Set readahead_buffer_size default to 0 for ArrayRecordReader.

Read-ahead is only useful when performing sequential read,
which is much slower than the parallel-read feature.

The read-ahead buffer occupies extra memory space with little
benefit to both random access and batch access, hence
disables it by default.

PiperOrigin-RevId: 842735797

Author: dryman

cpp/array_record_reader.cc

@@ -167,11 +167,11 @@ ArrayRecordReaderBase::Options::FromString(absl::string_view text) {
   // ReadaheadBuffer
   options_parser.AddOption(
       "readahead_buffer_size",
-      ValueParser::Or(
-          ValueParser::Enum({{"auto", kDefaultReadaheadBufferSize}},
-                            &options.readahead_buffer_size_),
-          ValueParser::Bytes(0, std::numeric_limits<uint64_t>::max(),
-                             &options.readahead_buffer_size_)));
+      ValueParser::Or(ValueParser::Enum({{"auto", kDefaultReadaheadBufferSize}},
+                                        &options.readahead_buffer_size_),
+                      ValueParser::Bytes(/*min_value=*/0,
+                                         /*max_value=*/1024 * 1024 * 1024,
+                                         &options.readahead_buffer_size_)));
   // Index storage option
   options_parser.AddOption(
       "index_storage_option",

cpp/array_record_reader.h

@@ -81,8 +81,8 @@ class ArrayRecordReaderBase : public riegeli::Object {
     //     "max_parallelism" ":" max_parallelism
     //     "index_storage_option" ":" index_storage_option
     //   readahead_buffer_size ::= non-negative integer expressed as real with
-    //     optional suffix [BkKMGTPE]. (Default 16MB). Set to 0 optimizes random
-    //     access performance.
+    //     optional suffix [BKMG]. (Example 16MB). Set to 0 optimizes random
+    //     access performance. Defaults 0.
     //   max_parallelism ::= `auto` or non-negative integer. Each parallel
     //     thread owns its readhaed buffer with the size
     //     `readahead_buffer_size`.  (Default thread pool size) Set to 0
@@ -95,7 +95,7 @@ class ArrayRecordReaderBase : public riegeli::Object {
     // Readahead speeds up sequential reads, but harms random access. When using
     // ArrayRecord for random access, user should configure the buffer size with
     // 0.
-    static constexpr uint64_t kDefaultReadaheadBufferSize = 1L << 24;
+    static constexpr uint64_t kDefaultReadaheadBufferSize = 0;
     Options& set_readahead_buffer_size(uint64_t readahead_buffer_size) {
       readahead_buffer_size_ = readahead_buffer_size;
       return *this;