Evaluating Parallel Programs in Computing
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What is the primary factor that affects communication time in parallel programming?

  • Network structure and network contention (correct)
  • Startup time only
  • Network contention alone
  • Transmission time to send one data word

    • What is the formula to calculate the communication time (tcomm) in parallel programming?

  • tcomm = tstartup + ntdata + tcomp
  • tcomm = tstartup * ntdata
  • tcomm = tstartup / ntdata
  • tcomm = tstartup + ntdata (correct)

    • What is the unit of measurement for startup time and data transmission time in parallel programming?

  • Bytes
  • Computational steps (correct)
  • Seconds
  • Milliseconds

    • What is the purpose of adding tcomp and tcomm together in parallel programming?

    <p>To calculate the parallel execution time</p> Signup and view all the answers

    Why is it sufficient to consider only one process when calculating the final communication time?

    <p>Because all processes have the same communication pattern</p> Signup and view all the answers

    What is the assumption made about routing when calculating communication time?

    <p>Routing is ignored</p> Signup and view all the answers

    What is the term for the time it takes to send a message with no data?

    <p>Startup time</p> Signup and view all the answers

    What is the term for the time it takes to send one data word?

    <p>Transmission time</p> Signup and view all the answers

    What is the primary purpose of a space-time diagram in parallel programming?

    <p>To debug and evaluate the parallel programs empirically</p> Signup and view all the answers

    Which of the following is a characteristic of visualization tools in parallel programming?

    <p>They imply software probes into the execution</p> Signup and view all the answers

    What is the purpose of the MPI_Wtime() routine in parallel programming?

    <p>To return the time in seconds</p> Signup and view all the answers

    How can the execution time between two points in the code be measured in parallel programming?

    <p>By using a construction such as L1: time(&amp;t1);..L2: time(&amp;t2);</p> Signup and view all the answers

    What is the primary advantage of using hardware performance monitors in parallel programming?

    <p>They do not affect the performance of the program</p> Signup and view all the answers

    Which of the following is a type of visualization tool available for MPI implementations?

    <p>Upshot program visualization system</p> Signup and view all the answers

    What is the primary purpose of processor synchronization in parallel execution?

    <p>To ensure that processors execute in a coordinated manner</p> Signup and view all the answers

    What is a common challenge in parallel programming?

    <p>Managing communication overhead between processes</p> Signup and view all the answers

    What is a key drawback of using Amdahl's law and empirical observations in evaluating parallel programs?

    <p>They are unable to explain available observations and predict future circumstances</p> Signup and view all the answers

    What is the main reason why conventional modeling is not practical for parallel programmers?

    <p>It is too detailed and requires significant computational resources</p> Signup and view all the answers

    What is the primary component of parallel execution time, tp, in addition to the number of computational steps, tcomp?

    <p>Communication overhead</p> Signup and view all the answers

    What is the purpose of counting the number of computational steps in estimating the computational time, tcomp?

    <p>To identify the most complex process</p> Signup and view all the answers

    What is a common assumption made in analyzing the computation time, tcomp?

    <p>All processors are the same and operate at the same speed</p> Signup and view all the answers

    What is the main benefit of using a performance modeling technique with intermediate-level detail?

    <p>It captures the complexity of the system without being too detailed</p> Signup and view all the answers

    What is the relationship between the sequential execution time, ts, and the number of computational steps?

    <p>ts is estimated by counting the number of computational steps</p> Signup and view all the answers

    What is the purpose of breaking down the computation time, tcomp, into parts?

    <p>To analyze the computation time in detail</p> Signup and view all the answers

    What can be established using ts, tcomp, and tcomm?

    <p>Speedup factor and computation/communication ratio</p> Signup and view all the answers

    What is the primary challenge in getting a parallel program to work properly?

    <p>Addressing the intellectual challenge of parallel programming</p> Signup and view all the answers

    What is the effect of instrumentation code on parallel programs?

    <p>It can cause instructions to be executed in a different order</p> Signup and view all the answers

    What is a limitation of traditional debugging tools in parallel programming?

    <p>They are of little use due to varying orders of execution</p> Signup and view all the answers

    What is the purpose of the computation/communication ratio?

    <p>To highlight the effect of communication on parallel program execution</p> Signup and view all the answers

    What is the relationship between the number of processors, p, and the speedup factor?

    <p>The speedup factor is directly proportional to p</p> Signup and view all the answers

    What is the primary challenge in evaluating parallel programs empirically?

    <p>Addressing the varying orders of execution possible</p> Signup and view all the answers

    What is the purpose of establishing the speedup factor and computation/communication ratio?

    <p>To evaluate the scalability of parallel solutions</p> Signup and view all the answers

    What is the primary characteristic of asynchronous message passing routines?

    <p>They require local storage for messages.</p> Signup and view all the answers

    What is the main difference between blocking and non-blocking operations in MPI?

    <p>Blocking operations return after local actions complete, while non-blocking operations return immediately.</p> Signup and view all the answers

    What is the purpose of using asynchronous message passing routines in parallel programming?

    <p>To reduce communication overhead and allow processes to move forward sooner.</p> Signup and view all the answers

    What is the implication of using non-blocking operations in MPI?

    <p>The programmer must ensure that data storage is not modified before transfer.</p> Signup and view all the answers

    What is the primary benefit of using asynchronous message passing routines in parallel programming?

    <p>Reduced parallel execution time.</p> Signup and view all the answers

    What is the main consideration when using asynchronous message passing routines?

    <p>They must be used with care to avoid data inconsistencies.</p> Signup and view all the answers

    What is the difference between asynchronous message passing routines and other message passing routines?

    <p>Asynchronous message passing routines return immediately, while other message passing routines wait.</p> Signup and view all the answers

    What is the implication of using blocking operations in MPI?

    <p>The operation returns after the local actions complete, but the message transfer may not have been completed.</p> Signup and view all the answers

    Study Notes

    Evaluating Parallel Programs

    • A good performance model should explain available observations and predict future circumstances.
    • Amdahl's law, empirical observations, and asymptotic analysis do not satisfy these requirements.
    • Conventional modeling is not practical for parallel programmers due to detailed simulations of hardware components.

    Performance Modeling

    • Sequential execution time (ts) is estimated by counting the computational steps of the best sequential algorithm.
    • Parallel execution time (tp) is the sum of computational time (tcomp) and communication overhead (tcomm): tp = tcomp + tcomm.

    Computational Time

    • Computational time (tcomp) is counted by the number of computational steps.
    • When multiple processes are executed simultaneously, the computational steps of the most complex process are counted.
    • tcomp is generally a function of n and p, i.e., tcomp = f(n, p).
    • Computational time can be broken down into parts: tcomp = tcomp1 + tcomp2 + tcomp3 + …

    Debugging and Evaluating Parallel Programs

    • Visualization tools can be used to debug and evaluate parallel programs by watching them execute in a space-time diagram.
    • Implementations of visualization tools are available for MPI, such as the Upshot program visualization system.
    • Hardware performance monitors (which do not usually affect performance) are also available.

    Evaluating Programs Empirically

    • Measuring execution time can be done by using the MPI_Wtime() routine or by using the time() function to calculate the elapsed time between two points in the code.

    Communication Time

    • Communication time (tcomm) is the sum of startup time (tstartup) and transmission time (tdata): tcomm = tstartup + ntdata.
    • tstartup is the time to send a message with no data, and tdata is the transmission time to send one data word.

    Benchmark Factors

    • The speedup factor and computation/communication ratio can be established using ts, tcomp, and tcomm.
    • These ratios are functions of the number of processors (p) and the number of data elements (n).

    Low-level Debugging

    • Debugging parallel programs can be challenging due to the varying orders of execution possible.
    • Instrumentation code can slow down a parallel program and cause nonworking programs to work.

    Asynchronous Message Passing

    • Asynchronous message passing routines do not wait for actions to complete before returning.
    • They usually require local storage for messages and must be used with care.

    MPI Definitions of Blocking and Non-Blocking

    • Blocking routines return after local actions are complete, though the message transfer may not have been completed.
    • Non-blocking routines return immediately, assuming that data storage used for transfer is not modified by subsequent statements prior to being used for transfer.

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    Description

    This quiz assesses understanding of evaluating parallel programs, including limitations of Amdahl's law, empirical observations, and asymptotic analysis in modeling parallelism. It covers the importance of a good performance model in explaining observations and predicting future circumstances.

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