Quiz 2

Question 2

A program X has a constant sequential fraction value regardless of problem size. Amdahl’s and Gustafson’s Laws predict different maximum speedups.

• False
• CONSTANT SEQUENTIAL FRACTION value
• Gustafson’s assumption: sequential fraction becomes smaller with larger problem size.

Question 4

Given a program with 95% arithmeticinstructions and 5% memory instructions, Amdahl’s Law instructs programmer to optimize arithmetic calculation before looking into memory accessses

• FALSE
• 95% of the computation is arithmetic BUT Sequential fraction is about the time taken by a part of a program
• Amdahl’s law tells us to optimize the bottle neck but this question doesn’t tell us what the bottle neck is.

Question 6

Data parallelism: Within the main simulation there are no parallel heterogenous tasks. Ad d pragma omp above for loop in line 18

Parallel patterns:

• Task pool: not good, Task pool does not have coordination and usually handles heterogenous tasks
• Master worker: homogenous work, and have a coordinator.

Quiz 3

Question 1 + 2

• False sharing
• Different data members of a struct are laid out contiguously, and they are writing to the same entity
• Overhead from updating the same cache line
• When thread A writes to position, thread B’s cache will be invalidated and have to be updated before the state can be updated.

Assumptions:

• Write allocate cache being used
  • A cache with a write-through policy (and write-allocate) reads an entire block (cacheline) from memory on a cache miss and writes only the updated item to memory for a store. Evictions do not need to write to memory.

Question 3

Relaxed consistency models address the concern that software complexity is increased by the need for unnecessary sychronization.

• False
• In fact, dealing with relaxed consistency requires more attention to synchronization
• It’s a way to improve performance rather than complexity
• Software complexity: How difficult it is to write the software to run.

Question 4 + 6

Cache coherence is the property that multiple caches contain the same set of items.

• False
• Caches don’t have the contain the same set of itmes