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Frequently Asked Questions

What versions of Apache Spark does the RAPIDS Accelerator for Apache Spark support?

The RAPIDS Accelerator for Apache Spark requires version 3.0.1, 3.0.2, 3.1.1 or 3.1.2 of Apache Spark. Because the plugin replaces parts of the physical plan that Apache Spark considers to be internal the code for those plans can change even between bug fix releases. As a part of our process, we try to stay on top of these changes and release updates as quickly as possible.

Which distributions are supported?

The RAPIDS Accelerator for Apache Spark officially supports:

Most distributions based on a supported Apache Spark version should work, but because the plugin replaces parts of the physical plan that Apache Spark considers to be internal the code for those plans can change from one distribution to another. We are working with most cloud service providers to set up testing and validation on their distributions.

What CUDA versions are supported?

CUDA 11.0 and 11.2 are currently supported. Please look here for download links for the latest release.

What hardware is supported?

The plugin is tested and supported on V100, T4, A10, A30 and A100 datacenter GPUs. It is possible to run the plugin on GeForce desktop hardware with Volta or better architectures. GeForce hardware does not support CUDA enhanced compatibility, and will need CUDA 11.2 installed. If not, the following error will be displayed:

ai.rapids.cudf.CudaException: forward compatibility was attempted on non supported HW
        at ai.rapids.cudf.Cuda.getDeviceCount(Native Method)
        at com.nvidia.spark.rapids.GpuDeviceManager$.findGpuAndAcquire(GpuDeviceManager.scala:78)

How can I check if the RAPIDS Accelerator is installed and which version is running?

On startup the RAPIDS Accelerator will log a warning message on the Spark driver showing the version with a message that looks something like this:

21/04/14 22:14:55 WARN SQLExecPlugin: RAPIDS Accelerator 21.06.0 using cudf 21.06.0. To disable GPU support set `spark.rapids.sql.enabled` to false

The full RAPIDS Accelerator and cudf build properties are logged at INFO level in the Spark driver and executor logs with messages that are similar to the following:

21/04/14 17:20:20 INFO RapidsExecutorPlugin: RAPIDS Accelerator build: {version=0.5.0-SNAPSHOT, user=jlowe, url=, date=2021-04-14T22:12:14Z, revision=79a5cf8acd615587b2c7835072b0d8b0d4604f8b, cudf_version=0.19-SNAPSHOT, branch=branch-0.5}
21/04/14 17:20:20 INFO RapidsExecutorPlugin: cudf build: {version=0.19-SNAPSHOT, user=, date=2021-04-13T08:42:40Z, revision=a5d2407b93de444a6a7faf9db4b7dbf4ecbfe9ed, branch=HEAD}

What is the right hardware setup to run GPU accelerated Spark?

Reference architectures should be available around Q1 2021.

What parts of Apache Spark are accelerated?

Currently a limited set of SQL and DataFrame operations are supported, please see the configs and supported operations for a more complete list of what is supported. Some of structured streaming is likely to be accelerated, but it has not been an area of focus right now. Other areas like MLLib, GraphX or RDDs are not accelerated.

Is the Spark Dataset API supported?

The RAPIDS Accelerator supports the DataFrame API which is implemented in Spark as Dataset[Row]. If you are using Dataset[Row] that is equivalent to the DataFrame API. However using custom classes or types with Dataset is not supported. Such queries should still execute correctly when using the RAPIDS Accelerator, but it is likely most query operations will not be performed on the GPU.

With custom types the Dataset API generates query plans that use opaque lambda expressions to access the custom types. The opaque expressions prevent the RAPIDS Accelerator from translating any operation with these opaque expressions to the GPU, since the RAPIDS Accelerator cannot determine how the expression operates.

What is the road-map like?

Please look at the github repository https://github.com/nvidia/spark-rapids. It contains issue tracking and planning for sprints and releases.

How much faster will my query run?

Any single operator isn’t a fixed amount faster. So there is no simple algorithm to see how much faster a query will run. In addition, Apache Spark can store intermediate data to disk and send it across the network, both of which we typically see as bottlenecks in real world queries. Generally for complicated queries where all the processing can run on the GPU we see between 3x and 7x speedup, with a 4x speedup typical. We have seen as high as 100x in some specific cases.

What operators are best suited for the GPU?

  • Group by operations with high cardinality
  • Joins with a high cardinality
  • Sorts with a high cardinality
  • Window operations, especially for large windows
  • Complicated processing
  • Writing Parquet/ORC
  • Reading CSV
  • Transcoding (reading an input file and doing minimal processing before writing it out again, possibly in a different format, like CSV to Parquet)

Are there initialization costs?

From our tests the GPU typically takes about 2 to 3 seconds to initialize when an executor first starts. If you are only going to run a single query that only takes a few seconds to run this can be problematic. In general if you are going to do 30 seconds or more of processing within a single session the overhead can be amortized.

How can I tell what will run on the GPU and what will not run on it?

An Apache Spark plan is transformed and optimized into a set of operators called a physical plan. This plan is then run through a set of rules to translate it to a version that runs on the GPU. If you want to know what will run on the GPU and what will not along with an explanation why you can set spark.rapids.sql.explain to ALL. If you just want to see the operators not on the GPU you may set it to NOT_ON_GPU. Be aware that some queries end up being broken down into multiple jobs, and in those cases a separate log message might be output for each job. These are logged each time a query is compiled into an RDD, not just when the job runs. Because of this calling explain on a DataFrame will also trigger this to be logged.

The format of each line follows the pattern

indicator operation<NAME> operator? explanation

In this indicator is one of the following

  • * for operations that will run on the GPU
  • @ for operations that could run on the GPU but will not because they are a part of a larger section of the plan that will not run on the GPU
  • # for operations that have been removed from the plan. The reason they are removed will be in the explanation.
  • ! for operations that cannot run on the GPU

operation indicates the type of the operator.

  • Expression These are typically functions that operate on columns of data and produce a column of data.
  • Exec These are higher level operations that operate on an entire table at a time.
  • Partitioning These are different types of partitioning used when reorganizing data to move to different tasks.
  • Input These are different input formats used with a few input statements, but not all.
  • Output These are different output formats used with a few output statements, but not all.
  • NOT_FOUND These are for anything that the plugin has no replacement rule for.

NAME is the name of the operator given by Spark.

operator? is an optional string representation of the operator given by Spark.

explanation is a text explanation saying if this will

  • run on the GPU
  • could run on the GPU but will not because of something outside this operator and an explanation why
  • will not run on the GPU with an explanation why
  • will be removed from the plan with a reason why

Generally if an operator is not compatible with Spark for some reason and is off the explanation will include information about how it is incompatible and what configs to set to enable the operator if you can accept the incompatibility.

Why does the plan for the GPU query look different from the CPU query?

Typically, there is a one to one mapping between CPU stages in a plan and GPU stages. There are a few places where this is not the case.

  • WholeStageCodeGen - The GPU plan typically does not do code generation, and does not support generating code for an entire stage in the plan. Code generation reduces the cost of processing data one row at a time. The GPU plan processes the data in a columnar format, so the costs of processing a batch is amortized over the entire batch of data and code generation is not needed.

  • ColumnarToRow and RowToColumnar transitions - The CPU version of Spark plans typically process data in a row based format. The main exception to this is reading some kinds of columnar data, like Parquet. Transitioning between the CPU and the GPU also requires transitioning between row and columnar formatted data.

  • GpuCoalesceBatches and GpuShuffleCoalesce - Processing data on the GPU scales sublinearly. That means doubling the data does often takes less than half the time. Because of this we want to process larger batches of data when possible. These operators will try to combine smaller batches of data into fewer, larger batches to process more efficiently.

  • SortMergeJoin - The RAPIDS Accelerator does not support sort merge joins yet. For now, we translate sort merge joins into shuffled hash joins. Because of this there are times when sorts may be removed or other sorts added to meet the ordering requirements of the query.

  • TakeOrderedAndProject - The TakeOrderedAndProject operator will take the top N entries in each task, shuffle the results to a single executor and then take the top N results from that. The GPU plan often has more metrics than the CPU versions do, and when we tried to combine all of these operations into a single stage the metrics were confusing to understand. Instead, we split the single stage up into multiple smaller parts, so the metrics are clearer.

Why does explain() show that the GPU will be used even after setting spark.rapids.sql.enabled to false?

Apache Spark caches what is used to build the output of the explain() function. That cache has no knowledge about configs, so it may return results that are not up to date with the current config settings. This is true of all configs in Spark. If you changed spark.sql.autoBroadcastJoinThreshold after running explain() on a DataFrame, the resulting query would not change to reflect that config and still show a SortMergeJoin even though the new config might have changed to be a BroadcastHashJoin instead. When actually running something like with collect, show or write a new DataFrame is constructed causing Spark to re-plan the query. This is why spark.rapids.sql.enabled is still respected when running, even if explain shows stale results.

How are failures handled?

The RAPIDS Accelerator does not change the way failures are normally handled by Apache Spark.

How does the Spark scheduler decide what to do on the GPU vs the CPU?

Technically the Spark scheduler does not make those decisions. The plugin has a set of rules that decide if an operation can safely be replaced by a GPU enabled version. We are working on some cost based optimizations to try and improve performance for some situations where it might be more efficient to stay on the CPU instead of going back and forth.

Is Dynamic Partition Pruning (DPP) Supported?

Yes, DPP still works. It might not be as efficient as it could be, and we are working to improve it.

Is Adaptive Query Execution (AQE) Supported?

In the 0.2 release, AQE is supported but all exchanges will default to the CPU. As of the 0.3 release, running on Spark 3.0.1 and higher any operation that is supported on GPU will now stay on the GPU when AQE is enabled.

Why does my query show as not on the GPU when Adaptive Query Execution is enabled?

When running an explain() on a query where AQE is on, it is possible that AQE has not finalized the plan. In this case a message stating AdaptiveSparkPlan isFinalPlan=false will be printed at the top of the physical plan, and the explain output will show the query plan with CPU operators. As the query runs, the plan on the UI will update and show operations running on the GPU. This can happen for any AdaptiveSparkPlan where isFinalPlan=false.

== Physical Plan ==
AdaptiveSparkPlan isFinalPlan=false
+- ...

Are cache and persist supported?

Yes cache and persist are supported, but they are not GPU accelerated yet. We are working with the Spark community on changes that would allow us to accelerate compression when caching data.

Can I cache data into GPU memory?

No, that is not currently supported. It would require much larger changes to Apache Spark to be able to support this.

Is PySpark supported?

Yes

Are the R APIs for Spark supported?

Yes, but we don’t actively test them.

Are the Java APIs for Spark supported?

Yes, but we don’t actively test them.

Are the Scala APIs for Spark supported?

Yes

Is the GPU needed on the driver? Are there any benefits to having a GPU on the driver?

The GPU is not needed on the driver and there is no benefit to having one available on the driver for the RAPIDS plugin.

How does the performance compare to Databricks’ DeltaEngine?

We have not evaluated the performance yet. DeltaEngine is not open source, so any analysis needs to be done with Databricks in some form. When DeltaEngine is generally available and the terms of service allow it, we will look into doing a comparison.

How many tasks can I run per executor? How many should I run per executor?

There is no limit on the number of tasks per executor that you can run. Generally we recommend 2 to 6 tasks per executor and 1 GPU per executor. The GPU typically benefits from having 2 tasks run in parallel on it at a time, assuming your GPU has enough memory to support that. Having 2 to 3 times as many tasks off of the GPU as on the GPU allows for I/O to be run in parallel with the processing. If you increase the tasks too high you can overload the I/O and starting the initial processing can suffer. But if you have a lot of processing that cannot be done on the GPU, like complex UDFs, the more tasks you have the more CPU processing you can throw at it.

Why are multiple GPUs per executor not supported?

The RAPIDS Accelerator only supports a single GPU per executor because that was a limitation of RAPIDS cudf, the foundation of the Accelerator. Basic support for working with multiple GPUs has only recently been added to RAPIDS cudf, and there are no plans for its individual operations to leverage multiple GPUs (e.g.: a single task’s join operation processed by multiple GPUs).

Many Spark setups avoid allocating too many concurrent tasks to the same executor, and often multiple executors are run per node on the cluster. Therefore this feature has not been prioritized, as there has not been a compelling use-case that requires it.

Why are multiple executors per GPU not supported?

There are multiple reasons why this a problematic configuration:

  • Apache Spark does not support scheduling a fractional number of GPUs to an executor
  • CUDA context switches between processes sharing a single GPU can be expensive
  • Each executor would have a fraction of the GPU memory available for processing

Is Multi-Instance GPU (MIG) supported?

Yes, but it requires support from the underlying cluster manager to isolate the MIG GPU instance for each executor (e.g.: by setting CUDA_VISIBLE_DEVICES or other means).

Note that MIG is not recommended for use with the RAPIDS Accelerator since it significantly reduces the amount of GPU memory that can be used by the Accelerator for each executor instance. If the cluster is purpose-built to run Spark with the RAPIDS Accelerator then we recommend running without MIG. Also note that the UCX-based shuffle plugin will not work as well in this configuration because MIG does not support direct GPU to GPU transfers.

How can I run custom expressions/UDFs on the GPU?

The RAPIDS Accelerator provides the following solutions for running user-defined functions on the GPU:

RAPIDS Accelerated UDFs

UDFs can provide a RAPIDS accelerated implementation which allows the RAPIDS Accelerator to perform the operation on the GPU. See the RAPIDS accelerated UDF documentation for details.

Automatic Translation of Scala UDFs to Apache Spark Operations

The RAPIDS Accelerator has an experimental byte-code analyzer which can translate some simple Scala UDFs into equivalent Apache Spark operations in the query plan. The RAPIDS Accelerator then translates these operations into GPU operations just like other query plan operations.

The Scala UDF byte-code analyzer is disabled by default and must be enabled by the user via the spark.rapids.sql.udfCompiler.enabled configuration setting.

Why is the size of my output Parquet/ORC file different?

This can come down to a number of factors. The GPU version often compresses data in smaller chunks to get more parallelism and performance. This can result in larger files in some instances. We have also seen instances where the ordering of the data can have a big impact on the output size of the files. Spark tends to prefer sort based joins, and in some cases sort based aggregations, whereas the GPU versions are all hash based. This means that the resulting data can come out in a different order for the CPU and the GPU. This is not wrong, but can make the size of the output data different because of compression. Users can turn on spark.rapids.sql.hashOptimizeSort.enabled to have the GPU try to replicate more closely what the output ordering would have been if sort were used, like on the CPU.

Why am I getting an error when trying to use pinned memory?

Caused by: ai.rapids.cudf.CudaException: OS call failed or operation not supported on this OS
	at ai.rapids.cudf.Cuda.hostAllocPinned(Native Method)
	at ai.rapids.cudf.PinnedMemoryPool.<init>(PinnedMemoryPool.java:254)
	at ai.rapids.cudf.PinnedMemoryPool.lambda$initialize$1(PinnedMemoryPool.java:185)
	at java.util.concurrent.FutureTask.run(FutureTask.java:264)

This is typically caused by the IOMMU being enabled. Please see the CUDA docs for this issue.

To fix it you can either disable the IOMMU, or you can disable using pinned memory by setting spark.rapids.memory.pinnedPool.size to 0.

Is speculative execution supported?

Yes, speculative execution in Spark is fine with the RAPIDS Accelerator plugin.

As with all speculative execution, it may or may not be beneficial depending on the nature of why a particular task is slow and how easily speculation is triggered. You should monitor your Spark jobs to see how often task speculation occurs and how often the speculating task (i.e.: the one launched later) finishes before the slow task that triggered speculation. If the speculating task often finishes first then that’s good, it is working as intended. If many tasks are speculating, but the original task always finishes first then this is a pure loss, the speculation is adding load to the Spark cluster with no benefit.