1 files changed, 120 insertions, 0 deletions
diff --git a/src/rocksdb/table/block_based/block_prefetcher.cc b/src/rocksdb/table/block_based/block_prefetcher.cc
new file mode 100644
index 000000000..83ec2cb06
--- /dev/null
+++ b/src/rocksdb/table/block_based/block_prefetcher.cc
@@ -0,0 +1,120 @@
+//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+//
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+#include "table/block_based/block_prefetcher.h"
+
+#include "rocksdb/file_system.h"
+#include "table/block_based/block_based_table_reader.h"
+
+namespace ROCKSDB_NAMESPACE {
+void BlockPrefetcher::PrefetchIfNeeded(
+    const BlockBasedTable::Rep* rep, const BlockHandle& handle,
+    const size_t readahead_size, bool is_for_compaction,
+    const bool no_sequential_checking,
+    const Env::IOPriority rate_limiter_priority) {
+  // num_file_reads is used  by FilePrefetchBuffer only when
+  // implicit_auto_readahead is set.
+  if (is_for_compaction) {
+    rep->CreateFilePrefetchBufferIfNotExists(
+        compaction_readahead_size_, compaction_readahead_size_,
+        &prefetch_buffer_, /*implicit_auto_readahead=*/false,
+        /*num_file_reads=*/0, /*num_file_reads_for_auto_readahead=*/0);
+    return;
+  }
+
+  // Explicit user requested readahead.
+  if (readahead_size > 0) {
+    rep->CreateFilePrefetchBufferIfNotExists(
+        readahead_size, readahead_size, &prefetch_buffer_,
+        /*implicit_auto_readahead=*/false, /*num_file_reads=*/0,
+        /*num_file_reads_for_auto_readahead=*/0);
+    return;
+  }
+
+  // Implicit readahead.
+
+  // If max_auto_readahead_size is set to be 0 by user, no data will be
+  // prefetched.
+  size_t max_auto_readahead_size = rep->table_options.max_auto_readahead_size;
+  if (max_auto_readahead_size == 0 || initial_auto_readahead_size_ == 0) {
+    return;
+  }
+
+  if (initial_auto_readahead_size_ > max_auto_readahead_size) {
+    initial_auto_readahead_size_ = max_auto_readahead_size;
+  }
+
+  // In case of no_sequential_checking, it will skip the num_file_reads_ and
+  // will always creates the FilePrefetchBuffer.
+  if (no_sequential_checking) {
+    rep->CreateFilePrefetchBufferIfNotExists(
+        initial_auto_readahead_size_, max_auto_readahead_size,
+        &prefetch_buffer_, /*implicit_auto_readahead=*/true,
+        /*num_file_reads=*/0,
+        rep->table_options.num_file_reads_for_auto_readahead);
+    return;
+  }
+
+  size_t len = BlockBasedTable::BlockSizeWithTrailer(handle);
+  size_t offset = handle.offset();
+
+  // If FS supports prefetching (readahead_limit_ will be non zero in that case)
+  // and current block exists in prefetch buffer then return.
+  if (offset + len <= readahead_limit_) {
+    UpdateReadPattern(offset, len);
+    return;
+  }
+
+  if (!IsBlockSequential(offset)) {
+    UpdateReadPattern(offset, len);
+    ResetValues(rep->table_options.initial_auto_readahead_size);
+    return;
+  }
+  UpdateReadPattern(offset, len);
+
+  // Implicit auto readahead, which will be enabled if the number of reads
+  // reached `table_options.num_file_reads_for_auto_readahead` (default: 2)  and
+  // scans are sequential.
+  num_file_reads_++;
+  if (num_file_reads_ <= rep->table_options.num_file_reads_for_auto_readahead) {
+    return;
+  }
+
+  if (rep->file->use_direct_io()) {
+    rep->CreateFilePrefetchBufferIfNotExists(
+        initial_auto_readahead_size_, max_auto_readahead_size,
+        &prefetch_buffer_, /*implicit_auto_readahead=*/true, num_file_reads_,
+        rep->table_options.num_file_reads_for_auto_readahead);
+    return;
+  }
+
+  if (readahead_size_ > max_auto_readahead_size) {
+    readahead_size_ = max_auto_readahead_size;
+  }
+
+  // If prefetch is not supported, fall back to use internal prefetch buffer.
+  // Discarding other return status of Prefetch calls intentionally, as
+  // we can fallback to reading from disk if Prefetch fails.
+  Status s = rep->file->Prefetch(
+      handle.offset(),
+      BlockBasedTable::BlockSizeWithTrailer(handle) + readahead_size_,
+      rate_limiter_priority);
+  if (s.IsNotSupported()) {
+    rep->CreateFilePrefetchBufferIfNotExists(
+        initial_auto_readahead_size_, max_auto_readahead_size,
+        &prefetch_buffer_, /*implicit_auto_readahead=*/true, num_file_reads_,
+        rep->table_options.num_file_reads_for_auto_readahead);
+    return;
+  }
+
+  readahead_limit_ = offset + len + readahead_size_;
+  // Keep exponentially increasing readahead size until
+  // max_auto_readahead_size.
+  readahead_size_ = std::min(max_auto_readahead_size, readahead_size_ * 2);
+}
+}  // namespace ROCKSDB_NAMESPACE