CSE 221: Homework 3

Winter 2017

Hardcopy due Thursday, March 16 at the start of class

Answer the following questions. For questions asking for short answers, there may not necessarily be a "right" answer, although some answers may be more compelling and/or much easier to justify. But I am interested in your explanation (the "why") as much as the answer itself. Also, do not use shorthand: write your answers using complete sentences.

When grading homeworks, we will grade one question in detail and assign full credit for technical answers to the others.

1. The Scheduler Activations paper states that deadlock is potentially an issue when activations perform an upcall:

   "One issue we have not yet addressed is that a user-level thread could be executing in a critical section at the instant when it is blocked or preempted...[a] possible ill effect ... [is] deadlock (e.g., the preempted thread could be holding a lock on the user-level thread ready list; if so, deadlock would occur if the upcall attempted to place the preempted thread onto the ready list)." (p. 102)

   Why is this not a concern with standard kernel threads, i.e., why do scheduler activations have to worry about this deadlock issue, but standard kernel threads implementations do not have to?

2. Both the BN-RPC and IX systems include a variety of optimizations to improve communication performance. For each of the optimizations from BN-RPC below:
   • State whether IX also optimized this aspect of the system.
   • If IX did not optimize it, explain why.
   • If IX did optimize it, compare and contrast IX's optimization relative to BN-RPC. Also, if the papers report the performance benefits of the optimization, quote the performance results reported.
   1. Frequent vs. infrequent requests
   2. Connection management
   3. Process/thread management
   4. Communication protocol

3. The FFS, LFS, and Soft Updates file systems introduced new designs and optimizations to improve upon a previous file system implementation. Consider the following three changes in underlying workload and storage technology. For each of the three file systems, explain whether the improvements they found with their design and optimizations would still hold under each of these changes. For instance, would FFS still see similar improvements relative to the old Unix file system under a read-dominated workload?
   1. Read-dominated workload (100x reads per write)
   2. Latency improves by 10x, bandwidth improves by 10x ("SSD")
   3. Latency degrades by 10x, bandwidth degrades by 10x ("Internet Cloud storage")