NAME: STUDENT ID: MIDTERM 235 POINTS. Closed book, closed notes 70 minutes

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1 NAME: STUDENT ID: MIDTERM 235 POINTS Closed book, closed notes 70 minutes 1. Name three types of failures in a distributed system. (15 points) 5 points for each correctly names failure type Valid answers are: Omission failure Process or communication omission Failstop (or crash) Timing Arbitrary or Byzantine 2. In an asynchronous distributed system the client can always tell if the server has crashed or if it is just being slow if a proper protocol with keep-alive messages and acknowledgements is used and if that protocol provides at-most-once semantics. True of False? (20 points) FALSE.

2 3. Describe two modes of consistency validation in a distributed file system. (20 points) 10 points for each correct mode. The correct answer should look something like: Mode #1: Client validation A client asks the server if the data is valid. This can also be called client polling. Mode #2: Server validation A server tells clients when the data becomes invalid. This can also be called callback validation. 4. Why is support for threads and processes so important for building scalable distributed systems? (15 points) Distributed systems do lots of I/O. Threads/processes help overlap I/O and computation, allowing for a more efficient utilization of system resources.

3 5. Recall the implementation of the TAS lock. Recall also the experiment on a shared-bus multiprocessor where N threads increment a shared counter protected by a TAS lock, each thread performing 1,000,000/N increments. Recall, finally, that the performance of this test degraded as the number of threads increased. (Note that N was less than or equal to the number of processors.) How would you explain degradation in performance? (30 points) TAS lock does a continuous read-and-write of a shared memory location. On a multiprocessor system, this causes lots of invalidations. Invalidations delay all activities on the shared bus and thus delay acquisition and release of locks. That is why TAS lock operations become less efficient as the number of threads increases. 30 points for a full correct answer. 15 points for a partial answer.

4 6. Describe the implementation of Anderson s Queue lock. How did it address the performance problem of the TAS lock? (30 points) Each thread was doing read-only spinning on its own memory location. This prevented continuous invalidations characteristic of a TAS lock. The thread releasing the lock notified only one thread, the one at the top of the queue. This prevented a storm of invalidations when the lock was released. 30 points for a full correct answer. 15 points for a partial answer. 7. Name three ways in which inter-process communication may happen on Unix systems. (15 points) Pipes Shared memory Memory-mapped files (5 points for each correct IPC method)

5 8. Some scientists became frustrated with (MP) or multithreaded (MT) server architectures because of poor performance and scalability. In response to this problem, alternative server architectures were proposed. Those architectures relied less heavily on threads. Name at least one such architecture and briefly explain how it was designed (one-two sentences). (20 points) Valid answers are: SEDA. Staged event-driven architecture. If they forget the name, give full credit if they give the right explanation: an architecture consisting of multiple stages. Each stage is responsible for a particular task. There is a queue for each stage. Each stage can be handled by one or more threads. AMPED. Asymmetric Multiprocess Event Driven. If they forget the name, give full credit if they give the right explanation: An event-driven architecture that performs asynchronous I/O operations when possible and spawns additional processes to handle operations that are likely to block. SPED. Single-process event driven. If they forget the name, give full credit if they give the right explanation: All I/O is done asynchronously. A single process handles requests from all clients.

6 9. Give two reasons why providing complete transparency in RPC is difficult. (20 points) 10 points for each correct reason. Valid answers include: A programmer may want to have the procedure invoked at a specific server. In this case server name must be passed to the function call Name binding happens at runtime as opposed to link/load time Client and server can fail independently RPC generates new types of errors that a local call would not generate Global variables and context sensitive variables (such as file descriptors) cannot be used in a remote procedure Pointer pickling, required for proper sending of arguments to the server, is difficult in a weakly typed language such as C Argument aliasing and use of IN/OUT parameters may cause RPC to return incorrect results.

7 10. Sketch a request-response protocol for asynchronous system with exactly-once semantics. The protocol must be designed to work in presence of failures. (20 points) Such a protocol cannot be constructed for an asynchronous system.

8 11. Recall that design of a distributed file system is usually driven by patterns in which clients use the file system. Therefore, extensive studies have been done to analyze file system usage patterns. Those patterns later drove designs of systems such as AFS and NFS. Describe three usage patterns that we discussed in class and explain how those patterns drove the design for AFS or NFS. (30 points) 5 points for each valid usage pattern. Valid responses include: Most files are small Read operations are much more frequent than write operations Most accesses are sequential, random access is rare Files are usually read in their entirety Recently written data tends to be overwritten Most files are read and written by one user When users share a file, typically only one user modifies the file Fine-grained read/write sharing is rare (in research/academic environments) File references show substantial temporal locality Explanation of how those patterns drove performance: 5 points for each correctly explained pattern. Examples are: Most files are small NFS used block-level granularity Read operations are much more frequent than write operations NFS and AFS chose weak consistency and delayed propagation Most accesses are sequential, random access is rare NFS chose to do pre-fetching Files are usually read in their entirety AFS chose to do whole-file caching Recently written data tends to be overwritten Justifies client caching and delayed propagation Most files are read and written by one user weak consistency is acceptable, ditto for delayed propagation When users share a file, typically only one user modifies the file weak consistency Fine-grained read/write sharing is rare (in research/academic environments) weak consistency File references show substantial temporal locality caching