How to Speed Up Spark Jobs on Small Test Datasets

Dealing with small datasets (under a million records) can be a peculiar challenge when you've chosen Apache Spark as your go-to tool. Apache Spark is known for its strength in handling massive datasets through distributed computing. However, using it for smaller datasets may not always be the most efficient choice. This is especially true for writing tests, and I've noticed that people frequently overlook some key optimizations — after all, who knows your work better than you?

In this blog post, we'll explore various optimization techniques to fine-tune Apache Spark for small datasets and discuss when it might be worthwhile to consider alternative tools.

By default, Apache Spark allocates multiple executors per Spark session, which can be inefficient for small datasets. To address this, configure Spark to use only one local executor by specifying local[1] when creating a session:

.master("local[1]")

This ensures that your computational resources are optimally utilized.

The spark.sql.shuffle.partitions configuration controls the number of partitions used during shuffle operations. Shuffling redistributes data across partitions and is computationally expensive. For large datasets, the default value of 200 makes sense, but for smaller datasets, it's excessive.

Consider reducing spark.sql.shuffle.partitions to a smaller value, such as 20 or even 1, depending on your use case:

.config("spark.sql.shuffle.partitions", 1)

This modification prevents Spark from spending unnecessary time on shuffling operations and generating excessive tasks, resulting in much faster processing.

Think of your machine's CPU cores as eager workers waiting to tackle tasks. The spark.default.parallelism parameter determines how many tasks Spark can run concurrently. To make the most of your available cores, set it equal to the number of available cores in the processing environment:

.config("spark.default.parallelism", <number_of_cores>)

This configuration ensures that each core is efficiently utilized, leading to a significant speedup in processing.

Compression can be resource-intensive, and for small datasets, it may not provide significant benefits while consuming CPU cycles. Disabling compression can save computational resources and improve processing speed.

You can disable compression for both RDDs and shuffle operations using the following configurations:

.config("spark.rdd.compress", "false")
.config("spark.shuffle.compress", "false")

By setting these values to "false", you instruct Spark to skip unnecessary data compression.

Dynamic allocation is a feature in Spark that adjusts executor resources based on the workload. While beneficial for larger datasets and clusters, it may not be advantageous for smaller datasets.

You can disable dynamic allocation with the following configuration:

.config("spark.dynamicAllocation.enabled", "false")

This ensures that the number of allocated resources remains fixed and does not fluctuate based on workload.

In a smaller dataset scenario, fine-grained control over executor resources can be beneficial. You can explicitly set the number of CPU cores and executor instances using the following configurations:

.config("spark.executor.cores", 1)
.config("spark.executor.instances", 1)

Setting both values to 1 ensures that Spark efficiently utilizes available resources without overcommitting them. This is especially valuable when working with a single machine and small datasets.

For testing scenarios, you can further enhance efficiency by disabling Spark's user interface:

.config("spark.ui.enabled", "false")
.config("spark.ui.showConsoleProgress", "false")

This step streamlines your testing process, ensuring that resources are allocated solely to your computations.

While the above optimizations can work for certain scenarios, it's essential to understand the trade-offs involved.

For example, when you reduce the number of partitions, Spark may resort to more in-memory shuffling. This can create memory pressure, and in some cases, slow down the processing speed instead of speeding it up. As always, performance tuning is a balancing act, and it's crucial to monitor the impact of these changes on your specific application.

It's essential to ask yourself whether Spark is genuinely the best tool for handling small datasets. Spark is well-known for its prowess with massive datasets, leveraging distributed computing power to its advantage. However, for smaller datasets, the overhead of Apache Spark's infrastructure might overshadow its benefits.

Consider exploring alternative solutions, such as Pandas, Polars, Dask, or DuckDB (which also has Spark compatible API), which are more lightweight and tailored for working with smaller datasets on a single machine. These tools might offer more streamlined and efficient performance for your specific use case.

Remember, there's no one-size-fits-all solution. The choice of the right tool depends on the nature and scale of your data processing needs.

Additional materials

• High Performance Spark: Best Practices for Scaling and Optimizing Apache Spark by Holden Karau, Rachel Warren
• Spark: The Definitive Guide
• Learning Spark: Lightning-Fast Data Analytics