421/521 CSE - Operating Systems Fall 2011
Lecture - XXIII
Distributed Systems - I
Tevfik Koar
University at Buffalo
November 22nd, 2011
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Motivation
Distributed system is collection of loosely coupled processors that
do not share memory interconnected by a communications network
Reasons for distributed systems
Resource sharing
sharing and printing files at remote sites processing information in a distributed database using remote specialized hardware devices
Computation speedup load sharing Reliability detect and recover from site failure, function transfer, reintegrate failed site Communication message passing
Distributed-Operating Systems
Users not aware of multiplicity of machines
Access to remote resources similar to access to local resources
Data Migration transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task Computation Migration transfer the computation, rather than the data, across the system
Distributed-Operating Systems (Cont.)
Process Migration execute an entire process, or parts of it, at different sites
Load balancing distribute processes across network to even the workload Computation speedup subprocesses can run concurrently on different sites Hardware preference process execution may require specialized processor Software preference required software may be available at only a particular site Data access run process remotely, rather than transfer all data locally
Network Topology
Robustness in Distributed Systems
Failure detection Reconfiguration
Failure Detection
Detecting hardware failure is difficult To detect a link failure, a handshaking protocol can be used Assume Site A and Site B have established a link
At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running
If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost Site A can now send an Are-you-up? message to Site B If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B
Failure Detection (cont)
If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred Types of failures: - Site B is down - The direct link between A and B is down - The alternate link from A to B is down - The message has been lost However, Site A cannot determine exactly why the failure has occurred
Reconfiguration
When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available When the link or the site becomes available again, this information must again be broadcast to all other sites
Distributed Coordination
Ordering events and achieving synchronization in centralized systems is easier.
We can use common clock and memory
What about distributed systems?
No common clock or memory happened-before relationship provides partial ordering How to provide total ordering?
Event Ordering
Happened-before relation (denoted by )
If A and B are events in the same process (assuming sequential processes), and A was executed before B, then A B If A is the event of sending a message by one process and B is the event of receiving that message by another process, then A B If A B and B C then A C If two events A and B are not related by the relation, then these events are executed concurrently.
Relative Time for Three Concurrent Processes
Which events are concurrent and which ones are ordered?
Exercise
Which of the following event orderings true? are (a) p0 --> p3 (b) p1 --> q3 (c) q0 --> p3 (d) r0 --> p4 (e) p0 --> r4 : : : : :
Which of the following statements are true? (a) (b) (c) (d) (e) p2 and q2 are concurrent processes. q1 and r1 are concurrent processes. p0 and q3 are concurrent processes. r0 and p0 are concurrent processes. r0 and p4 are concurrent processes.
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Implementation of
Associate a timestamp with each system event
Require that for every pair of events A and B, if A B, then the timestamp of A is less than the timestamp of B The logical clock can be implemented as a simple counter that is incremented between any two successive events executed within a process
Within each process Pi, define a logical clock
Logical clock is monotonically increasing A process advances its logical clock when it receives a message whose timestamp is greater than the current value of its logical clock
Assume A sends a message to B, LC1(A)=200, LC2(B)=195 --> LC2(B)=201
If the timestamps of two events A and B are the same, then the events are concurrent
We may use the process identity numbers to break ties and to create a total ordering
Distributed Mutual Exclusion (DME)
Assumptions
The system consists of n processes; each process Pi resides at a different processor Each process has a critical section that requires mutual exclusion
Requirement
If Pi is executing in its critical section, then no other process Pj is executing in its critical section
We present two algorithms to ensure the mutual exclusion execution of processes in their critical sections
DME: Centralized Approach
One of the processes in the system is chosen to coordinate the entry to the critical section A process that wants to enter its critical section sends a request message to the coordinator The coordinator decides which process can enter the critical section next, and its sends that process a reply message When the process receives a reply message from the coordinator, it enters its critical section After exiting its critical section, the process sends a release message to the coordinator and proceeds with its execution This scheme requires three messages per critical-section entry:
request reply release
DME: Fully Distributed Approach
When process Pi wants to enter its critical section, it generates a new timestamp, TS, and sends the message request (Pi, TS) to all processes in the system When process Pj receives a request message, it may reply immediately or it may defer sending a reply back When process Pi receives a reply message from all other processes in the system, it can enter its critical section After exiting its critical section, the process sends reply messages to all its deferred requests
DME: Fully Distributed Approach (Cont.)
The decision whether process Pj replies immediately to a request(Pi, TS) message or defers its reply is based on three factors:
If Pj is in its critical section, then it defers its reply to Pi If Pj does not want to enter its critical section, then it sends a reply immediately to Pi If Pj wants to enter its critical section but has not yet entered it, then it compares its own request timestamp with the timestamp TS
If its own request timestamp is greater than TS, then it sends a reply immediately to Pi (Pi asked first) Otherwise, the reply is deferred
Example: P1 sends a request to P2 and P3 (timestamp=10) P3 sends a request to P1 and P2 (timestamp=4)
Undesirable Consequences
The processes need to know the identity of all other processes in the system, which makes the dynamic addition and removal of processes more complex If one of the processes fails, then the entire scheme collapses
This can be dealt with by continuously monitoring the state of all the processes in the system, and notifying all processes if a process fails