GH-48636: [C++][Parquet] Improve parquet reading using multi threads

[OmBiradar]

Rationale for this change

Currently the parquet file structs are read sequentially, even when the use_threads option is used by the user. This PR aims to bridge this gap by making the struct reading truly parallel.

What changes are included in this PR?

Changes to the LoadBatch and BuildArray functions in the StructReader in the Parquet reader to enable multi threaded reads of structs in parquet files.

Are these changes tested?

• Yes I have tested them. Details can be found in this testing and benchmarking repo
• Weather benchmarks should be added for parallel reads is a question I would ask the reviewer.

Are there any user-facing changes?

This is purely a performance related PR. No changes to the user or API is needed.

• GitHub Issue: [Python] Enable multi-threaded reads of struct-list stored data in parquet files #48636

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates Parquet testing submodule pins and introduces optional parallelism when loading/building nested struct children readers to improve throughput when use_threads() is enabled.

Changes:

• Bump testing and cpp/submodules/parquet-testing submodule commits.
• Use ::arrow::internal::OptionalParallelFor to parallelize StructReader::LoadBatch.
• Parallelize StructReader::BuildArray child array construction before converting chunks to single arrays.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
testing	Updates submodule commit SHA.
cpp/submodules/parquet-testing	Updates submodule commit SHA for parquet-testing.
cpp/src/parquet/arrow/reader.cc	Adds optional parallel execution for struct child LoadBatch and BuildArray.

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[OmBiradar]

template <class FUNCTION>
Status OptionalParallelFor(bool use_threads, int num_tasks, FUNCTION&& func,
                           Executor* executor = internal::GetCpuThreadPool()) {
  if (use_threads) {
    return ParallelFor(num_tasks, std::forward<FUNCTION>(func), executor);
  } else {
    for (int i = 0; i < num_tasks; ++i) {
      RETURN_NOT_OK(func(i));
    }
    return Status::OK();
  }
}

At the end, it's always cast to an int so truncating is unavoidable.

[OmBiradar]

template <class FUNCTION>
Status OptionalParallelFor(bool use_threads, int num_tasks, FUNCTION&& func,
                           Executor* executor = internal::GetCpuThreadPool()) {
  if (use_threads) {
    return ParallelFor(num_tasks, std::forward<FUNCTION>(func), executor);
  } else {
    for (int i = 0; i < num_tasks; ++i) {
      RETURN_NOT_OK(func(i));
    }
    return Status::OK();
  }
}

At the end, it's always cast to an int so truncating is unavoidable.

[OmBiradar]

@wgtmac @pitrou

Should I add benchmarks for the multi threaded reading scenario, I am asking as I have not seen any other multi threaded performance benchmarks.