Bugfix for issue 33; reduce lock contention in Get(), parallel benchmarks.

- Fix for issue 33 (non-null-terminated result from leveldb_property_value()) - Support for running multiple instances of a benchmark in parallel. - Reduce lock contention on Get(): (1) Do not hold the lock while searching memtables. (2) Shard block and table caches 16-ways. Benchmark for evaluating this change: $ db_bench --benchmarks=fillseq1,readrandom --threads=$n (fillseq1 is a small hack to make sure fillseq runs once regardless of number of threads specified on the command line). git-svn-id: https://leveldb.googlecode.com/svn/trunk@49 62dab493-f737-651d-591e-8d6aee1b9529
13 lat temu · e3584f9c28
--- 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;
  }
--- 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) {
--- 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;
 }
--- 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:
  explicitn> 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);
 }

 }
--- 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) {
--- 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);
 }
--- 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;