Avoid doing a exhaustive search when looking for overlapping files.

Summary:
The Version::GetOverlappingInputs() is called multiple times in
the compaction code path. Eack invocation does a binary search
for overlapping files in the specified key range.
This patch remembers the offset of an overlapped file when
GetOverlappingInputs() is called the first time within
a compaction run. Suceeding calls to GetOverlappingInputs()
uses the remembered index to avoid the binary search.

I measured that 1000 iterations of GetOverlappingInputs
takes around 4500 microseconds without this patch. If I use
this patch with the hint on every invocation, then 1000
iterations take about 3900 microsecond.

Test Plan: make check OPT=-g

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: MarkCallaghan, emayanke, sheki

Differential Revision: https://reviews.facebook.net/D6513

db/version_set.cc

@@ -453,11 +453,16 @@ int Version::PickLevelForMemTableOutput(
 }
 
 // Store in "*inputs" all files in "level" that overlap [begin,end]
+// If hint_index is specified, then it points to a file in the 
+// overlapping range.
+// The file_index returns a pointer to any file in an overlapping range.
 void Version::GetOverlappingInputs(
     int level,
     const InternalKey* begin,
     const InternalKey* end,
-    std::vector<FileMetaData*>* inputs) {
+    std::vector<FileMetaData*>* inputs,
+    int hint_index,
+    int* file_index) {
   inputs->clear();
   Slice user_begin, user_end;
   if (begin != NULL) {
@@ -468,7 +473,8 @@ void Version::GetOverlappingInputs(
   }
   const Comparator* user_cmp = vset_->icmp_.user_comparator();
   if (begin != NULL && end != NULL && level > 0) {
-    GetOverlappingInputsBinarySearch(level, user_begin, user_end, inputs);
+    GetOverlappingInputsBinarySearch(level, user_begin, user_end, inputs,
+      hint_index, file_index);
     return;
   }
   for (size_t i = 0; i < files_[level].size(); ) {
@@ -493,6 +499,8 @@ void Version::GetOverlappingInputs(
           inputs->clear();
           i = 0;
         }
+      } else if (file_index) {
+        *file_index = i;
       }
     }
   }
@@ -506,14 +514,24 @@ void Version::GetOverlappingInputsBinarySearch(
     int level,
     const Slice& user_begin,
     const Slice& user_end,
-    std::vector<FileMetaData*>* inputs) {
+    std::vector<FileMetaData*>* inputs,
+    int hint_index,
+    int* file_index) {
   assert(level > 0);
   int min = 0;
   int mid = 0;
   int max = files_[level].size() -1;
   bool foundOverlap = false;
   const Comparator* user_cmp = vset_->icmp_.user_comparator();
-  while (min <= max) {
+
+  // if the caller already knows the index of a file that has overlap,
+  // then we can skip the binary search.
+  if (hint_index != -1) {
+    mid = hint_index;
+    foundOverlap = true;
+  }
+
+  while (!foundOverlap && min <= max) {
     mid = (min + max)/2;
     FileMetaData* f = files_[level][mid];
     const Slice file_start = f->smallest.user_key();
@@ -532,6 +550,10 @@ void Version::GetOverlappingInputsBinarySearch(
   if (!foundOverlap) {
     return;
   }
+  // returns the index where an overlap is found
+  if (file_index) {
+    *file_index = mid;
+  }
   ExtendOverlappingInputs(level, user_begin, user_end, inputs, mid);
 }
   
@@ -546,13 +568,21 @@ void Version::ExtendOverlappingInputs(
     std::vector<FileMetaData*>* inputs,
     int midIndex) {
 
-  // assert that the file at midIndex overlaps with the range
   const Comparator* user_cmp = vset_->icmp_.user_comparator();
-  assert(midIndex < files_[level].size());
+#ifndef NDEBUG
-  assert((user_cmp->Compare(files_[level][midIndex]->largest.user_key(),
+  {
-                           user_begin) >= 0) ||
+    // assert that the file at midIndex overlaps with the range
-         (user_cmp->Compare(files_[level][midIndex]->smallest.user_key(),
+    assert(midIndex < files_[level].size());
-                           user_end) <= 0));
+    FileMetaData* f = files_[level][midIndex];
+    const Slice fstart = f->smallest.user_key();
+    const Slice flimit = f->largest.user_key();
+    if (user_cmp->Compare(fstart, user_begin) >= 0) {
+      assert(user_cmp->Compare(fstart, user_end) <= 0);
+    } else {
+      assert(user_cmp->Compare(flimit, user_begin) >= 0);
+    }
+  }
+#endif
 
   // check backwards from 'mid' to lower indices
   for (size_t i = midIndex; i < files_[level].size(); i--) {
@@ -1487,12 +1517,14 @@ Compaction* VersionSet::PickCompaction() {
       if (compact_pointer_[level].empty() ||
           icmp_.Compare(f->largest.Encode(), compact_pointer_[level]) > 0) {
         c->inputs_[0].push_back(f);
+        c->base_index_ = i;
         break;
       }
     }
     if (c->inputs_[0].empty()) {
       // Wrap-around to the beginning of the key space
       c->inputs_[0].push_back(current_->files_[level][0]);
+      c->base_index_ = 0;
     }
   } else if (seek_compaction) {
     level = current_->file_to_compact_level_;
@@ -1527,7 +1559,8 @@ void VersionSet::SetupOtherInputs(Compaction* c) {
   InternalKey smallest, largest;
   GetRange(c->inputs_[0], &smallest, &largest);
 
-  current_->GetOverlappingInputs(level+1, &smallest, &largest, &c->inputs_[1]);
+  current_->GetOverlappingInputs(level+1, &smallest, &largest, &c->inputs_[1],
+                                 c->parent_index_, &c->parent_index_);
 
   // Get entire range covered by compaction
   InternalKey all_start, all_limit;
@@ -1537,7 +1570,8 @@ void VersionSet::SetupOtherInputs(Compaction* c) {
   // changing the number of "level+1" files we pick up.
   if (!c->inputs_[1].empty()) {
     std::vector<FileMetaData*> expanded0;
-    current_->GetOverlappingInputs(level, &all_start, &all_limit, &expanded0);
+    current_->GetOverlappingInputs(level, &all_start, &all_limit, &expanded0,
+                                   c->base_index_, NULL);
     const int64_t inputs0_size = TotalFileSize(c->inputs_[0]);
     const int64_t inputs1_size = TotalFileSize(c->inputs_[1]);
     const int64_t expanded0_size = TotalFileSize(expanded0);
@@ -1548,7 +1582,8 @@ void VersionSet::SetupOtherInputs(Compaction* c) {
       GetRange(expanded0, &new_start, &new_limit);
       std::vector<FileMetaData*> expanded1;
       current_->GetOverlappingInputs(level+1, &new_start, &new_limit,
-                                     &expanded1);
+                                     &expanded1, c->parent_index_,
+                                     &c->parent_index_);
       if (expanded1.size() == c->inputs_[1].size()) {
         Log(options_->info_log,
             "Expanding@%d %d+%d (%ld+%ld bytes) to %d+%d (%ld+%ld bytes)\n",
@@ -1632,7 +1667,9 @@ Compaction::Compaction(int level, uint64_t target_file_size,
       seek_compaction_(seek_compaction),
       grandparent_index_(0),
       seen_key_(false),
-      overlapped_bytes_(0) {
+      overlapped_bytes_(0),
+      base_index_(-1),
+      parent_index_(-1) {
   edit_ = new VersionEdit(number_levels_);
   level_ptrs_ = new size_t[number_levels_];
   for (int i = 0; i < number_levels_; i++) {

db/version_set.h

@@ -87,13 +87,17 @@ class Version {
       int level,
       const InternalKey* begin,         // NULL means before all keys
       const InternalKey* end,           // NULL means after all keys
-      std::vector<FileMetaData*>* inputs);
+      std::vector<FileMetaData*>* inputs,
+      int hint_index = -1,              // index of overlap file
+      int* file_index = NULL);          // return index of overlap file
 
   void GetOverlappingInputsBinarySearch(
       int level,
       const Slice& begin,         // NULL means before all keys
       const Slice& end,           // NULL means after all keys
-      std::vector<FileMetaData*>* inputs);
+      std::vector<FileMetaData*>* inputs,
+      int hint_index,             // index of overlap file
+      int* file_index);           // return index of overlap file
 
   void ExtendOverlappingInputs(
       int level,
@@ -430,6 +434,8 @@ class Compaction {
   bool seen_key_;             // Some output key has been seen
   int64_t overlapped_bytes_;  // Bytes of overlap between current output
                               // and grandparent files
+  int base_index_;   // index of the file in files_[level_]
+  int parent_index_; // index of some file with same range in files_[level_+1]
 
   // State for implementing IsBaseLevelForKey