perf: Add Comet config for native Iceberg reader's data file concurrency (#3584)

diff --git a/common/src/main/scala/org/apache/comet/CometConf.scala b/common/src/main/scala/org/apache/comet/CometConf.scala
index 480eafd..5ee777f 100644
--- a/common/src/main/scala/org/apache/comet/CometConf.scala
+++ b/common/src/main/scala/org/apache/comet/CometConf.scala

@@ -148,6 +148,17 @@
       .booleanConf
       .createWithDefault(false)
 
+  val COMET_ICEBERG_DATA_FILE_CONCURRENCY_LIMIT: ConfigEntry[Int] =
+    conf("spark.comet.scan.icebergNative.dataFileConcurrencyLimit")
+      .category(CATEGORY_SCAN)
+      .doc(
+        "The number of Iceberg data files to read concurrently within a single task. " +
+          "Higher values improve throughput for tables with many small files by overlapping " +
+          "I/O latency, but increase memory usage. Values between 2 and 8 are suggested.")
+      .intConf
+      .checkValue(v => v > 0, "Data file concurrency limit must be positive")
+      .createWithDefault(1)
+
   val COMET_CSV_V2_NATIVE_ENABLED: ConfigEntry[Boolean] =
     conf("spark.comet.scan.csv.v2.enabled")
       .category(CATEGORY_TESTING)

diff --git a/native/core/src/execution/operators/iceberg_scan.rs b/native/core/src/execution/operators/iceberg_scan.rs
index bc20592..39ce250 100644
--- a/native/core/src/execution/operators/iceberg_scan.rs
+++ b/native/core/src/execution/operators/iceberg_scan.rs

@@ -61,6 +61,8 @@
     catalog_properties: HashMap<String, String>,
     /// Pre-planned file scan tasks
     tasks: Vec<FileScanTask>,
+    /// Number of data files to read concurrently
+    data_file_concurrency_limit: usize,
     /// Metrics
     metrics: ExecutionPlanMetricsSet,
 }
@@ -71,6 +73,7 @@
         schema: SchemaRef,
         catalog_properties: HashMap<String, String>,
         tasks: Vec<FileScanTask>,
+        data_file_concurrency_limit: usize,
     ) -> Result<Self, ExecutionError> {
         let output_schema = schema;
         let plan_properties = Self::compute_properties(Arc::clone(&output_schema), 1);
@@ -83,6 +86,7 @@
             plan_properties,
             catalog_properties,
             tasks,
+            data_file_concurrency_limit,
             metrics,
         })
     }
@@ -158,7 +162,7 @@
 
         let reader = iceberg::arrow::ArrowReaderBuilder::new(file_io)
             .with_batch_size(batch_size)
-            .with_data_file_concurrency_limit(context.session_config().target_partitions())
+            .with_data_file_concurrency_limit(self.data_file_concurrency_limit)
             .with_row_selection_enabled(true)
             .build();
 

diff --git a/native/core/src/execution/planner.rs b/native/core/src/execution/planner.rs
index f84d6cc..ef81cdf 100644
--- a/native/core/src/execution/planner.rs
+++ b/native/core/src/execution/planner.rs

@@ -1176,12 +1176,14 @@
                     .collect();
                 let metadata_location = common.metadata_location.clone();
                 let tasks = parse_file_scan_tasks_from_common(common, &scan.file_scan_tasks)?;
+                let data_file_concurrency_limit = common.data_file_concurrency_limit as usize;
 
                 let iceberg_scan = IcebergScanExec::new(
                     metadata_location,
                     required_schema,
                     catalog_properties,
                     tasks,
+                    data_file_concurrency_limit,
                 )?;
 
                 Ok((

diff --git a/native/proto/src/proto/operator.proto b/native/proto/src/proto/operator.proto
index 93872b4..bf2752b 100644
--- a/native/proto/src/proto/operator.proto
+++ b/native/proto/src/proto/operator.proto

@@ -175,6 +175,9 @@
   repeated PartitionData partition_data_pool = 9;
   repeated DeleteFileList delete_files_pool = 10;
   repeated spark.spark_expression.Expr residual_pool = 11;
+
+  // Number of data files to read concurrently within a single task
+  uint32 data_file_concurrency_limit = 12;
 }
 
 message IcebergScan {

diff --git a/spark/src/main/scala/org/apache/comet/serde/operator/CometIcebergNativeScan.scala b/spark/src/main/scala/org/apache/comet/serde/operator/CometIcebergNativeScan.scala
index c86b2a5..9f1a015 100644
--- a/spark/src/main/scala/org/apache/comet/serde/operator/CometIcebergNativeScan.scala
+++ b/spark/src/main/scala/org/apache/comet/serde/operator/CometIcebergNativeScan.scala

@@ -31,7 +31,7 @@
 import org.apache.spark.sql.execution.datasources.v2.{BatchScanExec, DataSourceRDD, DataSourceRDDPartition}
 import org.apache.spark.sql.types._
 
-import org.apache.comet.ConfigEntry
+import org.apache.comet.{CometConf, ConfigEntry}
 import org.apache.comet.iceberg.{CometIcebergNativeScanMetadata, IcebergReflection}
 import org.apache.comet.serde.{CometOperatorSerde, OperatorOuterClass}
 import org.apache.comet.serde.ExprOuterClass.Expr
@@ -757,6 +757,8 @@
     var totalTasks = 0
 
     commonBuilder.setMetadataLocation(metadata.metadataLocation)
+    commonBuilder.setDataFileConcurrencyLimit(
+      CometConf.COMET_ICEBERG_DATA_FILE_CONCURRENCY_LIMIT.get())
     metadata.catalogProperties.foreach { case (key, value) =>
       commonBuilder.putCatalogProperties(key, value)
     }