CPU usage.
– Mach
• 128 priority levels
• Hands-off scheduling: a receiver blocked on a message will
get scheduled immediately, regardless of the scheduling
queues.
– Linux
• Three scheduler algorithms: RR & FIFO (used in real time
applications) and OTHER; based on POSIX work.
• See “man sched_setscheduler” for more information.
• Deadlock
– Situation where two or more processes are each
waiting for a resource that another process in the
group holds.
– Figure 8.9 illustrates deadlock; this is an example of a
resource allocation graph:
• Arrows FROM process TO resource indicate a resource
request.
• Arrows TO process FROM resource indicate an allocated
resource.
• In this case, a cycle exists with no other resource instances,
thus deadlock exists!
– Similar deadlock exists with 2-process
Send()/Receive() cycle & 4-process cycle.
• Deadlock
– Conditions necessary for deadlock to occur:
1. Resources are not preemptable.
2. Resources cannot be shared.
3. A process can hold one resource & request another.
4. Circular wait is possible (a cycle exists in the
request/allocated graph)
• Dealing with deadlocks (3 techniques)
– Prevention: place restrictions on resource requests so
that deadlock cannot occur.
– Avoidance: Pretend to allocate the resource, run an
avoidance algorithm that detects cycles, don’t permit
the allocation if deadlock possible.
– Recovery: Let deadlock happen & recover from it.
• Deadlock Prevention
– Involves breaking one of the four necessary
conditions:
1. Allow preemption. Not always possible (how to
preempt a printer, tape drive, or other non-shareable
resource?)
2. Avoid mutual exclusion: Create virtual instances,
where each processes has the illusion of mutex
(again, not always possible).
3. Avoid “Hold & Wait”: One technique is to force
processes to acquire all their resources at one time,
preventing any future waiting. This is inefficient and
a process may not always know what it’s future needs
are.
• Deadlock Prevention
4. Avoid circular wait: One technique is to prioritize
resource usage by assigning a unique positive integer
to a resource; a process can only request resources in
numerically increasing order. Good solution, but can
still lead to inefficient usage of resources.
• Deadlock Avoidance
– Before a resource allocation is granted an algorithm
is executed that pretends to perform the allocation
and then detects cycles. A classic example is the
Banker’s Algorithm (used by banks to make sure they
have enough money to cover a loan, for instance).
– Unfortunately, execution of deadlock avoidance
algorithms is computationally expensive (O(n2)).
• Deadlock Avoidance
– Another problem is that algorithms like the Banker’s
Algorithm only work if a process knows the
maximum number of each type of resource before
beginning execution.
• Deadlock Recovery
– 2 steps: detect the deadlock first, then figure out how
to untangle the resource allocations to arrive at an
undeadlocked state.
– Deadlock detection algorithms are similar to
deadlock avoidance algorithms, which means they are
also computationally expensive, but at least they
don’t have to be invoked at each resource request.
• Deadlock Recovery
– Deadlock detection & recovery is optimistic; assumes
no deadlock will occur. VMS uses a timer of 10
seconds after a resource request is started. If when
the timer goes off the request is still not granted then
the deadlock detection/recovery algorithm is invoked.
– Recovery from the deadlock can involve a number of
techniques:
• Rollback - return a process back to an earlier point in
execution before the deadlock occurs; perhaps a future
allocation will avoid the deadlock. This is an expensive
proposition!
• Process termination - kill one or more processes until the
deadlock is broken.
• Six approaches to the deadlock problem (from
extreme to liberal)
– Never grant resource requests!
– Serialize process executions (one process at a time
only)!
– One-shot allocation - a process must request all it’s
resources at once.
– Hierarchical allocation - a process must request
resources in a known order.
– Advance claim - a process must indicate it’s
maximum requests at one time (needed for deadlock
avoidance algorithm, such as the Banker’s
algorithm).
– Always allocate & hope detect/recover works!
• Interesting that a number of operating systems
choose to not bother with deadlock
prevention/avoidance/recovery at all! This may
explain machine hangs that occur now & then;
the cost of any deadlock algorithm far outweighs
the frequency of actual deadlocks.
• Two-phase locking
– Common database method for record access that
avoids deadlock.
– 1st phase = locking phase: process will attempt to
lock a record or set of records for update; can only
hold these set of locks (thus, no “Hold/Wait”).
• Two-phase locking
– 2nd phase = changing phase: database records are
actually updated at this time; since no other locks are
permitted to be acquired, no deadlock is possible. All
locks are released when this phase is complete.
• Starvation
– Case where a process may never get to proceed due
to an inefficient allocation algorithm or unfair
deadlock algorithm (aging can solve starvation).
– Random scheduling or even FCFS (DAT & CDROM
example) can cause process resource starvation.
– Solutions to the deadlock problem can make
starvation more likely and solutions to the starvation
problem can make deadlock more likely!
• Message-passing variations
– Some useful lessons come out of tweaking the basic
message passing SOS system calls.
– First variation: rather than have anonymous message
queues that any process can send/receive, have
messages go directly to a process via the processes’
PID. Note this is less flexible - Figure 8.10 (a).
– Second variation: Message passing with nonblocking receives. What if you want to be able to
receive from multiple queues? The original
ReceiveMessage() call only allowed you to look at
one queue at a time; it would block the caller if a
message wasn’t available. If we make the system call
non-blocking, then the process can “poll”.
• Message-passing variations
– Call this new variation ReceiveMessageNoWait() and
have a return value indicate there is no message on
the queue. Problem now becomes one of much busy
waiting as the receiver polls multiple queues in a loop
(Figure 8.11 (b)).
– Can create children to handle each queue you want to
receive on using traditional blocking (Figure 8.11
(a)).
– Can create a new system call that permits a process to
block waiting for a SendMessage() on multiple
queues (Figure 8.11 <c>). In most UNIXes, this is
done via the select() system call (see “man -s 3c
select” on xi).
• Message-passing variations
– The next variation works on the sender. Make the
SendMessage() system call block waiting for a
receiver (no need for queues)!
– Can extend the client-server model of communication
to Send & Receive:
• Client sends a message to the server (SendMessage()) and
blocks.
• Server receives the messages (ReceiveMessage()).
• Server sends a reply to the client (SendReply()), which
unblocks the client.
– Figure 8.12 illustrates this.
– Receiver is blocked until the SendMessage(); sender
is blocked until the SendReply().
• Message passing with blocking sends
– To re-iterate, here’s the steps (from page 302):
1. Sender calls SendMessage() and blocks.
2. Receiver calls ReceiveMessage().
3. Message is transferred directly from the sender’s
buffer to the receiver’s buffer (no intermediate
message queue is used).
4. Receiver handles the message.
5. Receiver calls SendReply(). Message reply is copied
from the receiver’s buffer directly into the sender’s
buffer.
6. Sender is unblocked.
• Remote Procedure Calls
– The SendMessage/ReceiveMessage/SendReply
mechanism is similar to a procedure call made
between two processes. The next logical variation is
to extend this mechanism to RPC (Figure 8.13):
•
•
•
•
•
Client makes an RPC call.
Client-side stub turns it into a message to a server.
Server-side stub calls the actual procedure on the server.
The real procedure performs the work.
The server-side stub sends the result back in a message to
the client.
• The client-side stub receives the result message.
• The client-side stub returns to the RPC caller.
• Remote Procedure Calls
– Advantages to RPC: easily understood by
programmers, supports typed data types for better
error checking.
– Implementation issues with RPC:
• How are the calling and returning arguments sent? Can’t
pass pointers; must pass the entire object! Sun’s RPC
mechanism uses XDR (see “man xdr”) to translate machine
differences in data type encodings between heterogeneous
machines.
• How do you connect a client with a particular server?
Must have some sort of RPC registration system; Sun’s
RPC uses a flat file (/etc/rpc on xi) as well as a background
daemon process (rpcbind on xi) and a public method for
displaying RPC mappings (try “rpcinfo” on xi).
• Remote Procedure Calls
– More implementation issues with RPC:
• Another popular RPC system is used within the Distributed
Computing Environment (DCE).
• How to handle errors? What if messages get lost between
client and server?
– RPC libraries - common set of routines that don’t
require “reinventing the wheel” for RPC
programmers.
– RPC is the backbone of a number of network
services. Sun’s RPC is used, for instance, to support
the Network File System (NFS), which is how your
home directories are shared between machines.
– DCE also uses RPC heavily.
• Synchronization
– A process waits for notification of an event that will
occur in another process.
– The IPC patterns in Chapter 7 were examples of
synchronization patterns.
– We will concentrate on just the mutual exclusion
(mutex) style of IPC, since it presents unique
problems.
– Page 305: In signaling, one process waits for another
process to do something (signal an event) and in
mutual exclusion, one process waits for another
process to not be doing something (not be in it’s
critical section).
• Synchronization
– Signaling is the most basic form of process
cooperation and mutex is the most basic form of
process competition.
– Figure 8.14: nice table of different styles of
synchronization found in the book.
– Race conditions can exist in O/S code as well as user
code. A common technique for enforcing
serialization in O/S code is via disabling interrupts or
using spin locks. These are too extreme for user-level
programs, so we use semaphores and monitors to
create critical sections and prevent race conditions.
• Semaphores
– Semaphore: Solves both the signaling and mutex
problems.
– SOS: We will use an integer value to “name” a
semaphore. The values are global (e.g., SID == 4 is
the same semaphore for all process).
– New semaphore SOS calls:
• int AttachSemaphore(int sema_gid) - Attach this process to
the global semaphore specified by sema_gid. Return a
local identifier to be used in later semaphore calls. The
first call using sema_gid will create the semaphore and set
it’s state to busy.
• int Wait(int sema_id) - If the semaphore specified by local
identifier sema_id is not busy then set it to busy and return.
If the semaphore is busy, block the calling process on the
semaphore’s wait queue.
• Semaphores
– New semaphore SOS calls:
• int Signal(int sema_id) - If the wait queue of the specified
ID is empty, set the state of the semaphore to not busy. If
the wait queue is not empty, remove some process from the
queue and unblock that process.
• int DetachSemaphore(int sema_id) - indicate to the
operating system that this process no longer is using
sema_id.
– These are binary semaphores, meaning they are either
busy or not busy. Pseudo-code for their
implementation is on page 310. To work, Wait() and
Signal() must be atomic (allowed to execute
completely before the process is switched out).
• Semaphores
– Analogy: bathroom key at a gas station! Only one
key, thus only one person allowed at a time in
bathroom. Many may wait for the key :)
• Using semaphores instead of Message Queues to
solve IPC patterns
– Two process mutex with semaphores (page 311).
– Two process rendezvous with semaphores (page 312).
– 1 Buffer Producer/Consumer with semaphores (page
312-313).
• Counting Semaphores
– The semaphore is a counter rather than a binary flag.
– Permits more complex process synchronization, like
allowing more than one process at a time into the
critical section.
– Page 313/314 show pseudo code for Wait()/Signal()
using counting semaphores.
– Page 314/315 show Producer/Consumer with N
buffers.
– Notice how the synchronization and data transfer is
decoupled using semaphores, unlike using message
queues. Semaphores are easier to implement,
however. Threads commonly use semaphores.
– Using semaphores is logically equivalent to
sending/receiving empty messages, without the
overhead of managing message queues.
– Message are more appropriate for IPC between
processes that do not share memory.
– Semaphores are more appropriate for IPC between
processes (or more likely threads) that do share
memory.
• Implementing Semaphores
– Pages 316 through 319 provide changes to support
counting semaphores in SOS. Easily applicable to
JavaSOS!
• Using semaphores in SOS
– Semaphores are used to create critical sections around
SOS kernel code that uses shared data, such as:
• Accessing the process table in SelectProcessToRun().
• Accessing the disk request queue in DiskIO(),
ScheduleDisk() and the disk interrupt handler.
• Monitors
– Monitors are an example of synchronization added to
a programming language, rather than having to be
explicitly programmed via system calls.
– Monitors fit naturally into languages that support
modular/abstract data types.
– This type of monitor is not something you connect to
the video adapter of a computer!
• Monitors
– A monitor is a programming language module with:
• variables - usually private variables.
• condition variables - used for signaling inside the monitor.
• Procedures - public procedures callable from outside the
monitor.
– The monitor ensures that only one procedure at a time
within the monitor module can be called at a time. It,
by definition, provides mutual exclusion.
– Besides mutex, monitors provide signaling through
their condition variables.
– Condition variables have two operations allowed on
them -- wait and signal.
• Monitors
– Note that the semantics of wait and signal are
different than for semaphore’s Wait() and Signal().
– A condition variable wait will cause a process to
block automatically.
– A condition variable signal will unblock all the
processes that are waiting on the condition variable.
– CVs do not count or keep a busy/non-busy flag as
semaphores do; if signal is called on a CV 10 times
and then a process waits on the CV it will still block.
A wait waits for the next signal call.
– Unlike semaphores, a process that calls a CV wait
gives up the lock on the monitor.
• Monitors
– The behavior of CV wait and signal becomes clearer
after going through the signaling monitor code (pages
324-325) as well as the monitor solution to the
Producer/Consumer problem (pages 325-326).
– One could even implement semaphores as a monitor,
assuming no O/S-level support for semaphores was
available yet you had a language that supported
monitors (page 327-328).
– Note that CVs are not used soley in monitors.
– Section 8.19.2 (Synchronization primitives in Ada95)
is not covered.
– Neither is section 8.20 (message-passing design
issues).
• IPC in Mach
– Interesting IPC structure that has influenced OS
design.
– Mach task = traditional process (at least one thread
plus memory objects, etc.).
– Mach port = message queue variation:
• Only one task can receive messages on a port, but any task
can send a message to a port.
• Children can inherit ports.
• Messages are sent down ports. Messages can be either:
– Data
– Out-of-line data - pass part of the sender’s address space to the
receiver.
– Ports - can pass port addresses back and forth.
• IPC in Mach
– Mach is object-oriented.
– A Mach port represents an object.
– You perform operations on an object by sending
messages to the port that represents the object.
• IPC and Synchronization Examples
– UNIX signals are used to indicate many possible
events between processes; see “man -s 5 signal” on xi
for a complete list of events.
– Users can install their own signal handlers via the
signal() system call.
• IPC and Synchronization Examples
– SVR4 UNIX: implements UNIX signals,
semaphores, semaphore arrays, message queues,
shared memory.
– Windows NT:
• alerts & asynchronous procedure calls provides a signaling
mechanism.
• LPC (Local Procedure Call) mechanism used for message
passing.
• Standard RPC, named pipes are available for network
communication.
• Spin locks are used for multiprocessor synchronization.
• Range of sync. primitives (threads can wait on I/O
completion or on another thread exit; binary semaphores;
general semaphores & general events).
• IPC and Synchronization Examples
– OS/2: Shared memory, binary semaphores, general
semaphores, message queues, signals & pipes.
– Solaris: binary semaphores (mutexes), general
semaphores, special reader/writer lock, nonblocking
locks, condition variables that can be used with
mutexes to wait for arbitrary conditions.