Case Study 1: AMOEBA
History of Amoeba n 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. n 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 n n n 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 File server Print server
The Amoeba Microkernel Client Microkernel Process management Memory management Communication I/O Server Thread
The Amoeba Servers n n n 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 n n n 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 Bits 48 24 8 48 Server port Object Rights Check
Capability n When the server creates an object, it generates a capability. port n n object 1111 C (random number) 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.
Restricted capability Capability port object 1111 C New rights mask 0000 0001 Exclusive OR One-way function Restricted capability port object 00000001 f(C XOR 00000001)
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 n 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.
n n 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 = 386 Capability for exit status Segment descriptors Thread 1 Thread 2 Thead 3 Text Shared data PC 1 SP 1 1 PC 2 SP 2 2 Stacks 3 SP 1 PC 2 PC 3 SP 2 Private data PC 3 Segments SP 3
Library procedures n n n 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 n n 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 n n 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 Process virtual address space S D T Memory segments S D T
Communication in Amoeba supports two forms of communication: RPC, using point-to-point message passing and group communication.
Remote Procedure Call n 1. 2. 3. n RPC Primitives: get_request – indicates a server’s willingness to listen on a port. put_reply – done by a server when it has a reply to send. trans – send a message from client to server and wait for the reply.
Group Communication in Amoeba Create. Group Join. Group Create a new group and set its parameters Make the caller a member of a group Leave. Group Remove the caller from a group Send. To. Group Reliably send a message to all members of a group Receive. From. G Block until a message arrives from a roup group Reset. Group Initiate recovery after a process crash
The Amoeba Reliable Broadcast Protocol 1. 2. 3. 4. 5. The user process traps to the kernel, passing it the message. The kernel accepts the message and blocks the user process. The kernel sends a point-to-point message to the sequencer. 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). When the sending kernel sees the broadcast message, it unblocks the calling process to let it continue execution.
System Structure Application programs A Kernel A S Kernel Sequencer enabled A Kernel Sequencer disabled Broadcast network
An example Sequencer machine A B M 25 A A A M Last = 24 M M 25 Last=24 history S M 25 Request for 24 A Last=23 M 25 buffered C
Sender’s action for sending The sender sends a message to the sequencer and starts a timer: n (a) the broadcast comes back before the timer runs out. (normal case). n the sender just stops the timer. n (b) the broadcast has not come back before the timer expires. (either the message or the broadcast has been lost). n 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 n
n n (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 n n 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 n n n 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 n n n 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 2 A 1 1 2 2 S 2 2 1. Message sent to the sequencer 2. The sequencer broadcasts it B 1 2 1 1 1. 2. 2 S 1 A 2 2 2 A broadcast M S broadcasts Accept B
n n 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 n 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 n n 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 (a) Sequencer dies coordinator 40 43 41 44 40 X 0 1 2 3 4 5 Dead sequencer coordinator (b) 40 43 41 44 40 X 0 1 2 3 4 5 X new sequencer (c ) 44 44 44 0 1 2 3 4 (a) The sequencer crashes (b) A coordinator is selected (c) Recovery
n n n 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 (nonfault 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.
n n 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 n n 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 Read Create a new file; optionally commit it as well 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 or append n bytes to an uncommitted file Delete n bytes from an uncommitted file Delete
The Directory Server Create a new directory Delete a directory or an entry in a directory Append Add a new directory entry to a specified directory Replace a single directory entry Lookup Return the capability set corresponding to a specified name Return the rights masks for the specified entry Getmasks Chmod Change the rights bits in an existing directory entry
n n n n n 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