There are some common questions about developing, debugging and testing been asked again and again. In order to help the developers to contribute to Gluten as soon as possible, we collected these frequently asked questions, and organized them in the form of Q&A. It’s convenient for the developers to check and learn.

when you encountered a new problem and then resolved it, please add a new item to this document if you think it may be helpful to the other developers.

We use gluten_home to represent the home directory of Gluten in this document.

How to understand the key work of Gluten?

The Gluten worked as the role of bridge, it’s a middle layer between the Spark and the native execution library.

The Gluten is responsibility for validating whether the operators of the Spark plan can be executed by the native engine or not. If yes, the Gluten transforms Spark plan to Substrait plan, and then send the Substrait plan to the native engine.

The Gluten codes consist of two parts: the C++ codes and the Java/Scala codes.

All C++ codes are placed under the directory of gluten_home/cpp, the Java/Scala codes are placed under several directories, such as gluten_home/gluten-core gluten_home/gluten-data gluten_home/backends-velox.
The Java/Scala codes are responsibility for validating and transforming the execution plan. Source data should also be provided, the source data may come from files or other forms such as networks.
The C++ codes take the Substrait plan and the source data as inputs and transform the Substrait plan to the corresponding backend plan. If the backend is Velox, the Substrait plan will be transformed to the Velox plan, and then be executed.

JNI is a programming technology of invoking C++ from Java. All JNI interfaces are defined in the file JniWrapper.cc under the directory jni.

How to debug in Gluten?

1 How to debug C++

If you don’t concern about the Scala/Java codes and just want to debug the C++ codes executed in native engine, you may debug the C++ via benchmarks with GDB.

To debug C++, you have to generate the example files, the example files consist of:

A file contained Substrait plan in JSON format
One or more input data files in Parquet format

You can generate the example files by the following steps:

build Velox and Gluten CPP
```
gluten_home/dev/builddeps-veloxbe.sh --build_tests=ON --build_benchmarks=ON --build_type=Debug
```
- Compiling with --build_type=Debug is good for debugging.
- The executable file generic_benchmark will be generated under the directory of gluten_home/cpp/build/velox/benchmarks/.

build Gluten and generate the example files

cd gluten_home
mvn clean package -Pspark-3.2 -Pbackends-velox -Prss
mvn test -Pspark-3.2 -Pbackends-velox -Prss -pl backends-velox -am -DtagsToInclude="io.glutenproject.tags.GenerateExample" -Dtest=none -DfailIfNoTests=false -Darrow.version=11.0.0-gluten -Dexec.skip

After the above operations, the examples files are generated under gluten_home/backends-velox
You can check it by the command tree gluten_home/backends-velox/generated-native-benchmark/

You may replace -Pspark-3.2 with -Pspark-3.3 if your spark’s version is 3.3

$ tree gluten_home/backends-velox/generated-native-benchmark/
gluten_home/backends-velox/generated-native-benchmark/
├── example.json
├── example_lineitem
│   ├── part-00000-3ec19189-d20e-4240-85ae-88631d46b612-c000.snappy.parquet
│   └── _SUCCESS
└── example_orders
 ├── part-00000-1e66fb98-4dd6-47a6-8679-8625dbc437ee-c000.snappy.parquet
 └── _SUCCESS

now, run benchmarks with GDB
```
cd gluten_home/cpp/build/velox/benchmarks/
gdb generic_benchmark
```
- When GDB load generic_benchmark successfully, you can set breakpoint on the main function with command b main, and then run with command r, then the process generic_benchmark will start and stop at the main function.
- You can check the variables’ state with command p variable_name, or execute the program line by line with command n, or step-in the function been called with command s.
- Actually, you can debug generic_benchmark with any gdb commands as debugging normal C++ program, because the generic_benchmark is a pure C++ executable file in fact.
gdb-tui is a valuable feature and is worth trying. You can get more help from the online docs. gdb-tui
you can start generic_benchmark with specific JSON plan and input files
- If you omit them, the example.json, example_lineitem + example_orders under the directory of gluten_home/backends-velox/generated-native-benchmark will be used as default.
- You can also edit the file example.json to custom the Substrait plan or specify the inputs files placed in the other directory.
get more detail information about benchmarks from MicroBenchmarks

2 How to debug plan validation process

Gluten will validate generated plan before execute it, and validation usually happens in native side, so we provide a utility to help debug validation process in native side.

Run query with conf spark.gluten.sql.debug=true, and you will find generated plan be printed in stderr with json format, save it as plan.json for example.
Compile cpp part with --build_benchmarks=ON, then check plan_validator_util executable file in gluten_home/cpp/build/velox/benchmarks/.
Run or debug with ./plan_validator_util <path>/plan.json

3 How to debug Java/Scala

wait to add

4 How to debug with core-dump

wait to complete

cd the_directory_of_core_file_generated
gdb gluten_home/cpp/build/releases/libgluten.so 'core-Executor task l-2000883-1671542526'

the core-Executor task l-2000883-1671542526 represents the core file name.

How to run TPC-H on Velox backend

Now, both Parquet and DWRF format files are supported, related scripts and files are under the directory of gluten_home/backends-velox/workload/tpch. The file README.md under gluten_home/backends-velox/workload/tpch offers some useful help but it’s still not enough and exact.

One way of run TPC-H test is to run velox-be by workflow, you can refer to velox_be.yml

Here will explain how to run TPC-H on Velox backend with the Parquet file format.

First step, prepare the datasets, you have two choices.
- One way, generate Parquet datasets using the script under gluten_home/backends-velox/workload/tpch/gen_data/parquet_dataset, You can get help from the above mentioned README.md.
- The other way, using the small dataset under gluten_home/backends-velox/src/test/resources/tpch-data-parquet-velox directly, If you just want to make simple TPC-H testing, this dataset is a good choice.
Second step, run TPC-H on Velox backend testing.
- Modify gluten_home/backends-velox/workload/tpch/run_tpch/tpch_parquet.scala.
- set var parquet_file_path to correct directory. If using the small dataset directly in the step one, then modify it as below
```
 var parquet_file_path = "gluten_home/backends-velox/src/test/resources/tpch-data-parquet-velox"
```
- set var gluten_root to correct directory. If gluten_home is the directory of /home/gluten, then modify it as below
```
 var gluten_root = "/home/gluten"
```
```
  - Modify `gluten_home/backends-velox/workload/tpch/run_tpch/tpch_parquet.sh`.
```
- Set GLUTEN_JAR correctly. Please refer to the section of Build Gluten with Velox Backend
- Set SPARK_HOME correctly.
- Set the memory configurations appropriately. - Execute tpch_parquet.sh using the below command.
- cd gluten_home/backends-velox/workload/tpch/run_tpch/
- ./tpch_parquet.sh

How to run TPC-DS

wait to add

How to track the memory exhaust problem

When your gluten spark jobs failed because of OOM, you can track the memory allocation’s call stack by configuring spark.gluten.backtrace.allocation = true. The above configuration will use BacktraceAllocationListener wrapping from SparkAllocationListener to create VeloxMemoryManager.

BacktraceAllocationListener will check every allocation, if a single allocation bytes exceeds a fixed value or the accumulative allocation bytes exceeds 1/2/3…G, the call stack of memory allocation will be outputted to standard output, you can check the backtrace and get some valuable information about tracking the memory exhaust issues.

You can also adjust the policy to decide when to backtrace, such as the fixed value.