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RAPIDS Accelerator for Apache Spark Tuning Guide

Tuning a Spark job’s configuration settings from the defaults can often improve job performance, and this remains true for jobs leveraging the RAPIDS Accelerator plugin for Apache Spark. This document provides guidelines on how to tune a Spark job’s configuration settings for improved performance when using the RAPIDS Accelerator plugin.

Number of Executors

The RAPIDS Accelerator plugin only supports a one-to-one mapping between GPUs and executors.

Number of Tasks per Executor

Running multiple, concurrent tasks per executor is supported in the same manner as standard Apache Spark. For example, if the cluster nodes each have 24 CPU cores and 4 GPUs then setting spark.executor.cores=6 will run each executor with 6 cores and 6 concurrent tasks per executor, assuming the default setting of one core per task, i.e.: spark.task.cpus=1.

It is recommended to run more than one concurrent task per executor as this allows overlapping I/O and computation. For example one task can be communicating with a distributed filesystem to fetch an input buffer while another task is decoding an input buffer on the GPU. Configuring too many concurrent tasks on an executor can lead to excessive I/O and overload host memory. Counter-intuitively leaving some CPU cores idle may actually speed up your overall job. We typically find that two times the number of concurrent GPU tasks is a good starting point.

The number of concurrent tasks running on a GPU is configured separately.

Pooled Memory

Configuration key: spark.rapids.memory.gpu.pooling.enabled

Default value: true

Configuration key: spark.rapids.memory.gpu.allocFraction

Default value: 0.9

Allocating memory on a GPU can be an expensive operation. RAPIDS uses a pooling allocator called RMM to mitigate this overhead. By default, on startup the plugin will allocate 90% (0.9) of the memory on the GPU and keep it as a pool that can be allocated from. If the pool is exhausted more memory will be allocated and added to the pool. Most of the time this is a huge win, but if you need to share the GPU with other libraries that are not aware of RMM this can lead to memory issues, and you may need to disable pooling.

Pinned Memory

Configuration key: spark.rapids.memory.pinnedPool.size

Default value: 0

Pinned memory refers to memory pages that the OS will keep in system RAM and will not relocate or swap to disk. Using pinned memory significantly improves performance of data transfers between the GPU and host memory as the transfer can be performed asynchronously from the CPU. Pinned memory is relatively expensive to allocate and can cause system issues if too much memory is pinned, so by default no pinned memory is allocated.

It is recommended to use some amount of pinned memory when using the RAPIDS Accelerator if possible. Ideally the amount of pinned memory allocated would be sufficient to hold the input partitions for the number of concurrent tasks that Spark can schedule for the executor.

Note that the specified amount of pinned memory is allocated per executor. For example, if each node in the cluster has 4 GPUs and therefore 4 executors per node, a configuration setting of spark.rapids.memory.pinnedPool.size=4G will allocate a total of 16GB of memory on the system.

When running on YARN, make sure to account for the extra memory consumed by setting spark.executor.memoryOverhead to a value at least as large as the amount of pinned memory allocated by each executor. Note that pageable, off-heap host memory is used once the pinned memory pool is exhausted, and that would also need to be accounted for in the memory overhead setting.

Locality Wait

Configuration key: spark.locality.wait

Default value: 3s

This configuration setting controls how long Spark should wait to obtain better locality for tasks. When tasks complete quicker than this setting, the Spark scheduler can end up not leveraging all of the executors in the cluster during a stage. If you see stages in the job where it appears Spark is running tasks serially through a small subset of executors it is probably due to this setting. Some queries will see significant performance gains by setting this to 0.

Number of Concurrent Tasks per GPU

Configuration key: spark.rapids.sql.concurrentGpuTasks

Default value: 1

The number of concurrent tasks per executor can be further limited when tasks are sharing the GPU. This is useful for avoiding GPU out of memory errors while still allowing full concurrency for the portions of the job that are not executing on the GPU. Some queries benefit significantly from setting this to a value between 2 and 4, with 2 typically providing the most benefit, and higher numbers giving diminishing returns.

Setting this value to high can lead to GPU out of memory errors or poor runtime performance. Running multiple tasks concurrently on the GPU will reduce the memory available to each task as they will be sharing the GPU’s total memory. As a result, some queries that fail to run with a higher concurrent task setting may run successfully with a lower setting.

Shuffle Partitions

Configuration key: spark.sql.shuffle.partitions

Default value: 200

The number of partitions produced between Spark stages can have a significant performance impact on a job. Too few partitions and a task may run out of memory. Too many partitions and partition processing overhead dominates the task runtime.

Partitions have a higher incremental cost for GPU processing than CPU processing, so it is recommended to keep the number of partitions as low as possible without running out of memory in a task. This also has the benefit of increasing the amount of data each task will process which reduces the overhead costs of GPU processing. Note that setting the partition count too low could result in GPU out of memory errors. In that case the partition count will need to be increased.

Try setting the number of partitions to either the number of GPUs or the number of concurrent GPU tasks in the cluster. The number of concurrent GPU tasks is computed by multiplying the number of GPUs in the Spark cluster multiplied by the number of concurrent tasks allowed per GPU. This will set the number of partitions to matching the computational width of the Spark cluster which provides work for all GPUs.

Input Files

GPUs process data much more efficiently when they have a large amount of data to process in parallel. Loading data from fewer, large input files will perform better than loading data from many small input files. Ideally input files should be on the order of a few gigabytes rather than megabytes or smaller.

Note that the GPU can encode Parquet and ORC data much faster than the CPU, so the costs of writing large files can be significantly lower.

Input Partition Size

Similar to the discussion on input file size, many queries can benefit from using a larger input partition size than the default setting. This allows the GPU to process more data at once, amortizing overhead costs across a larger set of data. Many queries perform better when this is set to 256MB or 512MB. Note that setting this value too high can cause tasks to fail with GPU out of memory errors.

The configuration settings that control the input partition size depend upon the method used to read the input data.

Input Partition Size with DataSource API

Configuration key: spark.sql.files.maxPartitionBytes

Default value: 128MB

Using the SparkSession methods to read data (e.g.: spark.read.…) will go through the DataSource API.

Input Partition Size with Hive API

Configuration keys:

  • spark.hadoop.mapreduce.input.fileinputformat.split.minsize
  • spark.hadoop.mapred.min.split.size

Default value: 0

Columnar Batch Size

Configuration key: spark.rapids.sql.batchSizeBytes

Default value: 2147483648

The RAPIDS Accelerator plugin processes data on the GPU in a columnar format. Data is processed in a series of columnar batches, and during processing sometimes multiple batches are concatenated into a single batch to make the GPU processing more efficient. This setting controls the upper limit on the concatenation process. Setting this value too low can result in a large amount of GPU processing overhead and slower task execution. Setting this value too high can lead to GPU out of memory errors. If tasks fail due to GPU out of memory errors after the query input partitions have been read, try setting this to a lower value.

File Reader Batch Size

Configuration key: spark.rapids.sql.reader.batchSizeRows

Default value: 2147483648

Configuration key: spark.rapids.sql.reader.batchSizeBytes

Default value: 2147483648

When reading data from a file, this setting is used to control the maximum batch size separately from the main columnar batch size setting. Some transcoding jobs (e.g.: load CSV files then write Parquet files) need to lower this setting when using large task input partition sizes to avoid GPU out of memory errors.

Enable Incompatible Operations

Configuration key: spark.rapids.sql.incompatibleOps.enabled

Default value: false

There are several operators/expressions that are not 100% compatible with the CPU version. These incompatibilities are documented here and in the configuration documentation. Many of these incompatibilities are around corner cases that most queries do not encounter, or that would not result in any meaningful difference to the resulting output. By enabling these operations either individually or with the spark.rapids.sql.incompatibleOps.enabled config it can greatly improve performance of your queries. Over time, we expect the number of incompatible operators to reduce.

If you want to understand if an operation is or is not on the GPU and why see second on explain in the FAQ

The following configs all enable different types of incompatible operations that can improve performance.