Exams / SAA-C03 / Choosing compute / Question 3
SAA-C03 practice question: Choosing compute (3 of 3)
Design High-Performing Architectures A free question from our original SAA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.
An engineering firm runs a fluid dynamics simulation on 16 identical compute-optimised EC2 instances. Every few milliseconds, each instance exchanges results with all the others, so job run time is dominated by network latency between the nodes. The jobs write a checkpoint every 10 minutes, so if an Availability Zone fails the firm is happy to restart a job elsewhere. Which launch configuration will give the SHORTEST job run time?
Choose one answer.
- AA. Launch the 16 instances into a spread placement group that spans three Availability Zones in the Region.
- BB. Launch all 16 instances in a single request into a cluster placement group in one Availability Zone.
- CC. Launch the 16 instances into a partition placement group with seven partitions in a single Availability Zone.
- DD. Launch the 16 instances across two Availability Zones in the same VPC, with enhanced networking turned on.
Show the answer and explanation
Answer: B
- A. Wrong: A spread placement group puts each instance on distinct hardware to reduce correlated failures, and with up to seven running instances per Availability Zone, 16 instances would fit across three zones. But spreading nodes across racks and zones raises the latency between them, which is the opposite of what this job needs. Learn more: Placement strategies for your placement groups (opens in a new tab)
- B. Right: Correct. A cluster placement group packs instances close together inside one Availability Zone for the low-latency, high-throughput networking that tightly coupled HPC jobs need. Launching them in one request with one instance type reduces the chance of an insufficient capacity error, and checkpoints cover the single-zone risk. Learn more: Placement strategies for your placement groups (opens in a new tab)
- C. Wrong: Partition placement groups keep groups of instances on separate racks so that a hardware failure affects only one partition, which suits HDFS, HBase or Cassandra. Placing nodes on different racks does not minimise node-to-node latency, so it would not give the shortest run time. Learn more: Placement strategies for your placement groups (opens in a new tab)
- D. Wrong: Enhanced networking helps every instance, but traffic between Availability Zones has higher latency than traffic within one zone, and without a cluster placement group instances are not placed close together. The firm has already accepted the single-zone risk, so it does not need to pay this latency cost. Learn more: Placement groups for your Amazon EC2 instances (opens in a new tab)
Explanation
The workload is tightly coupled and latency-bound, and the firm accepts restarting from a checkpoint if a zone fails, so there is no reason to trade latency for resilience. A cluster placement group in one Availability Zone gives the lowest latency between instances; spread and partition groups deliberately separate hardware, and spanning zones adds latency.
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