Improve CPU Efficiency of ApproximateSize (part 1) (#5613)

Summary:
1. Avoid creating the iterator in order to call BlockBasedTable::ApproximateOffsetOf(). Instead, directly call into it.
2. Optimize BlockBasedTable::ApproximateOffsetOf() keeps the index block iterator in stack.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5613

Differential Revision: D16442660

Pulled By: elipoz

fbshipit-source-id: 9320be3e918c139b10e758cbbb684706d172e516

db/table_cache.cc

@@ -194,7 +194,7 @@ InternalIterator* TableCache::NewIterator(
   if (table_reader == nullptr) {
     s = FindTable(env_options, icomparator, fd, &handle, prefix_extractor,
                   options.read_tier == kBlockCacheTier /* no_io */,
-                  !for_compaction /* record read_stats */, file_read_hist,
+                  !for_compaction /* record_read_stats */, file_read_hist,
                   skip_filters, level);
     if (s.ok()) {
       table_reader = GetTableReaderFromHandle(handle);
@@ -505,4 +505,30 @@ void TableCache::Evict(Cache* cache, uint64_t file_number) {
   cache->Erase(GetSliceForFileNumber(&file_number));
 }
 
+uint64_t TableCache::ApproximateOffsetOf(
+    const Slice& key, const FileDescriptor& fd, TableReaderCaller caller,
+    const InternalKeyComparator& internal_comparator,
+    const SliceTransform* prefix_extractor) {
+  uint64_t result = 0;
+  TableReader* table_reader = fd.table_reader;
+  Cache::Handle* table_handle = nullptr;
+  if (table_reader == nullptr) {
+    const bool for_compaction = (caller == TableReaderCaller::kCompaction);
+    Status s = FindTable(env_options_, internal_comparator, fd, &table_handle,
+                         prefix_extractor, false /* no_io */,
+                         !for_compaction /* record_read_stats */);
+    if (s.ok()) {
+      table_reader = GetTableReaderFromHandle(table_handle);
+    }
+  }
+
+  if (table_reader != nullptr) {
+    result = table_reader->ApproximateOffsetOf(key, caller);
+  }
+  if (table_handle != nullptr) {
+    ReleaseHandle(table_handle);
+  }
+
+  return result;
+}
 }  // namespace rocksdb

db/table_cache.h

@@ -153,6 +153,12 @@ class TableCache {
       const FileDescriptor& fd,
       const SliceTransform* prefix_extractor = nullptr);
 
+  // Returns approximated offset of a key in a file represented by fd.
+  uint64_t ApproximateOffsetOf(
+      const Slice& key, const FileDescriptor& fd, TableReaderCaller caller,
+      const InternalKeyComparator& internal_comparator,
+      const SliceTransform* prefix_extractor = nullptr);
+
   // Release the handle from a cache
   void ReleaseHandle(Cache::Handle* handle);
 

db/version_set.cc

@@ -4974,19 +4974,12 @@ uint64_t VersionSet::ApproximateSize(Version* v, const FdWithKeyRange& f,
   } else {
     // "key" falls in the range for this table.  Add the
     // approximate offset of "key" within the table.
-    TableReader* table_reader_ptr;
-    InternalIterator* iter = v->cfd_->table_cache()->NewIterator(
-        ReadOptions(), v->env_options_, v->cfd_->internal_comparator(),
-        *f.file_metadata, nullptr /* range_del_agg */,
-        v->GetMutableCFOptions().prefix_extractor.get(), &table_reader_ptr,
-        /*file_read_hist=*/nullptr, caller,
-        /*arena=*/nullptr, /*skip_filters=*/false, /*level=*/-1,
-        /*smallest_compaction_key=*/nullptr,
-        /*largest_compaction_key=*/nullptr);
-    if (table_reader_ptr != nullptr) {
-      result = table_reader_ptr->ApproximateOffsetOf(key, caller);
-    }
-    delete iter;
+    TableCache* table_cache = v->cfd_->table_cache();
+    if (table_cache != nullptr) {
+      result = table_cache->ApproximateOffsetOf(
+          key, f.file_metadata->fd, caller, v->cfd()->internal_comparator(),
+          v->GetMutableCFOptions().prefix_extractor.get());
+    }
   }
   return result;
 }

db/version_set.h

@@ -655,7 +655,7 @@ class Version {
 
   uint64_t GetSstFilesSize();
 
-  MutableCFOptions GetMutableCFOptions() { return mutable_cf_options_; }
+  const MutableCFOptions& GetMutableCFOptions() { return mutable_cf_options_; }
 
  private:
   Env* env_;
@@ -981,7 +981,7 @@ class VersionSet {
   void AddLiveFiles(std::vector<FileDescriptor>* live_list);
 
   // Return the approximate size of data to be scanned for range [start, end)
-  // in levels [start_level, end_level). If end_level == 0 it will search
+  // in levels [start_level, end_level). If end_level == -1 it will search
   // through all non-empty levels
   uint64_t ApproximateSize(Version* v, const Slice& start, const Slice& end,
                            int start_level, int end_level,

table/block_based/block_based_table_reader.cc

@@ -4018,10 +4018,11 @@ Status BlockBasedTable::CreateIndexReader(
 uint64_t BlockBasedTable::ApproximateOffsetOf(const Slice& key,
                                               TableReaderCaller caller) {
   BlockCacheLookupContext context(caller);
-  std::unique_ptr<InternalIteratorBase<IndexValue>> index_iter(
+  IndexBlockIter iiter_on_stack;
+  auto index_iter =
       NewIndexIterator(ReadOptions(), /*need_upper_bound_check=*/false,
-                       /*input_iter=*/nullptr, /*get_context=*/nullptr,
-                       /*lookup_context=*/&context));
+                       /*input_iter=*/&iiter_on_stack, /*get_context=*/nullptr,
+                       /*lookup_context=*/&context);
 
   index_iter->Seek(key);
   uint64_t result;
@@ -4041,6 +4042,11 @@ uint64_t BlockBasedTable::ApproximateOffsetOf(const Slice& key,
       result = rep_->footer.metaindex_handle().offset();
     }
   }
+
+  if (index_iter != &iiter_on_stack) {
+    delete index_iter;
+  }
+
   return result;
 }
 

table/block_based/block_based_table_reader.h

@@ -318,8 +318,11 @@ class BlockBasedTable : public TableReader {
       BlockCacheLookupContext* lookup_context) const;
 
   // Get the iterator from the index reader.
-  // If input_iter is not set, return new Iterator
-  // If input_iter is set, update it and return it as Iterator
+  //
+  // If input_iter is not set, return a new Iterator.
+  // If input_iter is set, try to update it and return it as Iterator.
+  // However note that in some cases the returned iterator may be different
+  // from input_iter. In such case the returned iterator should be freed.
   //
   // Note: ErrorIterator with Status::Incomplete shall be returned if all the
   // following conditions are met: