Upload
graciela-julian
View
245
Download
3
Embed Size (px)
Citation preview
Case Study 1: AMOEBA
History of Amoeba Amoeba originated at the Vrije Universiteit, Amsterdam, The
Netherlands in 1981 as a research project in distributed and parallel computing. It was designed primarily by Andrew S. Tanenbaum and three of his Ph.D. students. By 1983, an initial prototype, Amoeba 1.0, was operational.
Starting in 1984, a second group was set up. This work used Amoeba 3.0, which was based on RPC. Using Amoeba 3.0, it was possible for clients in Tromso to access servers in Amsterdam transparently, and vice versa.
Research Goals The primary goal of the project was to build a
transparent distributed operating system. An important distinction between Amoeba and
most other distributed systems is that Amoeba has no concept of a “home machine”.
A secondary goal of Amoeba is to provide a testbed for doing distributed and parallel programming.
The Amoeba System Architecture
Processor pool
X-terminals
Fileserver Print server
The Amoeba Microkernel
Microkernel
Client
Process managementMemory managementCommunicationI/O
ServerThread
The Amoeba Servers Amoeba is based on the client-server
model. Probably the most important server is the
file server, known as the bullet server. Another important server is the directory
server, also known as the soap server.
Objects and Capabilities The basic unifying concept underlying all
the Amoeba servers and the services they provide is the object.
Each object is managed by a server process.
Objects are named and protected by capabilities.
A capability in Amoeba
Server port Object Rights Check
Bits 48 24 8 48
Capability When the server creates an object, it
generates a capability.
If a client wants to create a restricted capability, go through the following.
On subsequent operations, the client must present the capability to identify the object.
port object 1111 1111 C (random number)
Restricted capability
port object C1111 1111
port object 00000001 f(C XOR 00000001)
New rights mask
0000 0001
Exclusive OR
One-way function
Restricted capability
Capability
Standard Operations Age Perform a garbage collection cycle: starts a new garbage collection cycle to get rid of old objects
that are no longer accessible.
Copy Duplicate the object and return a capability for the copy: it is a shortcut that makes it possible to duplicate an object without actually transferring it. Without this operation, copying a file would require sending it over the network twice: from the server to the client and then back again.
Detroy Destroy the object and reclaim its storage: deletes the object
Getparams Get parameters associated with the server: allow the system administrator to read and write parameters that control server operation. For example, the algorithm used to choose processors can be selected using this mechanism.
Info Get an ASCII string briefly describing the object
Restrict Produce a new, restricted capability for the object
Setparams Set parameters associated with the server: same as Getparams
Status Get current status information from the server
Touch Pretend the object was just used: tells the server that the object touched is still in used.
Process Management in Amoeba A process is an object in Amoeba. When a
process is created, the parent process is given a capability for the child process. The child can be suspended, restarted, signaled, or destroyed.
Process management is handled at three different levels in Amoeba.
1. At the lowest level are the process servers, which are kernel threads running on every machine.
2. At the next level up we have a set of library procedures that provide a more convenient interface for user programs.
3. Finally, the simplest way to create a process is to use the run server, which does most of the work of determining where to run the new process.
Process Descriptor
Architecture = 386Capability for exit statusSegment descriptorsThread 1 PC1 SP1
Thread 2 PC2 SP2
Thead 3 PC 3 SP 3
Text Shared data 1 2 3
Private data
Stacks
PC1
PC2
PC3
Segments
SP1SP2
SP3
Library procedures exec: to do an RPC with the specified
process server asking it to run the process. getload: returns information about the CPU
speed, current load, and amount of memory free at the moment.
stun: to suspend a child process. Two kinds of stuns are supported: normal and emergency.
Threads When a process starts up, it has one thread. During execution, the process can create
additional threads, and existing threads can terminate.
Three methods are provided for threads to synchronize: signals, mutexes, and semaphores.
All threads are managed by the kernel.
Memory Management in Amoeba Amoeba has an extremely simple memory model. A process can have any number of segments it
wants to have, and they can be located wherever it wants in the process’ virtual address space.
Segments are not swapped or paged, so a process must be entirely memory resident to run.
Each segment is stored contiguously in memory.
Mapped segments
S
D
T
S DT
Process virtualaddress space Memory segments
Communication in Amoeba Amoeba supports two forms of
communication: RPC, using point-to-point message passing and group communication.
Remote Procedure Call RPC Primitives: 1. get_request – indicates a server’s
willingness to listen on a port.2. put_reply – done by a server when it has a
reply to send.3. trans – send a message from client to
server and wait for the reply.
Group Communication in Amoeba
CreateGroup Create a new group and set its parameters
JoinGroup Make the caller a member of a group
LeaveGroup Remove the caller from a group
SendToGroup Reliably send a message to all members of a group
ReceiveFromGroup
Block until a message arrives from a group
ResetGroup Initiate recovery after a process crash
The Amoeba Reliable Broadcast Protocol 1. The user process traps to the kernel, passing it the
message.2. The kernel accepts the message and blocks the user
process.3. The kernel sends a point-to-point message to the
sequencer.4. When the sequencer gets the message, it allocates the next
available sequence number, puts the sequence number in a header field reserved for it, and broadcasts the message (and sequence number).
5. When the sending kernel sees the broadcast message, it unblocks the calling process to let it continue execution.
System Structure
A
SKernel
A
Kernel
A
Kernel
Sequencer enabled Sequencer disabled
Application programs
Broadcast network
An example
A
Last = 24
A
SLast=24
A
Last=24
A
Last=23
AM
M
M25
B
C
history
M25
M25
M25
Sequencer machine
M25
M25
Request for 24
buffered
Sender’s action for sending The sender sends a message to the sequencer and starts a timer: (a) the broadcast comes back before the timer runs out.
(normal case). the sender just stops the timer. (b) the broadcast has not come back before the timer expires.
(either the message or the broadcast has been lost). the sender retransmits the message. if the original message is lost, no harm is done. if the sender missed the broadcast, the sequencer will
detect the retransmission as a duplicate and tell the sender everything is all right.
(c ) The broadcast comes back before the timer expires, but it is the wrong broadcast. This occurs when two processes attempt to broadcast simultaneously.
If message A gets to the sequencer first, and is broadcast. A sees the broadcast and unblocks its application program. However, B sees A’s broadcast and realizes it has failed to go first. B will accept A’s broadcast and wait.
Sequencer’s action If a Request for Broadcast arrives: (a) check to see if the message is a
retransmission. If so, inform the sender that the broadcast has been done.
(b) if the message is new, assign the next sequence number to it, and increment the sequencer counter by 1.
The message and its identifier are stored in a history buffer, and the message is then broadcast.
Sender’s action for receiving When the sender receives a broadcast: (a) if the sequence number is 1 higher than
the most recent one (normal case). No broadcast has been missed.
(b) if the sequence number is more than 1 higher (a broadcast has been missed), the sender will send a message to the sequencer asking for the lost broadcast.
Management of the history buffer If the history buffer fills up, if the sequencer knows that
all machines have received broadcasts, say, 0 through 23, correctly, it can delete these from its history buffer. There are several mechanisms to discover this information:
(a) each Request for Broadcast message sent to the sequencer carriers a piggybacked acknowledgement, k, meaning that all broadcasts up to and including k have been correctly received.
(b) the sequencer can broadcast a Request for Status message asking the number of the highest broadcast received in sequence.
Two methods for doing reliable broadcasting
A S BAB S1
2 2
2
22
2 2
2
22
11
1
11
1. Message sent to the sequencer2. The sequencer broadcasts it
1. A broadcast M2. S broadcasts Accept
In method 1, each message appears in full on the network twice. Each user machine is interrupted only once.
In method 2, the full message appears only once on the network. Each machine is interrupted twice.
Summary of the protocol This protocol allows reliable broadcasting
to be done on an unreliable network in just over two messages per reliable broadcast. Each broadcast is indivisible, and all applications receive all messages in the same order, no matter how many are lost.
Fault tolerance The protocol is also fault tolerant. When a processor crashes, sooner or later, some
kernel will detect that the crashed machine are not being acknowledged. The kernel will mark the crashed processor as dead and initiates a recovery.
In phase 1, one process is elected as coordinator. In phase 2, the coordinator rebuilds the group and
brings all the other processes up to date.
The protocol is also fault tolerant
40 43 4441 40 X
40 43 4441 40 X
44 44 4444 44 X
0 1 2 3 4 5
coordinator coordinator Sequencer dies
coordinator Dead sequencer
new sequencer
0 1 2 3 4 5
0 1 2 3 4
(a)
(b)
(c )
(a) The sequencer crashes (b) A coordinator is selected (c) Recovery
How does the coordinator get any message it has missed if the sequencer has crashed?
The solution lies in the value of k. If k is 0 (non-fault tolerant), only the sequencer maintains a history buffer.
If k >0, k+1 machines maintain an up-to-date history buffer. If k machines crash, there is still one left to supply the coordinator with any messages it needs.
How to implement? In method 2, when the sequencer sees a message,
M, that was just broadcast, it does not immediately broadcast an Accept message. Instead, it waits until the k lowest-numbered kernels have acknowledged that they have seen and stored it. Now k+1 machines have stored M in their history buffers.
The Fast Local Internet Protocol Amoeba uses a custom protocol called
FLIP for actual message transmission. The protocol handles both RPC and group communication and is below them in the protocol hierarchy.
FLIP is a network layer protocol.
A
RPC Group
FLIP layer
The Bullet Server Create Create a new file; optionally commit it as
well
Read Read all or part of a specified file
Size Return the size of a specified file
Modify Overwrite n bytes of an uncommitted file
Insert Insert or append n bytes to an uncommitted file
Delete Delete n bytes from an uncommitted file
The Directory Server Create Create a new directory
Delete Delete a directory or an entry in a directory
Append Add a new directory entry to a specified directory
Replace Replace a single directory entry
Lookup Return the capability set corresponding to a specified name
Getmasks Return the rights masks for the specified entry
Chmod Change the rights bits in an existing directory entry
The Replication Server The Run Server The Boot Server The TCP/IP Server Disk server I/O server A time-of-day server A random number server Swiss Army Knife server