diff --git a/db/c.cc b/db/c.cc
index 366dd2d..038e5c0 100644
--- a/db/c.cc
+++ b/db/c.cc
@@ -196,7 +196,8 @@ char* leveldb_property_value(
     const char* propname) {
   std::string tmp;
   if (db->rep->GetProperty(Slice(propname), &tmp)) {
-    return CopyString(tmp);
+    // We use strdup() since we expect human readable output.
+    return strdup(tmp.c_str());
   } else {
     return NULL;
   }
diff --git a/db/db_bench.cc b/db/db_bench.cc
index 7b4e41a..d3ec61b 100644
--- a/db/db_bench.cc
+++ b/db/db_bench.cc
@@ -14,6 +14,7 @@
 #include "port/port.h"
 #include "util/crc32c.h"
 #include "util/histogram.h"
+#include "util/mutexlock.h"
 #include "util/random.h"
 #include "util/testutil.h"
 
@@ -60,6 +61,9 @@ static int FLAGS_num = 1000000;
 // Number of read operations to do.  If negative, do FLAGS_num reads.
 static int FLAGS_reads = -1;
 
+// Number of concurrent threads to run.
+static int FLAGS_threads = 1;
+
 // Size of each value
 static int FLAGS_value_size = 100;
 
@@ -91,8 +95,9 @@ static const char* FLAGS_db = "/tmp/dbbench";
 
 namespace leveldb {
 
-// Helper for quickly generating random data.
 namespace {
+
+// Helper for quickly generating random data.
 class RandomGenerator {
  private:
   std::string data_;
@@ -136,6 +141,152 @@ static Slice TrimSpace(Slice s) {
   return Slice(s.data() + start, limit - start);
 }
 
+static void AppendWithSpace(std::string* str, Slice msg) {
+  if (msg.empty()) return;
+  if (!str->empty()) {
+    str->push_back(' ');
+  }
+  str->append(msg.data(), msg.size());
+}
+
+class Stats {
+ private:
+  double start_;
+  double finish_;
+  double seconds_;
+  int done_;
+  int next_report_;
+  int64_t bytes_;
+  double last_op_finish_;
+  Histogram hist_;
+  std::string message_;
+
+ public:
+  Stats() { Start(); }
+
+  void Start() {
+    next_report_ = 100;
+    last_op_finish_ = start_;
+    hist_.Clear();
+    done_ = 0;
+    bytes_ = 0;
+    seconds_ = 0;
+    start_ = Env::Default()->NowMicros();
+    finish_ = start_;
+    message_.clear();
+  }
+
+  void Merge(const Stats& other) {
+    hist_.Merge(other.hist_);
+    done_ += other.done_;
+    bytes_ += other.bytes_;
+    seconds_ += other.seconds_;
+    if (other.start_ < start_) start_ = other.start_;
+    if (other.finish_ > finish_) finish_ = other.finish_;
+
+    // Just keep the messages from one thread
+    if (message_.empty()) message_ = other.message_;
+  }
+
+  void Stop() {
+    finish_ = Env::Default()->NowMicros();
+    seconds_ = (finish_ - start_) * 1e-6;
+  }
+
+  void AddMessage(Slice msg) {
+    AppendWithSpace(&message_, msg);
+  }
+
+  void FinishedSingleOp() {
+    if (FLAGS_histogram) {
+      double now = Env::Default()->NowMicros();
+      double micros = now - last_op_finish_;
+      hist_.Add(micros);
+      if (micros > 20000) {
+        fprintf(stderr, "long op: %.1f micros%30s\r", micros, "");
+        fflush(stderr);
+      }
+      last_op_finish_ = now;
+    }
+
+    done_++;
+    if (done_ >= next_report_) {
+      if      (next_report_ < 1000)   next_report_ += 100;
+      else if (next_report_ < 5000)   next_report_ += 500;
+      else if (next_report_ < 10000)  next_report_ += 1000;
+      else if (next_report_ < 50000)  next_report_ += 5000;
+      else if (next_report_ < 100000) next_report_ += 10000;
+      else if (next_report_ < 500000) next_report_ += 50000;
+      else                            next_report_ += 100000;
+      fprintf(stderr, "... finished %d ops%30s\r", done_, "");
+      fflush(stderr);
+    }
+  }
+
+  void AddBytes(int64_t n) {
+    bytes_ += n;
+  }
+
+  void Report(const Slice& name) {
+    // Pretend at least one op was done in case we are running a benchmark
+    // that does not call FinishedSingleOp().
+    if (done_ < 1) done_ = 1;
+
+    std::string extra;
+    if (bytes_ > 0) {
+      // Rate is computed on actual elapsed time, not the sum of per-thread
+      // elapsed times.
+      double elapsed = (finish_ - start_) * 1e-6;
+      char rate[100];
+      snprintf(rate, sizeof(rate), "%6.1f MB/s",
+               (bytes_ / 1048576.0) / elapsed);
+      extra = rate;
+    }
+    AppendWithSpace(&extra, message_);
+
+    fprintf(stdout, "%-12s : %11.3f micros/op;%s%s\n",
+            name.ToString().c_str(),
+            seconds_ * 1e6 / done_,
+            (extra.empty() ? "" : " "),
+            extra.c_str());
+    if (FLAGS_histogram) {
+      fprintf(stdout, "Microseconds per op:\n%s\n", hist_.ToString().c_str());
+    }
+    fflush(stdout);
+  }
+};
+
+// State shared by all concurrent executions of the same benchmark.
+struct SharedState {
+  port::Mutex mu;
+  port::CondVar cv;
+  int total;
+
+  // Each thread goes through the following states:
+  //    (1) initializing
+  //    (2) waiting for others to be initialized
+  //    (3) running
+  //    (4) done
+
+  int num_initialized;
+  int num_done;
+  bool start;
+
+  SharedState() : cv(&mu) { }
+};
+
+// Per-thread state for concurrent executions of the same benchmark.
+struct ThreadState {
+  int tid;             // 0..n-1 when running in n threads
+  Random rand;         // Has different seeds for different threads
+  Stats stats;
+
+  ThreadState(int index)
+      : tid(index),
+        rand(1000 + index) {
+  }
+};
+
 }
 
 class Benchmark {
@@ -143,20 +294,11 @@ class Benchmark {
   Cache* cache_;
   DB* db_;
   int num_;
+  int value_size_;
+  int entries_per_batch_;
+  WriteOptions write_options_;
   int reads_;
   int heap_counter_;
-  double start_;
-  double last_op_finish_;
-  int64_t bytes_;
-  std::string message_;
-  std::string post_message_;
-  Histogram hist_;
-  RandomGenerator gen_;
-  Random rand_;
-
-  // State kept for progress messages
-  int done_;
-  int next_report_;     // When to report next
 
   void PrintHeader() {
     const int kKeySize = 16;
@@ -232,94 +374,15 @@ class Benchmark {
 #endif
   }
 
-  void Start() {
-    start_ = Env::Default()->NowMicros() * 1e-6;
-    bytes_ = 0;
-    message_.clear();
-    last_op_finish_ = start_;
-    hist_.Clear();
-    done_ = 0;
-    next_report_ = 100;
-  }
-
-  void FinishedSingleOp() {
-    if (FLAGS_histogram) {
-      double now = Env::Default()->NowMicros() * 1e-6;
-      double micros = (now - last_op_finish_) * 1e6;
-      hist_.Add(micros);
-      if (micros > 20000) {
-        fprintf(stderr, "long op: %.1f micros%30s\r", micros, "");
-        fflush(stderr);
-      }
-      last_op_finish_ = now;
-    }
-
-    done_++;
-    if (done_ >= next_report_) {
-      if      (next_report_ < 1000)   next_report_ += 100;
-      else if (next_report_ < 5000)   next_report_ += 500;
-      else if (next_report_ < 10000)  next_report_ += 1000;
-      else if (next_report_ < 50000)  next_report_ += 5000;
-      else if (next_report_ < 100000) next_report_ += 10000;
-      else if (next_report_ < 500000) next_report_ += 50000;
-      else                            next_report_ += 100000;
-      fprintf(stderr, "... finished %d ops%30s\r", done_, "");
-      fflush(stderr);
-    }
-  }
-
-  void Stop(const Slice& name) {
-    double finish = Env::Default()->NowMicros() * 1e-6;
-
-    // Pretend at least one op was done in case we are running a benchmark
-    // that does nto call FinishedSingleOp().
-    if (done_ < 1) done_ = 1;
-
-    if (bytes_ > 0) {
-      char rate[100];
-      snprintf(rate, sizeof(rate), "%6.1f MB/s",
-               (bytes_ / 1048576.0) / (finish - start_));
-      if (!message_.empty()) {
-        message_  = std::string(rate) + " " + message_;
-      } else {
-        message_ = rate;
-      }
-    }
-
-    fprintf(stdout, "%-12s : %11.3f micros/op;%s%s\n",
-            name.ToString().c_str(),
-            (finish - start_) * 1e6 / done_,
-            (message_.empty() ? "" : " "),
-            message_.c_str());
-    if (FLAGS_histogram) {
-      fprintf(stdout, "Microseconds per op:\n%s\n", hist_.ToString().c_str());
-    }
-    fflush(stdout);
-
-    if (!post_message_.empty()) {
-      fprintf(stdout, "\n%s\n", post_message_.c_str());
-      post_message_.clear();
-    }
-  }
-
  public:
-  enum Order {
-    SEQUENTIAL,
-    RANDOM
-  };
-  enum DBState {
-    FRESH,
-    EXISTING
-  };
-
   Benchmark()
   : cache_(FLAGS_cache_size >= 0 ? NewLRUCache(FLAGS_cache_size) : NULL),
     db_(NULL),
     num_(FLAGS_num),
+    value_size_(FLAGS_value_size),
+    entries_per_batch_(1),
     reads_(FLAGS_reads < 0 ? FLAGS_num : FLAGS_reads),
-    heap_counter_(0),
-    bytes_(0),
-    rand_(301) {
+    heap_counter_(0) {
     std::vector<std::string> files;
     Env::Default()->GetChildren(FLAGS_db, &files);
     for (int i = 0; i < files.size(); i++) {
@@ -353,98 +416,203 @@ class Benchmark {
         benchmarks = sep + 1;
       }
 
-      Start();
+      // Reset parameters that may be overriddden bwlow
+      num_ = FLAGS_num;
+      reads_ = num_;
+      value_size_ = FLAGS_value_size;
+      entries_per_batch_ = 1;
+      write_options_ = WriteOptions();
+
+      void (Benchmark::*method)(ThreadState*) = NULL;
+      bool fresh_db = false;
 
-      WriteOptions write_options;
-      bool known = true;
       if (name == Slice("fillseq")) {
-        Write(write_options, SEQUENTIAL, FRESH, num_, FLAGS_value_size, 1);
+        fresh_db = true;
+        method = &Benchmark::WriteSeq;
       } else if (name == Slice("fillbatch")) {
-        Write(write_options, SEQUENTIAL, FRESH, num_, FLAGS_value_size, 1000);
+        fresh_db = true;
+        entries_per_batch_ = 1000;
+        method = &Benchmark::WriteSeq;
       } else if (name == Slice("fillrandom")) {
-        Write(write_options, RANDOM, FRESH, num_, FLAGS_value_size, 1);
+        fresh_db = true;
+        method = &Benchmark::WriteRandom;
       } else if (name == Slice("overwrite")) {
-        Write(write_options, RANDOM, EXISTING, num_, FLAGS_value_size, 1);
+        fresh_db = false;
+        method = &Benchmark::WriteRandom;
       } else if (name == Slice("fillsync")) {
-        write_options.sync = true;
-        Write(write_options, RANDOM, FRESH, num_ / 1000, FLAGS_value_size, 1);
+        fresh_db = true;
+        num_ /= 1000;
+        write_options_.sync = true;
+        method = &Benchmark::WriteRandom;
       } else if (name == Slice("fill100K")) {
-        Write(write_options, RANDOM, FRESH, num_ / 1000, 100 * 1000, 1);
+        fresh_db = true;
+        num_ /= 1000;
+        value_size_ = 100 * 1000;
+        method = &Benchmark::WriteRandom;
       } else if (name == Slice("readseq")) {
-        ReadSequential();
+        method = &Benchmark::ReadSequential;
       } else if (name == Slice("readreverse")) {
-        ReadReverse();
+        method = &Benchmark::ReadReverse;
       } else if (name == Slice("readrandom")) {
-        ReadRandom();
+        method = &Benchmark::ReadRandom;
       } else if (name == Slice("readhot")) {
-        ReadHot();
+        method = &Benchmark::ReadHot;
       } else if (name == Slice("readrandomsmall")) {
-        int n = reads_;
         reads_ /= 1000;
-        ReadRandom();
-        reads_ = n;
+        method = &Benchmark::ReadRandom;
       } else if (name == Slice("compact")) {
-        Compact();
+        method = &Benchmark::Compact;
       } else if (name == Slice("crc32c")) {
-        Crc32c(4096, "(4K per op)");
+        method = &Benchmark::Crc32c;
       } else if (name == Slice("acquireload")) {
-        AcquireLoad();
+        method = &Benchmark::AcquireLoad;
       } else if (name == Slice("snappycomp")) {
-        SnappyCompress();
+        method = &Benchmark::SnappyCompress;
       } else if (name == Slice("snappyuncomp")) {
-        SnappyUncompress();
+        method = &Benchmark::SnappyUncompress;
       } else if (name == Slice("heapprofile")) {
         HeapProfile();
       } else if (name == Slice("stats")) {
         PrintStats();
       } else {
-        known = false;
         if (name != Slice()) {  // No error message for empty name
           fprintf(stderr, "unknown benchmark '%s'\n", name.ToString().c_str());
         }
       }
-      if (known) {
-        Stop(name);
+
+      if (fresh_db) {
+        if (FLAGS_use_existing_db) {
+          fprintf(stdout, "%-12s : skipped (--use_existing_db is true)\n",
+                  name.ToString().c_str());
+          method = NULL;
+        } else {
+          delete db_;
+          db_ = NULL;
+          DestroyDB(FLAGS_db, Options());
+          Open();
+        }
+      }
+
+      if (method != NULL) {
+        RunBenchmark(name, method);
       }
     }
   }
 
  private:
-  void Crc32c(int size, const char* label) {
+  struct ThreadArg {
+    Benchmark* bm;
+    SharedState* shared;
+    ThreadState* thread;
+    void (Benchmark::*method)(ThreadState*);
+  };
+
+  static void ThreadBody(void* v) {
+    ThreadArg* arg = reinterpret_cast<ThreadArg*>(v);
+    SharedState* shared = arg->shared;
+    ThreadState* thread = arg->thread;
+    {
+      MutexLock l(&shared->mu);
+      shared->num_initialized++;
+      if (shared->num_initialized >= shared->total) {
+        shared->cv.SignalAll();
+      }
+      while (!shared->start) {
+        shared->cv.Wait();
+      }
+    }
+
+    thread->stats.Start();
+    (arg->bm->*(arg->method))(thread);
+    thread->stats.Stop();
+
+    {
+      MutexLock l(&shared->mu);
+      shared->num_done++;
+      if (shared->num_done >= shared->total) {
+        shared->cv.SignalAll();
+      }
+    }
+  }
+
+  void RunBenchmark(Slice name, void (Benchmark::*method)(ThreadState*)) {
+    const int n = FLAGS_threads;
+    SharedState shared;
+    shared.total = n;
+    shared.num_initialized = 0;
+    shared.num_done = 0;
+    shared.start = false;
+
+    ThreadArg* arg = new ThreadArg[n];
+    for (int i = 0; i < n; i++) {
+      arg[i].bm = this;
+      arg[i].method = method;
+      arg[i].shared = &shared;
+      arg[i].thread = new ThreadState(i);
+      Env::Default()->StartThread(ThreadBody, &arg[i]);
+    }
+
+    shared.mu.Lock();
+    while (shared.num_initialized < n) {
+      shared.cv.Wait();
+    }
+
+    shared.start = true;
+    shared.cv.SignalAll();
+    while (shared.num_done < n) {
+      shared.cv.Wait();
+    }
+    shared.mu.Unlock();
+
+    for (int i = 1; i < n; i++) {
+      arg[0].thread->stats.Merge(arg[i].thread->stats);
+    }
+    arg[0].thread->stats.Report(name);
+
+    for (int i = 0; i < n; i++) {
+      delete arg[i].thread;
+    }
+    delete[] arg;
+  }
+
+  void Crc32c(ThreadState* thread) {
     // Checksum about 500MB of data total
+    const int size = 4096;
+    const char* label = "(4K per op)";
     std::string data(size, 'x');
     int64_t bytes = 0;
     uint32_t crc = 0;
     while (bytes < 500 * 1048576) {
       crc = crc32c::Value(data.data(), size);
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
       bytes += size;
     }
     // Print so result is not dead
     fprintf(stderr, "... crc=0x%x\r", static_cast<unsigned int>(crc));
 
-    bytes_ = bytes;
-    message_ = label;
+    thread->stats.AddBytes(bytes);
+    thread->stats.AddMessage(label);
   }
 
-  void AcquireLoad() {
+  void AcquireLoad(ThreadState* thread) {
     int dummy;
     port::AtomicPointer ap(&dummy);
     int count = 0;
     void *ptr = NULL;
-    message_ = "(each op is 1000 loads)";
+    thread->stats.AddMessage("(each op is 1000 loads)");
     while (count < 100000) {
       for (int i = 0; i < 1000; i++) {
         ptr = ap.Acquire_Load();
       }
       count++;
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
     }
     if (ptr == NULL) exit(1); // Disable unused variable warning.
   }
 
-  void SnappyCompress() {
-    Slice input = gen_.Generate(Options().block_size);
+  void SnappyCompress(ThreadState* thread) {
+    RandomGenerator gen;
+    Slice input = gen.Generate(Options().block_size);
     int64_t bytes = 0;
     int64_t produced = 0;
     bool ok = true;
@@ -453,22 +621,23 @@ class Benchmark {
       ok = port::Snappy_Compress(input.data(), input.size(), &compressed);
       produced += compressed.size();
       bytes += input.size();
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
     }
 
     if (!ok) {
-      message_ = "(snappy failure)";
+      thread->stats.AddMessage("(snappy failure)");
     } else {
       char buf[100];
       snprintf(buf, sizeof(buf), "(output: %.1f%%)",
                (produced * 100.0) / bytes);
-      message_ = buf;
-      bytes_ = bytes;
+      thread->stats.AddMessage(buf);
+      thread->stats.AddBytes(bytes);
     }
   }
 
-  void SnappyUncompress() {
-    Slice input = gen_.Generate(Options().block_size);
+  void SnappyUncompress(ThreadState* thread) {
+    RandomGenerator gen;
+    Slice input = gen.Generate(Options().block_size);
     std::string compressed;
     bool ok = port::Snappy_Compress(input.data(), input.size(), &compressed);
     int64_t bytes = 0;
@@ -477,14 +646,14 @@ class Benchmark {
       ok =  port::Snappy_Uncompress(compressed.data(), compressed.size(),
                                     uncompressed);
       bytes += input.size();
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
     }
     delete[] uncompressed;
 
     if (!ok) {
-      message_ = "(snappy failure)";
+      thread->stats.AddMessage("(snappy failure)");
     } else {
-      bytes_ = bytes;
+      thread->stats.AddBytes(bytes);
     }
   }
 
@@ -501,95 +670,97 @@ class Benchmark {
     }
   }
 
-  void Write(const WriteOptions& options, Order order, DBState state,
-             int num_entries, int value_size, int entries_per_batch) {
-    if (state == FRESH) {
-      if (FLAGS_use_existing_db) {
-        message_ = "skipping (--use_existing_db is true)";
-        return;
-      }
-      delete db_;
-      db_ = NULL;
-      DestroyDB(FLAGS_db, Options());
-      Open();
-      Start();  // Do not count time taken to destroy/open
-    }
+  void WriteSeq(ThreadState* thread) {
+    DoWrite(thread, true);
+  }
 
-    if (num_entries != num_) {
+  void WriteRandom(ThreadState* thread) {
+    DoWrite(thread, false);
+  }
+
+  void DoWrite(ThreadState* thread, bool seq) {
+    if (num_ != FLAGS_num) {
       char msg[100];
-      snprintf(msg, sizeof(msg), "(%d ops)", num_entries);
-      message_ = msg;
+      snprintf(msg, sizeof(msg), "(%d ops)", num_);
+      thread->stats.AddMessage(msg);
     }
 
+    RandomGenerator gen;
     WriteBatch batch;
     Status s;
     std::string val;
-    for (int i = 0; i < num_entries; i += entries_per_batch) {
+    int64_t bytes = 0;
+    for (int i = 0; i < num_; i += entries_per_batch_) {
       batch.Clear();
-      for (int j = 0; j < entries_per_batch; j++) {
-        const int k = (order == SEQUENTIAL) ? i+j : (rand_.Next() % FLAGS_num);
+      for (int j = 0; j < entries_per_batch_; j++) {
+        const int k = seq ? i+j : (thread->rand.Next() % FLAGS_num);
         char key[100];
         snprintf(key, sizeof(key), "%016d", k);
-        batch.Put(key, gen_.Generate(value_size));
-        bytes_ += value_size + strlen(key);
-        FinishedSingleOp();
+        batch.Put(key, gen.Generate(value_size_));
+        bytes += value_size_ + strlen(key);
+        thread->stats.FinishedSingleOp();
       }
-      s = db_->Write(options, &batch);
+      s = db_->Write(write_options_, &batch);
       if (!s.ok()) {
         fprintf(stderr, "put error: %s\n", s.ToString().c_str());
         exit(1);
       }
     }
+    thread->stats.AddBytes(bytes);
   }
 
-  void ReadSequential() {
+  void ReadSequential(ThreadState* thread) {
     Iterator* iter = db_->NewIterator(ReadOptions());
     int i = 0;
+    int64_t bytes = 0;
     for (iter->SeekToFirst(); i < reads_ && iter->Valid(); iter->Next()) {
-      bytes_ += iter->key().size() + iter->value().size();
-      FinishedSingleOp();
+      bytes += iter->key().size() + iter->value().size();
+      thread->stats.FinishedSingleOp();
       ++i;
     }
     delete iter;
+    thread->stats.AddBytes(bytes);
   }
 
-  void ReadReverse() {
+  void ReadReverse(ThreadState* thread) {
     Iterator* iter = db_->NewIterator(ReadOptions());
     int i = 0;
+    int64_t bytes = 0;
     for (iter->SeekToLast(); i < reads_ && iter->Valid(); iter->Prev()) {
-      bytes_ += iter->key().size() + iter->value().size();
-      FinishedSingleOp();
+      bytes += iter->key().size() + iter->value().size();
+      thread->stats.FinishedSingleOp();
       ++i;
     }
     delete iter;
+    thread->stats.AddBytes(bytes);
   }
 
-  void ReadRandom() {
+  void ReadRandom(ThreadState* thread) {
     ReadOptions options;
     std::string value;
     for (int i = 0; i < reads_; i++) {
       char key[100];
-      const int k = rand_.Next() % FLAGS_num;
+      const int k = thread->rand.Next() % FLAGS_num;
       snprintf(key, sizeof(key), "%016d", k);
       db_->Get(options, key, &value);
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
     }
   }
 
-  void ReadHot() {
+  void ReadHot(ThreadState* thread) {
     ReadOptions options;
     std::string value;
     const int range = (FLAGS_num + 99) / 100;
     for (int i = 0; i < reads_; i++) {
       char key[100];
-      const int k = rand_.Next() % range;
+      const int k = thread->rand.Next() % range;
       snprintf(key, sizeof(key), "%016d", k);
       db_->Get(options, key, &value);
-      FinishedSingleOp();
+      thread->stats.FinishedSingleOp();
     }
   }
 
-  void Compact() {
+  void Compact(ThreadState* thread) {
     DBImpl* dbi = reinterpret_cast<DBImpl*>(db_);
     dbi->TEST_CompactMemTable();
     int max_level_with_files = 1;
@@ -609,10 +780,9 @@ class Benchmark {
   void PrintStats() {
     std::string stats;
     if (!db_->GetProperty("leveldb.stats", &stats)) {
-      message_ = "(failed)";
-    } else {
-      post_message_ = stats;
+      stats = "(failed)";
     }
+    fprintf(stdout, "\n%s\n", stats.c_str());
   }
 
   static void WriteToFile(void* arg, const char* buf, int n) {
@@ -625,13 +795,13 @@ class Benchmark {
     WritableFile* file;
     Status s = Env::Default()->NewWritableFile(fname, &file);
     if (!s.ok()) {
-      message_ = s.ToString();
+      fprintf(stderr, "%s\n", s.ToString().c_str());
       return;
     }
     bool ok = port::GetHeapProfile(WriteToFile, file);
     delete file;
     if (!ok) {
-      message_ = "not supported";
+      fprintf(stderr, "heap profiling not supported\n");
       Env::Default()->DeleteFile(fname);
     }
   }
@@ -661,6 +831,8 @@ int main(int argc, char** argv) {
       FLAGS_num = n;
     } else if (sscanf(argv[i], "--reads=%d%c", &n, &junk) == 1) {
       FLAGS_reads = n;
+    } else if (sscanf(argv[i], "--threads=%d%c", &n, &junk) == 1) {
+      FLAGS_threads = n;
     } else if (sscanf(argv[i], "--value_size=%d%c", &n, &junk) == 1) {
       FLAGS_value_size = n;
     } else if (sscanf(argv[i], "--write_buffer_size=%d%c", &n, &junk) == 1) {
diff --git a/db/db_impl.cc b/db/db_impl.cc
index fff4eaf..c4c6a61 100644
--- a/db/db_impl.cc
+++ b/db/db_impl.cc
@@ -989,27 +989,37 @@ Status DBImpl::Get(const ReadOptions& options,
     snapshot = versions_->LastSequence();
   }
 
-  // First look in the memtable, then in the immutable memtable (if any).
-  LookupKey lkey(key, snapshot);
-  if (mem_->Get(lkey, value, &s)) {
-    return s;
-  }
-  if (imm_ != NULL && imm_->Get(lkey, value, &s)) {
-    return s;
-  }
-
-  // Not in memtable(s); try live files in level order
+  MemTable* mem = mem_;
+  MemTable* imm = imm_;
   Version* current = versions_->current();
+  mem->Ref();
+  if (imm != NULL) imm->Ref();
   current->Ref();
+
+  bool have_stat_update = false;
   Version::GetStats stats;
-  { // Unlock while reading from files
+
+  // Unlock while reading from files and memtables
+  {
     mutex_.Unlock();
-    s = current->Get(options, lkey, value, &stats);
+    // First look in the memtable, then in the immutable memtable (if any).
+    LookupKey lkey(key, snapshot);
+    if (mem_->Get(lkey, value, &s)) {
+      // Done
+    } else if (imm_ != NULL && imm_->Get(lkey, value, &s)) {
+      // Done
+    } else {
+      s = current->Get(options, lkey, value, &stats);
+      have_stat_update = true;
+    }
     mutex_.Lock();
   }
-  if (current->UpdateStats(stats)) {
+
+  if (have_stat_update && current->UpdateStats(stats)) {
     MaybeScheduleCompaction();
   }
+  mem->Unref();
+  if (imm != NULL) imm->Unref();
   current->Unref();
   return s;
 }
diff --git a/util/cache.cc b/util/cache.cc
index 5cff3dd..ce99f08 100644
--- a/util/cache.cc
+++ b/util/cache.cc
@@ -30,7 +30,8 @@ struct LRUHandle {
   LRUHandle* prev;
   size_t charge;      // TODO(opt): Only allow uint32_t?
   size_t key_length;
-  size_t refs;        // TODO(opt): Pack with "key_length"?
+  uint32_t refs;
+  uint32_t hash;      // Hash of key(); used for fast sharding and comparisons
   char key_data[1];   // Beginning of key
 
   Slice key() const {
@@ -54,12 +55,12 @@ class HandleTable {
   HandleTable() : length_(0), elems_(0), list_(NULL) { Resize(); }
   ~HandleTable() { delete[] list_; }
 
-  LRUHandle* Lookup(LRUHandle* h) {
-    return *FindPointer(h);
+  LRUHandle* Lookup(const Slice& key, uint32_t hash) {
+    return *FindPointer(key, hash);
   }
 
   LRUHandle* Insert(LRUHandle* h) {
-    LRUHandle** ptr = FindPointer(h);
+    LRUHandle** ptr = FindPointer(h->key(), h->hash);
     LRUHandle* old = *ptr;
     h->next_hash = (old == NULL ? NULL : old->next_hash);
     *ptr = h;
@@ -74,8 +75,8 @@ class HandleTable {
     return old;
   }
 
-  LRUHandle* Remove(LRUHandle* h) {
-    LRUHandle** ptr = FindPointer(h);
+  LRUHandle* Remove(const Slice& key, uint32_t hash) {
+    LRUHandle** ptr = FindPointer(key, hash);
     LRUHandle* result = *ptr;
     if (result != NULL) {
       *ptr = result->next_hash;
@@ -92,13 +93,12 @@ class HandleTable {
   LRUHandle** list_;
 
   // Return a pointer to slot that points to a cache entry that
-  // matches *h.  If there is no such cache entry, return a pointer to
-  // the trailing slot in the corresponding linked list.
-  LRUHandle** FindPointer(LRUHandle* h) {
-    Slice key = h->key();
-    uint32_t hash = Hash(key.data(), key.size(), 0);
+  // matches key/hash.  If there is no such cache entry, return a
+  // pointer to the trailing slot in the corresponding linked list.
+  LRUHandle** FindPointer(const Slice& key, uint32_t hash) {
     LRUHandle** ptr = &list_[hash & (length_ - 1)];
-    while (*ptr != NULL && key != (*ptr)->key()) {
+    while (*ptr != NULL &&
+           ((*ptr)->hash != hash || key != (*ptr)->key())) {
       ptr = &(*ptr)->next_hash;
     }
     return ptr;
@@ -117,7 +117,7 @@ class HandleTable {
       while (h != NULL) {
         LRUHandle* next = h->next_hash;
         Slice key = h->key();
-        uint32_t hash = Hash(key.data(), key.size(), 0);
+        uint32_t hash = h->hash;
         LRUHandle** ptr = &new_list[hash & (new_length - 1)];
         h->next_hash = *ptr;
         *ptr = h;
@@ -132,26 +132,30 @@ class HandleTable {
   }
 };
 
-class LRUCache : public Cache {
+// A single shard of sharded cache.
+class LRUCache {
  public:
-  explicit LRUCache(size_t capacity);
-  virtual ~LRUCache();
+  LRUCache();
+  ~LRUCache();
 
-  virtual Handle* Insert(const Slice& key, void* value, size_t charge,
-                         void (*deleter)(const Slice& key, void* value));
-  virtual Handle* Lookup(const Slice& key);
-  virtual void Release(Handle* handle);
-  virtual void* Value(Handle* handle);
-  virtual void Erase(const Slice& key);
-  virtual uint64_t NewId();
+  // Separate from constructor so caller can easily make an array of LRUCache
+  void SetCapacity(size_t capacity) { capacity_ = capacity; }
+
+  // Like Cache methods, but with an extra "hash" parameter.
+  Cache::Handle* Insert(const Slice& key, uint32_t hash,
+                        void* value, size_t charge,
+                        void (*deleter)(const Slice& key, void* value));
+  Cache::Handle* Lookup(const Slice& key, uint32_t hash);
+  void Release(Cache::Handle* handle);
+  void Erase(const Slice& key, uint32_t hash);
 
  private:
   void LRU_Remove(LRUHandle* e);
   void LRU_Append(LRUHandle* e);
   void Unref(LRUHandle* e);
 
-  // Constructor parameters
-  const size_t capacity_;
+  // Initialized before use.
+  size_t capacity_;
 
   // mutex_ protects the following state.
   port::Mutex mutex_;
@@ -165,9 +169,8 @@ class LRUCache : public Cache {
   HandleTable table_;
 };
 
-LRUCache::LRUCache(size_t capacity)
-    : capacity_(capacity),
-      usage_(0),
+LRUCache::LRUCache()
+    : usage_(0),
       last_id_(0) {
   // Make empty circular linked list
   lru_.next = &lru_;
@@ -206,32 +209,25 @@ void LRUCache::LRU_Append(LRUHandle* e) {
   e->next->prev = e;
 }
 
-Cache::Handle* LRUCache::Lookup(const Slice& key) {
+Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {
   MutexLock l(&mutex_);
-
-  LRUHandle dummy;
-  dummy.next = &dummy;
-  dummy.value = const_cast<Slice*>(&key);
-  LRUHandle* e = table_.Lookup(&dummy);
+  LRUHandle* e = table_.Lookup(key, hash);
   if (e != NULL) {
     e->refs++;
     LRU_Remove(e);
     LRU_Append(e);
   }
-  return reinterpret_cast<Handle*>(e);
+  return reinterpret_cast<Cache::Handle*>(e);
 }
 
-void* LRUCache::Value(Handle* handle) {
-  return reinterpret_cast<LRUHandle*>(handle)->value;
-}
-
-void LRUCache::Release(Handle* handle) {
+void LRUCache::Release(Cache::Handle* handle) {
   MutexLock l(&mutex_);
   Unref(reinterpret_cast<LRUHandle*>(handle));
 }
 
-Cache::Handle* LRUCache::Insert(const Slice& key, void* value, size_t charge,
-                             void (*deleter)(const Slice& key, void* value)) {
+Cache::Handle* LRUCache::Insert(
+    const Slice& key, uint32_t hash, void* value, size_t charge,
+    void (*deleter)(const Slice& key, void* value)) {
   MutexLock l(&mutex_);
 
   LRUHandle* e = reinterpret_cast<LRUHandle*>(
@@ -240,6 +236,7 @@ Cache::Handle* LRUCache::Insert(const Slice& key, void* value, size_t charge,
   e->deleter = deleter;
   e->charge = charge;
   e->key_length = key.size();
+  e->hash = hash;
   e->refs = 2;  // One from LRUCache, one for the returned handle
   memcpy(e->key_data, key.data(), key.size());
   LRU_Append(e);
@@ -254,35 +251,77 @@ Cache::Handle* LRUCache::Insert(const Slice& key, void* value, size_t charge,
   while (usage_ > capacity_ && lru_.next != &lru_) {
     LRUHandle* old = lru_.next;
     LRU_Remove(old);
-    table_.Remove(old);
+    table_.Remove(old->key(), old->hash);
     Unref(old);
   }
 
-  return reinterpret_cast<Handle*>(e);
+  return reinterpret_cast<Cache::Handle*>(e);
 }
 
-void LRUCache::Erase(const Slice& key) {
+void LRUCache::Erase(const Slice& key, uint32_t hash) {
   MutexLock l(&mutex_);
-
-  LRUHandle dummy;
-  dummy.next = &dummy;
-  dummy.value = const_cast<Slice*>(&key);
-  LRUHandle* e = table_.Remove(&dummy);
+  LRUHandle* e = table_.Remove(key, hash);
   if (e != NULL) {
     LRU_Remove(e);
     Unref(e);
   }
 }
 
-uint64_t LRUCache::NewId() {
-  MutexLock l(&mutex_);
-  return ++(last_id_);
-}
+static const int kNumShardBits = 4;
+static const int kNumShards = 1 << kNumShardBits;
+
+class ShardedLRUCache : public Cache {
+ private:
+  LRUCache shard_[kNumShards];
+  port::Mutex id_mutex_;
+  uint64_t last_id_;
+
+  static inline uint32_t HashSlice(const Slice& s) {
+    return Hash(s.data(), s.size(), 0);
+  }
+
+  static uint32_t Shard(uint32_t hash) {
+    return hash >> (32 - kNumShardBits);
+  }
+
+ public:
+  explicit ShardedLRUCache(size_t capacity) {
+    const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
+    for (int s = 0; s < kNumShards; s++) {
+      shard_[s].SetCapacity(per_shard);
+    }
+  }
+  virtual ~ShardedLRUCache() { }
+  virtual Handle* Insert(const Slice& key, void* value, size_t charge,
+                         void (*deleter)(const Slice& key, void* value)) {
+    const uint32_t hash = HashSlice(key);
+    return shard_[Shard(hash)].Insert(key, hash, value, charge, deleter);
+  }
+  virtual Handle* Lookup(const Slice& key) {
+    const uint32_t hash = HashSlice(key);
+    return shard_[Shard(hash)].Lookup(key, hash);
+  }
+  virtual void Release(Handle* handle) {
+    LRUHandle* h = reinterpret_cast<LRUHandle*>(handle);
+    shard_[Shard(h->hash)].Release(handle);
+  }
+  virtual void Erase(const Slice& key) {
+    const uint32_t hash = HashSlice(key);
+    shard_[Shard(hash)].Erase(key, hash);
+  }
+  virtual void* Value(Handle* handle) {
+    return reinterpret_cast<LRUHandle*>(handle)->value;
+  }
+  virtual uint64_t NewId() {
+    MutexLock l(&id_mutex_);
+    return ++(last_id_);
+  }
+};
 
 }  // end anonymous namespace
 
 Cache* NewLRUCache(size_t capacity) {
-  return new LRUCache(capacity);
+  return new ShardedLRUCache(capacity);
 }
 
 }
diff --git a/util/cache_test.cc b/util/cache_test.cc
index dbab988..8a7f1c4 100644
--- a/util/cache_test.cc
+++ b/util/cache_test.cc
@@ -32,7 +32,7 @@ class CacheTest {
     current_->deleted_values_.push_back(DecodeValue(v));
   }
 
-  static const int kCacheSize = 100;
+  static const int kCacheSize = 1000;
   std::vector<int> deleted_keys_;
   std::vector<int> deleted_values_;
   Cache* cache_;
@@ -137,23 +137,40 @@ TEST(CacheTest, EvictionPolicy) {
   Insert(200, 201);
 
   // Frequently used entry must be kept around
-  for (int i = 0; i < kCacheSize; i++) {
+  for (int i = 0; i < kCacheSize + 100; i++) {
     Insert(1000+i, 2000+i);
     ASSERT_EQ(2000+i, Lookup(1000+i));
     ASSERT_EQ(101, Lookup(100));
   }
   ASSERT_EQ(101, Lookup(100));
-  ASSERT_EQ(2, deleted_keys_.size());
-  ASSERT_EQ(200, deleted_keys_[0]);
-  ASSERT_EQ(201, deleted_values_[0]);
+  ASSERT_EQ(-1, Lookup(200));
 }
 
-TEST(CacheTest, HeavyEntry) {
-  Insert(100, 101);
-  Insert(200, 201, kCacheSize);
-  ASSERT_EQ(1, deleted_keys_.size());
-  ASSERT_EQ(100, deleted_keys_[0]);
-  ASSERT_EQ(101, deleted_values_[0]);
+TEST(CacheTest, HeavyEntries) {
+  // Add a bunch of light and heavy entries and then count the combined
+  // size of items still in the cache, which must be approximately the
+  // same as the total capacity.
+  const int kLight = 1;
+  const int kHeavy = 10;
+  int added = 0;
+  int index = 0;
+  while (added < 2*kCacheSize) {
+    const int weight = (index & 1) ? kLight : kHeavy;
+    Insert(index, 1000+index, weight);
+    added += weight;
+    index++;
+  }
+
+  int cached_weight = 0;
+  for (int i = 0; i < index; i++) {
+    const int weight = (i & 1 ? kLight : kHeavy);
+    int r = Lookup(i);
+    if (r >= 0) {
+      cached_weight += weight;
+      ASSERT_EQ(1000+i, r);
+    }
+  }
+  ASSERT_LE(cached_weight, kCacheSize + kCacheSize/10);
 }
 
 TEST(CacheTest, NewId) {
diff --git a/util/histogram.cc b/util/histogram.cc
index c5178ef..12ec3cf 100644
--- a/util/histogram.cc
+++ b/util/histogram.cc
@@ -55,6 +55,17 @@ void Histogram::Add(double value) {
   sum_squares_ += (value * value);
 }
 
+void Histogram::Merge(const Histogram& other) {
+  if (other.min_ < min_) min_ = other.min_;
+  if (other.max_ > max_) max_ = other.max_;
+  num_ += other.num_;
+  sum_ += other.sum_;
+  sum_squares_ += other.sum_squares_;
+  for (int b = 0; b < kNumBuckets; b++) {
+    buckets_[b] += other.buckets_[b];
+  }
+}
+
 double Histogram::Median() const {
   return Percentile(50.0);
 }
diff --git a/util/histogram.h b/util/histogram.h
index f72f122..32484c0 100644
--- a/util/histogram.h
+++ b/util/histogram.h
@@ -16,6 +16,7 @@ class Histogram {
 
   void Clear();
   void Add(double value);
+  void Merge(const Histogram& other);
 
   std::string ToString() const;