The answer is enabling shuffle compression by setting spark.shuffle.compress to true, as this directly reduces memory pressure without increasing cost. Compression shrinks the volume of intermediate shuffle data written to disk and transferred across the network, which in turn lowers the memory footprint required for storing and processing that data on Amazon EMR. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this question tests your understanding of Spark memory optimization on EMR, specifically how to mitigate out-of-memory errors in data transformation jobs without scaling up instance types. A common trap is choosing Kryo serialization, which reduces object size but is far less effective for shuffle-heavy workloads than compression. Remember the mnemonic: “Shrink the shuffle, save the memory” — compression is your cheapest and most direct lever for reducing memory usage on moderate-memory EMR clusters.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A data engineering team uses Amazon EMR with Spark to transform large datasets in S3. The team notices that the Spark jobs on the EMR cluster are failing with out-of-memory errors. The cluster uses instance types with moderate memory. Which configuration change would MOST effectively reduce memory pressure without increasing cost?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable shuffle compression and set spark.shuffle.compress to true.
Option C is correct because enabling compression reduces the amount of data shuffled over the network and stored in memory, thus reducing memory usage. Option A is wrong because increasing the number of executor cores may increase parallelism but does not directly reduce memory per task. Option B is wrong because using more instances would increase cost. Option D is wrong because Kryo serialization reduces object size but is not as effective as compression for shuffle data.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use a larger EMR cluster with more instances of the same type.
Why it's wrong here
Adding instances increases cost.
✗
Increase the number of executor cores to improve parallelism.
Why it's wrong here
More cores can increase concurrency but may increase memory usage per executor.
✗
Switch from Java serialization to Kryo serialization.
Why it's wrong here
Kryo reduces object serialization size but does not directly reduce shuffle data volume as much as compression.
✓
Enable shuffle compression and set spark.shuffle.compress to true.
Why this is correct
Compression reduces the amount of data stored in memory during shuffles.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Data Engineering — This question tests Data Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable shuffle compression and set spark.shuffle.compress to true. — Option C is correct because enabling compression reduces the amount of data shuffled over the network and stored in memory, thus reducing memory usage. Option A is wrong because increasing the number of executor cores may increase parallelism but does not directly reduce memory per task. Option B is wrong because using more instances would increase cost. Option D is wrong because Kryo serialization reduces object size but is not as effective as compression for shuffle data.
What should I do if I get this MLS-C01 question wrong?
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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