PCDE Optimising Service Performance — 25 Questions

Question 1

Your GKE cluster is running a critical web application that experiences predictable traffic spikes during business hours. You want to minimize latency and avoid pod startup delays during scaling. The application uses CPU-intensive image processing. Which scaling strategy should you use?

Accepted Answer

Configure HPA with a minimum of 2 replicas and scale on CPU utilization; enable cluster autoscaler for node provisioning.. To avoid cold starts while ensuring pods can handle CPU spikes, you need a baseline of pods and dynamic scaling responsive to CPU. HPA with a minimum replicas of 2 ensures baseline capacity; HPA scales on CPU. Cluster autoscaler adds nodes if needed, but does not directly address pod startup delay. VPA adjusts resource requests, which can help but does not prevent cold starts. Using HPA alone with min replicas avoids pod creation latency.

Answer

Set a high number of static pods equal to peak traffic; use cluster autoscaler to add nodes.

Answer

Use VPA with updateMode: Auto to automatically adjust pod resources; enable cluster autoscaler to add nodes as required.

Answer

Deploy a CronJob to scale up replicas before business hours; rely on HPA to handle the rest.

Question 2

You are running a Cloud Run service that performs background image processing tasks after responding to HTTP requests. Users report that tasks are sometimes not completed. You have set CPU to always-on, but you notice the service's concurrency is set to the default of 80. What is the most likely cause of incomplete tasks?

Accepted Answer

The concurrency setting of 80 allows too many requests to share the instance's CPU, starving background tasks.. Cloud Run allows up to 80 concurrent requests per instance by default. If the concurrency is too high, the instance's CPU may be overwhelmed, causing background tasks to be preempted or not completed. Setting CPU to always-on ensures CPU is available but does not limit concurrency. Reducing concurrency to a lower value (e.g., 1) ensures each request gets dedicated CPU for background work.

Answer

The service should use Gen2 execution environment for better background processing.

Answer

The CPU always-on setting is not sufficient; you need to enable background CPU.

Answer

You need to increase the request timeout beyond 60 minutes.

Question 3

Your Cloud Spanner instance is experiencing high write latency and hot spots on a table that uses an auto-incrementing integer as the primary key. Which change would best mitigate the hot spots?

Accepted Answer

Change the primary key to a random UUID.. Monotonically increasing keys (like auto-incrementing integers) cause all writes to go to the same tablet (hot spot) in Cloud Spanner. Using a key with uniform distribution, such as a random UUID or a hash prefix, spreads writes across splits. Bit-reverse index is an alternative for sequential keys, but a random key is simpler. Interleaved tables and secondary indexes do not address the root cause.

Answer

Add a secondary index on the auto-incrementing column.

Answer

Increase the number of splits by raising the number of nodes.

Answer

Use interleaved tables to store related data together.

Question 4

Your organization uses Cloud SQL for PostgreSQL for a reporting application with read-heavy workloads. Queries are slow and you need to reduce load on the primary instance. You also need to ensure that all read queries from the reporting tool are isolated from the primary. What should you do?

Accepted Answer

Create a Cloud SQL read replica, and configure the reporting tool to connect to the replica's IP address.. Cloud SQL read replicas serve read traffic and offload the primary. For isolation, the reporting tool must connect to the replica's IP. Cloud SQL Auth Proxy is a secure tunnel, but does not provide connection pooling. PgBouncer can be used with proxies for connection pooling, but the question asks for isolation, which is achieved by pointing the reporting tool to the replica. The best answer is to create a read replica and configure the reporting tool to connect to it.

Answer

Create a Cloud SQL clone and point the reporting tool to the clone.

Answer

Enable connection pooling using Cloud SQL Auth Proxy and PgBouncer on the primary instance.

Answer

Configure the reporting tool to use the primary instance with a lower priority.

Question 5

You are deploying a batch job on Compute Engine that processes large datasets and can tolerate interruptions. The job runs for about 6 hours every night. Which option would minimize cost while ensuring the job completes within the 6-hour window?

Accepted Answer

Use preemptible VMs to get up to 80% discount, with checkpointing to resume if preempted.. Preemptible VMs are significantly cheaper (up to 80% discount) but can be terminated at any time. For fault-tolerant batch jobs, preemptible VMs are ideal; if preempted, the job can resume on another preemptible VM. Committed use discounts provide cost savings for steady-state usage (1- or 3-year commitment), but are not suitable for a nightly 6-hour job. Custom machine types or standard VMs would be more expensive.

Answer

Use standard VMs with committed use discounts for 1 year.

Answer

Use custom machine types to optimize cost.

Answer

Use sole-tenant nodes for dedicated hardware.

Question 6

Your Cloud Run service processes requests from an external API that sends a burst of up to 100 requests per second. You want to maximize throughput while minimizing instances. The service is CPU-bound. What configuration should you use?

Accepted Answer

Set concurrency to 1 to give each request full CPU, and enable CPU always-on.. For CPU-bound services, the default concurrency of 80 may be too high, causing resource contention and slowing down each request. Reducing concurrency to a lower value, such as 1, dedicates the entire CPU to each request, potentially improving throughput per instance. CPU always-on is needed for background tasks but not necessarily for request handling. Gen2 execution environment can help with higher memory but not CPU-bound throughput directly.

Answer

Use the Gen2 execution environment with higher memory.

Answer

Set concurrency to 1000 to handle burst efficiently.

Answer

Set concurrency to 80 (default) and CPU always-on.

Question 7

Your GKE cluster runs a stateful application that requires persistent storage. You want to use Vertical Pod Autoscaler (VPA) to optimize resource requests, but you notice that pods are being terminated and recreated when VPA updates resource recommendations. You want to avoid pod restarts. Which VPA updateMode should you use?

Accepted Answer

updateMode: Initial. VPA updateMode: Auto will evict and recreate pods to apply new resource limits. To avoid restarts, you can use Initial (applies only to new pods) or Off (no changes). The question asks to avoid pod restarts, so Initial is appropriate if you can accept that existing pods keep their old limits. Recreate is the same as Auto. Off disables VPA.

Answer

updateMode: Recreate

Answer

updateMode: Off

Answer

updateMode: Auto

Question 8

Your Pub/Sub subscription is not keeping up with the message publishing rate. The subscriber is a Cloud Run service that processes each message in about 2 seconds. You have already increased the number of subscribers to 10. What is the next best step to increase throughput?

Accepted Answer

Increase the acknowledgement deadline to 30 seconds.. If increasing subscribers does not help, the issue may be the acknowledgement deadline. If the deadline is too short, messages are redelivered before processing completes, causing duplicates and wasted work. Increasing the deadline (from default 10 seconds to something higher like 30 seconds) gives time to process. Flow control limits throughput. Ordering keys can reduce throughput.

Answer

Enable message ordering to ensure sequential processing.

Answer

Switch to a pull subscription instead of push.

Answer

Reduce the flow control max outstanding messages to 100.

Question 9

You want to ensure that a critical deployment on GKE has minimal downtime during rolling updates. You also want to ensure that at least 2 pods are always available. Which Kubernetes resource should you configure?

Accepted Answer

PodDisruptionBudget with minAvailable: 2. PodDisruptionBudget (PDB) specifies the minimum number or percentage of pods that must be available during voluntary disruptions like rolling updates. Setting minAvailable: 2 ensures at least 2 pods are running during updates. HPA, VPA, and cluster autoscaler do not control pod availability during updates.

Answer

Cluster autoscaler with minNodes: 2

Answer

HorizontalPodAutoscaler with minReplicas: 2

Answer

VerticalPodAutoscaler with updateMode: Auto

Question 10

Your Cloud Run service experiences cold starts that add 5 seconds of latency to user requests. You want to eliminate cold starts for a baseline traffic level of 10 requests per second. The service uses about 2 GiB of memory. What should you do?

Accepted Answer

Set minInstances to 2 to keep instances always warm.. Cloud Run cold starts can be eliminated by setting a minimum number of instances that are always warm. The minInstances flag ensures that the specified number of instances are always running and ready to serve requests. For a baseline of 10 RPS, you need to estimate how many instances are needed; but the question asks to eliminate cold starts, so setting minInstances to at least 1 (or more) will keep instances warm. Setting CPU always-on does not eliminate cold starts. Gen2 execution environment may reduce cold start time but not eliminate them.

Answer

Use the Gen2 execution environment to reduce cold start time.

Answer

Increase the concurrency to 1 to minimize instance count.

Answer

Set CPU always-on to keep the instance active.

Question 11

Your Cloud SQL for MySQL instance is experiencing high CPU usage due to a burst of concurrent connections. You want to handle up to 500 concurrent connections without over-provisioning the instance. What should you do?

Accepted Answer

Right-size the instance to a tier that supports 500 concurrent connections.. Cloud SQL for MySQL has a maximum connections limit (based on tier). To handle 500 concurrent connections, you need to choose a tier with sufficient vCPU and memory. The max_connections is typically calculated as (available memory)/1257280 * 500, but the simplest approach is to use a tier that supports at least 500 connections (e.g., db-n1-standard-2 or higher). However, using Cloud SQL Auth Proxy with PgBouncer is for PostgreSQL, not MySQL. Connection pooling with ProxySQL or a similar tool could help, but the question says 'without over-provisioning', so right-sizing the tier is key.

Answer

Create read replicas to distribute read traffic.

Answer

Enable automatic storage increase to handle the load.

Answer

Use Cloud SQL Auth Proxy with a connection pooler like PgBouncer.

Question 12

Your Cloud Spanner database has a table with a secondary index that is used for range queries. You notice that the index queries are slow because they require back-and-forth between the index and the base table. How can you optimize the index to reduce this overhead?

Accepted Answer

Use the STORING clause to include frequently queried columns in the index.. In Cloud Spanner, you can store additional columns from the base table in the secondary index using the STORING clause. This allows the index to satisfy queries without accessing the base table (covering index). INTERLEAVE IN PARENT for the index can also improve locality if the index is interleaved with a parent table. The question asks to reduce overhead of back-and-forth, which is solved by storing columns in the index.

Answer

Convert the table to use hash-prefixed keys.

Answer

Create a new interleaved table and move the index data there.

Answer

Use the spanner_interleave_in_parent option when creating the index.

Question 13

Your GKE cluster uses node auto-provisioning to automatically create node pools. However, you notice that the cluster autoscaler is removing nodes too aggressively, causing frequent pod evictions. Which TWO configuration changes can you make to reduce the frequency of scale-down events? (Choose two)

Accepted Answer

Increase the scale-down delay to 10 minutes.. The cluster autoscaler scale-down delay options control how long a node must be underutilized before it is removed. Increasing the scale-down delay (e.g., to 10 minutes) gives pods more time to stabilize. Additionally, setting a minimum number of nodes ensures that even if nodes are underutilized, they are not scaled down below a certain threshold, reducing evictions. Decreasing max nodes or enabling node auto-provisioning would not help.

Answer

Decrease the max node count to limit scaling.

Answer

Enable node auto-provisioning to automatically create new node pools.

Answer

Use PodDisruptionBudget to protect critical pods.

Question 14

Your Cloud SQL for PostgreSQL database is experiencing slow query performance. You want to identify and optimize the slowest queries. Which TWO actions should you take? (Choose two)

Accepted Answer

Enable the slow query log by setting the log_min_duration_statement flag.. To identify slow queries, enable the slow query log (via database flags) and use EXPLAIN ANALYZE to analyze query plans. Right-sizing the instance can help after identifying queries, but identifying them first requires logging and analysis. Connection pooling does not help identify slow queries.

Answer

Set up connection pooling using Cloud SQL Auth Proxy and PgBouncer.

Answer

Create read replicas to offload read traffic.

Answer

Increase the instance memory to the next tier.

Question 15

Your organization uses Cloud Spanner for a global application with strong consistency requirements. You need to design a table schema to avoid hot spots while supporting queries that join two related entities (e.g., Customers and Orders). Which THREE design choices should you implement? (Choose three)

Accepted Answer

Use hash prefixes or random UUIDs as the primary key to distribute writes.. To avoid hot spots and support efficient joins in Cloud Spanner, three design choices are recommended. First, use hash prefixes or random UUIDs as the primary key (B) to distribute writes evenly across nodes, preventing hot spots caused by monotonically increasing keys. Second, create secondary indexes with the INTERLEAVE IN PARENT option (D) to store index data alongside the parent table, reducing read latency for queries that filter by the indexed columns. Third, use interleaved tables (E) to store related rows (e.g., Customers and Orders) physically together, enabling efficient joins without cross-node lookups. These choices leverage Spanner's distributed architecture for scalability and strong consistency.

Answer

Use monotonically increasing keys (e.g., auto-increment) for easy ordering.

Answer

Disable strict consistency to allow faster writes.

Question 16

A company runs a GKE cluster for a web application. During peak traffic, the application experiences increased latency. The team has enabled the Horizontal Pod Autoscaler (HPA) based on CPU utilization, but the scaling is not fast enough. Which approach would improve the responsiveness of the HPA?

Accepted Answer

Decrease the HPA sync period from 15 seconds to 5 seconds.. Decreasing the HPA sync period makes the controller evaluate metrics more frequently, reducing the time to react to load changes. Increasing stabilization window delays scaling up, making it worse.

Answer

Decrease the CPU target utilization from 80% to 50%.

Answer

Increase the HPA sync period from 15 seconds to 60 seconds.

Answer

Increase the stabilization window from 5 minutes to 10 minutes.

Question 17

A company is using Cloud Run to deploy a service that processes background tasks. The service takes a few seconds to initialize, and users experience high latency on cold starts. How can the company eliminate cold starts for this service?

Accepted Answer

Set the minimum number of instances to a value based on the baseline traffic.. Setting a minimum number of instances ensures that at least that many instances are always running and ready to serve requests, eliminating cold starts.

Answer

Set the maximum number of instances to a higher value.

Answer

Use the gen1 execution environment.

Answer

Set the concurrency to 1.

Question 18

A company uses Cloud Spanner for a global application. They notice high write latency and occasional hotspotting on a table with a monotonically increasing integer primary key. Which schema design change would best prevent hotspotting while maintaining read performance?

Accepted Answer

Use bit-reversed indexes for the primary key.. Using a hash prefix of the primary key distributes writes across different splits, preventing hotspots. UUIDs also work, but bit-reverse is better for sequential keys. Interleaved tables do not help with hotspotting.

Answer

Use a UUID as the primary key.

Answer

Create an interleaved table with the primary key as the parent.

Answer

Add a secondary index on the primary key.

Question 19

A company is running a Cloud SQL for PostgreSQL database and wants to improve read performance for reporting queries. They have already optimized the queries but still see high CPU usage on the primary instance. What is the most cost-effective solution?

Accepted Answer

Add a read replica and direct reporting traffic to it.. Read replicas offload read traffic from the primary instance, reducing CPU usage without requiring a larger primary instance.

Answer

Enable connection pooling with PgBouncer on the primary.

Answer

Increase the memory of the primary instance.

Answer

Create a Cloud SQL Auth Proxy and use it for all connections.

Question 20

A company uses Pub/Sub to ingest events from multiple services. They notice a backlog of unacknowledged messages and want to increase the throughput of their subscriber. The subscriber is a single process running on Compute Engine. What is the most effective way to increase throughput?

Accepted Answer

Add more subscriber instances running in parallel.. Running multiple subscriber instances (parallel pull consumers) increases the overall throughput by allowing more messages to be pulled and processed concurrently.

Answer

Decrease the flow control max outstanding messages.

Answer

Increase the acknowledgement deadline to 10 minutes.

Answer

Enable ordering keys on the subscription.