ARROW-17299: [C++][Python] Expose the Scanner kDefaultBatchReadahead and kDefaultFragmentReadahead parameters

cpp/src/arrow/dataset/scanner.cc

@@ -826,15 +826,6 @@ Status ScannerBuilder::UseThreads(bool use_threads) {
   return Status::OK();
 }
 
-Status ScannerBuilder::FragmentReadahead(int fragment_readahead) {
-  if (fragment_readahead <= 0) {
-    return Status::Invalid("FragmentReadahead must be greater than 0, got ",
-                           fragment_readahead);
-  }
-  scan_options_->fragment_readahead = fragment_readahead;
-  return Status::OK();
-}
-
 Status ScannerBuilder::BatchSize(int64_t batch_size) {
   if (batch_size <= 0) {
     return Status::Invalid("BatchSize must be greater than 0, got ", batch_size);
@@ -843,6 +834,24 @@ Status ScannerBuilder::BatchSize(int64_t batch_size) {
   return Status::OK();
 }
 
+Status ScannerBuilder::BatchReadahead(int32_t batch_readahead) {
+  if (batch_readahead < 0) {
+    return Status::Invalid("BatchReadahead must be greater than or equal 0, got ",
+                           batch_readahead);
+  }
+  scan_options_->batch_readahead = batch_readahead;
+  return Status::OK();
+}
+
+Status ScannerBuilder::FragmentReadahead(int32_t fragment_readahead) {
+  if (fragment_readahead < 0) {
+    return Status::Invalid("FragmentReadahead must be greater than or equal 0, got ",
+                           fragment_readahead);
+  }
+  scan_options_->fragment_readahead = fragment_readahead;
+  return Status::OK();
+}
+
 Status ScannerBuilder::Pool(MemoryPool* pool) {
   scan_options_->pool = pool;
   return Status::OK();

cpp/src/arrow/dataset/scanner.h

@@ -373,9 +373,6 @@ class ARROW_DS_EXPORT ScannerBuilder {
   ///        ThreadPool found in ScanOptions;
   Status UseThreads(bool use_threads = true);
 
-  /// \brief Limit how many fragments the scanner will read at once
-  Status FragmentReadahead(int fragment_readahead);
-
   /// \brief Set the maximum number of rows per RecordBatch.
   ///
   /// \param[in] batch_size the maximum number of rows.
@@ -384,6 +381,23 @@ class ARROW_DS_EXPORT ScannerBuilder {
   /// This option provides a control limiting the memory owned by any RecordBatch.
   Status BatchSize(int64_t batch_size);
 
+  /// \brief Set the number of batches to read ahead within a file.
+  ///
+  /// \param[in] batch_readahead How many batches to read ahead within a file,
+  ///  might not work for all formats. 
+  /// \returns An error if this number is less than 0.
+  ///
+  /// This option provides a control on RAM vs I/O tradeoff.
+  Status BatchReadahead(int32_t batch_readahead);
+
+  /// \brief Set the number of fragments to read ahead
+  ///
+  /// \param[in] fragment_readahead How many fragments to read ahead
+  /// \returns An error if this number is less than 0.
+  ///
+  /// This option provides a control on RAM vs IO tradeoff.
+  Status FragmentReadahead(int32_t fragment_readahead);
+
   /// \brief Set the pool from which materialized and scanned arrays will be allocated.
   Status Pool(MemoryPool* pool);
 

python/pyarrow/_dataset.pyx

@@ -2168,11 +2168,14 @@ cdef class TaggedRecordBatchIterator(_Weakrefable):
 
 
 _DEFAULT_BATCH_SIZE = 2**17
-
+_DEFAULT_BATCH_READAHEAD = 16
+_DEFAULT_FRAGMENT_READAHEAD = 4
 
 cdef void _populate_builder(const shared_ptr[CScannerBuilder]& ptr,
                             object columns=None, Expression filter=None,
                             int batch_size=_DEFAULT_BATCH_SIZE,
+                            int batch_readahead=_DEFAULT_BATCH_READAHEAD,
+                            int fragment_readahead=_DEFAULT_FRAGMENT_READAHEAD,
                             bint use_threads=True, MemoryPool memory_pool=None,
                             FragmentScanOptions fragment_scan_options=None)\
         except *:
@@ -2207,6 +2210,8 @@ cdef void _populate_builder(const shared_ptr[CScannerBuilder]& ptr,
             )
 
     check_status(builder.BatchSize(batch_size))
+    check_status(builder.BatchReadahead(batch_readahead))
+    check_status(builder.FragmentReadahead(fragment_readahead))
     check_status(builder.UseThreads(use_threads))
     if memory_pool:
         check_status(builder.Pool(maybe_unbox_memory_pool(memory_pool)))
@@ -2254,6 +2259,12 @@ cdef class Scanner(_Weakrefable):
         The maximum row count for scanned record batches. If scanned
         record batches are overflowing memory then this method can be
         called to reduce their size.
+    batch_readahead : int, default 16
+        The number of batches to read ahead in a file. Increasing this number 
+        will increase RAM usage but also improve IO utilization.
+    fragment_readahead : int, default 4
+        The number of files to read ahead. Increasing this number will increase
+        RAM usage but also improve IO utilization.
     use_threads : bool, default True
         If enabled, then maximum parallelism will be used determined by
         the number of available CPU cores.
@@ -2291,6 +2302,8 @@ cdef class Scanner(_Weakrefable):
                      MemoryPool memory_pool=None,
                      object columns=None, Expression filter=None,
                      int batch_size=_DEFAULT_BATCH_SIZE,
+                     int batch_readahead=_DEFAULT_BATCH_READAHEAD,
+                     int fragment_readahead=_DEFAULT_FRAGMENT_READAHEAD,
                      FragmentScanOptions fragment_scan_options=None):
         """
         Create Scanner from Dataset,
@@ -2328,6 +2341,13 @@ cdef class Scanner(_Weakrefable):
             The maximum row count for scanned record batches. If scanned
             record batches are overflowing memory then this method can be
             called to reduce their size.
+        batch_readahead : int, default 16
+            The number of batches to read ahead in a file. This might not work
+            for all file formats like CSV. Increasing this number will increase
+            RAM usage but also improve IO utilization.
+        fragment_readahead : int, default 4
+            The number of files to read ahead. Increasing this number will increase
+            RAM usage but also improve IO utilization.
         use_threads : bool, default True
             If enabled, then maximum parallelism will be used determined by
             the number of available CPU cores.
@@ -2354,7 +2374,8 @@ cdef class Scanner(_Weakrefable):
 
         builder = make_shared[CScannerBuilder](dataset.unwrap(), options)
         _populate_builder(builder, columns=columns, filter=filter,
-                          batch_size=batch_size, use_threads=use_threads,
+                          batch_size=batch_size, batch_readahead=batch_readahead,
+                          fragment_readahead=fragment_readahead, use_threads=use_threads,
                           memory_pool=memory_pool,
                           fragment_scan_options=fragment_scan_options)
 
@@ -2367,6 +2388,7 @@ cdef class Scanner(_Weakrefable):
                       MemoryPool memory_pool=None,
                       object columns=None, Expression filter=None,
                       int batch_size=_DEFAULT_BATCH_SIZE,
+                      int batch_readahead=_DEFAULT_BATCH_READAHEAD,
                       FragmentScanOptions fragment_scan_options=None):
         """
         Create Scanner from Fragment,
@@ -2406,6 +2428,10 @@ cdef class Scanner(_Weakrefable):
             The maximum row count for scanned record batches. If scanned
             record batches are overflowing memory then this method can be
             called to reduce their size.
+        batch_readahead : int, default 16
+            The number of batches to read ahead in a file. This might not work
+            for all file formats like CSV. Increasing this number will increase
+            RAM usage but also improve IO utilization.
         use_threads : bool, default True
             If enabled, then maximum parallelism will be used determined by
             the number of available CPU cores.
@@ -2435,7 +2461,9 @@ cdef class Scanner(_Weakrefable):
         builder = make_shared[CScannerBuilder](pyarrow_unwrap_schema(schema),
                                                fragment.unwrap(), options)
         _populate_builder(builder, columns=columns, filter=filter,
-                          batch_size=batch_size, use_threads=use_threads,
+                          batch_size=batch_size, batch_readahead=batch_readahead,
+                          fragment_readahead=_DEFAULT_FRAGMENT_READAHEAD,
+                          use_threads=use_threads,
                           memory_pool=memory_pool,
                           fragment_scan_options=fragment_scan_options)
 
@@ -2508,7 +2536,8 @@ cdef class Scanner(_Weakrefable):
                           FutureWarning)
 
         _populate_builder(builder, columns=columns, filter=filter,
-                          batch_size=batch_size, use_threads=use_threads,
+                          batch_size=batch_size, batch_readahead=_DEFAULT_BATCH_READAHEAD,
+                          fragment_readahead=_DEFAULT_FRAGMENT_READAHEAD, use_threads=use_threads,
                           memory_pool=memory_pool,
                           fragment_scan_options=fragment_scan_options)
         scanner = GetResultValue(builder.get().Finish())

python/pyarrow/includes/libarrow_dataset.pxd

@@ -122,6 +122,8 @@ cdef extern from "arrow/dataset/api.h" namespace "arrow::dataset" nogil:
         CStatus UseThreads(c_bool use_threads)
         CStatus Pool(CMemoryPool* pool)
         CStatus BatchSize(int64_t batch_size)
+        CStatus BatchReadahead(int32_t batch_readahead)
+        CStatus FragmentReadahead(int32_t fragment_readahead)
         CStatus FragmentScanOptions(
             shared_ptr[CFragmentScanOptions] fragment_scan_options)
         CResult[shared_ptr[CScanner]] Finish()