40 Questions
What is the primary factor that affects communication time in parallel programming?
Network structure and network contention
What is the formula to calculate the communication time (tcomm) in parallel programming?
tcomm = tstartup + ntdata
What is the unit of measurement for startup time and data transmission time in parallel programming?
Computational steps
What is the purpose of adding tcomp and tcomm together in parallel programming?
To calculate the parallel execution time
Why is it sufficient to consider only one process when calculating the final communication time?
Because all processes have the same communication pattern
What is the assumption made about routing when calculating communication time?
Routing is ignored
What is the term for the time it takes to send a message with no data?
Startup time
What is the term for the time it takes to send one data word?
Transmission time
What is the primary purpose of a space-time diagram in parallel programming?
To debug and evaluate the parallel programs empirically
Which of the following is a characteristic of visualization tools in parallel programming?
They imply software probes into the execution
What is the purpose of the MPI_Wtime() routine in parallel programming?
To return the time in seconds
How can the execution time between two points in the code be measured in parallel programming?
By using a construction such as L1: time(&t1);..L2: time(&t2);
What is the primary advantage of using hardware performance monitors in parallel programming?
They do not affect the performance of the program
Which of the following is a type of visualization tool available for MPI implementations?
Upshot program visualization system
What is the primary purpose of processor synchronization in parallel execution?
To ensure that processors execute in a coordinated manner
What is a common challenge in parallel programming?
Managing communication overhead between processes
What is a key drawback of using Amdahl's law and empirical observations in evaluating parallel programs?
They are unable to explain available observations and predict future circumstances
What is the main reason why conventional modeling is not practical for parallel programmers?
It is too detailed and requires significant computational resources
What is the primary component of parallel execution time, tp, in addition to the number of computational steps, tcomp?
Communication overhead
What is the purpose of counting the number of computational steps in estimating the computational time, tcomp?
To identify the most complex process
What is a common assumption made in analyzing the computation time, tcomp?
All processors are the same and operate at the same speed
What is the main benefit of using a performance modeling technique with intermediate-level detail?
It captures the complexity of the system without being too detailed
What is the relationship between the sequential execution time, ts, and the number of computational steps?
ts is estimated by counting the number of computational steps
What is the purpose of breaking down the computation time, tcomp, into parts?
To analyze the computation time in detail
What can be established using ts, tcomp, and tcomm?
Speedup factor and computation/communication ratio
What is the primary challenge in getting a parallel program to work properly?
Addressing the intellectual challenge of parallel programming
What is the effect of instrumentation code on parallel programs?
It can cause instructions to be executed in a different order
What is a limitation of traditional debugging tools in parallel programming?
They are of little use due to varying orders of execution
What is the purpose of the computation/communication ratio?
To highlight the effect of communication on parallel program execution
What is the relationship between the number of processors, p, and the speedup factor?
The speedup factor is directly proportional to p
What is the primary challenge in evaluating parallel programs empirically?
Addressing the varying orders of execution possible
What is the purpose of establishing the speedup factor and computation/communication ratio?
To evaluate the scalability of parallel solutions
What is the primary characteristic of asynchronous message passing routines?
They require local storage for messages.
What is the main difference between blocking and non-blocking operations in MPI?
Blocking operations return after local actions complete, while non-blocking operations return immediately.
What is the purpose of using asynchronous message passing routines in parallel programming?
To reduce communication overhead and allow processes to move forward sooner.
What is the implication of using non-blocking operations in MPI?
The programmer must ensure that data storage is not modified before transfer.
What is the primary benefit of using asynchronous message passing routines in parallel programming?
Reduced parallel execution time.
What is the main consideration when using asynchronous message passing routines?
They must be used with care to avoid data inconsistencies.
What is the difference between asynchronous message passing routines and other message passing routines?
Asynchronous message passing routines return immediately, while other message passing routines wait.
What is the implication of using blocking operations in MPI?
The operation returns after the local actions complete, but the message transfer may not have been completed.
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.
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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