From aaf9c6203c21b9056b3bf4654fee58ad8473240b Mon Sep 17 00:00:00 2001
From: Siying Dong <siying.d@fb.com>
Date: Wed, 11 Dec 2013 11:56:36 -0800
Subject: [PATCH] [RocksDB][Performance Branch]Iterator Cleanup method only
 tries to find obsolete files if it has the last reference to a version

Summary: When deconstructing an iterator, no need to check obsolete file if it doesn't hold last reference of any version.

Test Plan: make all check

Reviewers: haobo, igor, dhruba, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14595
---
 db/db_impl.cc     | 7 ++++---
 db/version_set.cc | 4 +++-
 db/version_set.h  | 4 +++-
 3 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/db/db_impl.cc b/db/db_impl.cc
index 895bb20cc..192232483 100644
--- a/db/db_impl.cc
+++ b/db/db_impl.cc
@@ -2515,9 +2515,10 @@ static void CleanupIteratorState(void* arg1, void* arg2) {
       deletion_state.memtables_to_free.push_back(m);
     }
   }
-  state->version->Unref();
-  // fast path FindObsoleteFiles
-  state->db->FindObsoleteFiles(deletion_state, false, true);
+  if (state->version->Unref()) {
+    // fast path FindObsoleteFiles
+    state->db->FindObsoleteFiles(deletion_state, false, true);
+  }
   state->mu->Unlock();
   state->db->PurgeObsoleteFiles(deletion_state);
   delete state;
diff --git a/db/version_set.cc b/db/version_set.cc
index 653695caf..74a33b6fb 100644
--- a/db/version_set.cc
+++ b/db/version_set.cc
@@ -591,13 +591,15 @@ void Version::Ref() {
   ++refs_;
 }
 
-void Version::Unref() {
+bool Version::Unref() {
   assert(this != &vset_->dummy_versions_);
   assert(refs_ >= 1);
   --refs_;
   if (refs_ == 0) {
     delete this;
+    return true;
   }
+  return false;
 }
 
 bool Version::OverlapInLevel(int level,
diff --git a/db/version_set.h b/db/version_set.h
index bf466a932..aab4b82bc 100644
--- a/db/version_set.h
+++ b/db/version_set.h
@@ -89,7 +89,9 @@ class Version {
   // Reference count management (so Versions do not disappear out from
   // under live iterators)
   void Ref();
-  void Unref();
+  // Decrease reference count. Delete the object if no reference left
+  // and return true. Otherwise, return false.
+  bool Unref();
 
   void GetOverlappingInputs(
       int level,