Improve performance of long range scans with readahead

Summary:
This change improves the performance of iterators doing long range scans (e.g. big/full table scans in MyRocks) by using readahead and prefetching additional data on each disk IO. This prefetching is automatically enabled on noticing more than 2 IOs for the same table file during iteration. The readahead size starts with 8KB and is exponentially increased on each additional sequential IO, up to a max of 256 KB. This helps in cutting down the number of IOs needed to complete the range scan.

Constraints:
- The prefetched data is stored by the OS in page cache. So this currently works only for non direct-reads use-cases i.e applications which use page cache. (Direct-I/O support will be enabled in a later PR).
- This gets currently enabled only when ReadOptions.readahead_size = 0 (which is the default value).

Thanks to siying for the original idea and implementation.

**Benchmarks:**
Data fill:
```
TEST_TMPDIR=/data/users/$USER/benchmarks/iter ./db_bench -benchmarks=fillrandom -num=1000000000 -compression_type="none" -level_compaction_dynamic_level_bytes
```
Do a long range scan: Seekrandom with large number of nexts
```
TEST_TMPDIR=/data/users/$USER/benchmarks/iter ./db_bench -benchmarks=seekrandom -duration=60 -num=1000000000 -use_existing_db -seek_nexts=10000 -statistics -histogram
```

Page cache was cleared before each experiment with the command:
```
sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"
```
```
Before:
seekrandom   :   34020.945 micros/op 29 ops/sec;   32.5 MB/s (1636 of 1999 found)
With this change:
seekrandom   :    8726.912 micros/op 114 ops/sec;  126.8 MB/s (5702 of 6999 found)
```
~3.9X performance improvement.

Also verified with strace and gdb that the readahead size is increasing as expected.
```
strace -e readahead -f -T -t -p <db_bench process pid>
```
Closes https://github.com/facebook/rocksdb/pull/3282

Differential Revision: D6586477

Pulled By: sagar0

fbshipit-source-id: 8a118a0ed4594fbb7f5b1cafb242d7a4033cb58c

HISTORY.md

@@ -2,6 +2,10 @@
 ## Unreleased
 ### Public API Change
 * Iterator::SeekForPrev is now a pure virtual method. This is to prevent user who implement the Iterator interface fail to implement SeekForPrev by mistake.
+
+### New Features
+* Improve the performance of iterators doing long range scans by using readahead.
+
 ### Bug Fixes
 * Fix `DisableFileDeletions()` followed by `GetSortedWalFiles()` to not return obsolete WAL files that `PurgeObsoleteFiles()` is going to delete.
 * Fix DB::Flush() keep waiting after flush finish under certain condition.

table/block_based_table_reader.cc

@@ -1594,6 +1594,9 @@ BlockBasedTable::BlockEntryIteratorState::BlockEntryIteratorState(
       is_index_(is_index),
       block_map_(block_map) {}
 
+const size_t BlockBasedTable::BlockEntryIteratorState::kMaxReadaheadSize =
+    256 * 1024;
+
 InternalIterator*
 BlockBasedTable::BlockEntryIteratorState::NewSecondaryIterator(
     const Slice& index_value) {
@@ -1618,6 +1621,28 @@ BlockBasedTable::BlockEntryIteratorState::NewSecondaryIterator(
           &rep->internal_comparator, nullptr, true, rep->ioptions.statistics);
     }
   }
+
+  // Automatically prefetch additional data when a range scan (iterator) does
+  // more than 2 sequential IOs. This is enabled only when
+  // ReadOptions.readahead_size is 0.
+  if (read_options_.readahead_size == 0) {
+    if (num_file_reads_ < 2) {
+      num_file_reads_++;
+    } else if (handle.offset() + static_cast<size_t>(handle.size()) +
+                   kBlockTrailerSize >
+               readahead_limit_) {
+      num_file_reads_++;
+      // Do not readahead more than kMaxReadaheadSize.
+      readahead_size_ =
+          std::min(BlockBasedTable::BlockEntryIteratorState::kMaxReadaheadSize,
+                   readahead_size_);
+      table_->rep_->file->Prefetch(handle.offset(), readahead_size_);
+      readahead_limit_ = handle.offset() + readahead_size_;
+      // Keep exponentially increasing readahead size until kMaxReadaheadSize.
+      readahead_size_ *= 2;
+    }
+  }
+
   return NewDataBlockIterator(rep, read_options_, handle,
                               /* input_iter */ nullptr, is_index_,
                               /* get_context */ nullptr, s);

table/block_based_table_reader.h

@@ -376,6 +376,14 @@ class BlockBasedTable::BlockEntryIteratorState : public TwoLevelIteratorState {
   bool is_index_;
   std::unordered_map<uint64_t, CachableEntry<Block>>* block_map_;
   port::RWMutex cleaner_mu;
+
+  static const size_t kInitReadaheadSize = 8 * 1024;
+  // Found that 256 KB readahead size provides the best performance, based on
+  // experiments.
+  static const size_t kMaxReadaheadSize;
+  size_t readahead_size_ = kInitReadaheadSize;
+  size_t readahead_limit_ = 0;
+  int num_file_reads_ = 0;
 };
 
 // CachableEntry represents the entries that *may* be fetched from block cache.