Bugfixes: for Get(), don't hold mutex while writing log.

- Fix bug in Get: when it triggers a compaction, it could sometimes
  mark the compaction with the wrong level (if there was a gap
  in the set of levels examined for the Get).

- Do not hold mutex while writing to the log file or to the
  MANIFEST file.

  Added a new benchmark that runs a writer thread concurrently with
  reader threads.

  Percentiles
  ------------------------------
  micros/op: avg  median 99   99.9  99.99  99.999 max
  ------------------------------------------------------
  before:    42   38     110  225   32000  42000  48000
  after:     24   20     55   65    130    1100   7000

- Fixed race in optimized Get.  It should have been using the
  pinned memtables, not the current memtables.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@50 62dab493-f737-651d-591e-8d6aee1b9529

db/db_impl.cc

@@ -113,6 +113,8 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
       logfile_(NULL),
       logfile_number_(0),
       log_(NULL),
+      logger_(NULL),
+      logger_cv_(&mutex_),
       bg_compaction_scheduled_(false),
       manual_compaction_(NULL) {
   mem_->Ref();
@@ -308,6 +310,11 @@ Status DBImpl::Recover(VersionEdit* edit) {
     std::sort(logs.begin(), logs.end());
     for (size_t i = 0; i < logs.size(); i++) {
       s = RecoverLogFile(logs[i], edit, &max_sequence);
+
+      // The previous incarnation may not have written any MANIFEST
+      // records after allocating this log number.  So we manually
+      // update the file number allocation counter in VersionSet.
+      versions_->MarkFileNumberUsed(logs[i]);
     }
 
     if (s.ok()) {
@@ -485,7 +492,7 @@ Status DBImpl::CompactMemTable() {
   if (s.ok()) {
     edit.SetPrevLogNumber(0);
     edit.SetLogNumber(logfile_number_);  // Earlier logs no longer needed
-    s = versions_->LogAndApply(&edit);
+    s = versions_->LogAndApply(&edit, &mutex_);
   }
 
   if (s.ok()) {
@@ -523,7 +530,10 @@ void DBImpl::TEST_CompactRange(
 
 Status DBImpl::TEST_CompactMemTable() {
   MutexLock l(&mutex_);
+  LoggerId self;
+  AcquireLoggingResponsibility(&self);
   Status s = MakeRoomForWrite(true /* force compaction */);
+  ReleaseLoggingResponsibility(&self);
   if (s.ok()) {
     // Wait until the compaction completes
     while (imm_ != NULL && bg_error_.ok()) {
@@ -600,7 +610,7 @@ void DBImpl::BackgroundCompaction() {
     c->edit()->DeleteFile(c->level(), f->number);
     c->edit()->AddFile(c->level() + 1, f->number, f->file_size,
                        f->smallest, f->largest);
-    status = versions_->LogAndApply(c->edit());
+    status = versions_->LogAndApply(c->edit(), &mutex_);
     VersionSet::LevelSummaryStorage tmp;
     Log(options_.info_log, "Moved #%lld to level-%d %lld bytes %s: %s\n",
         static_cast<unsigned long long>(f->number),
@@ -748,7 +758,7 @@ Status DBImpl::InstallCompactionResults(CompactionState* compact) {
   }
   compact->outputs.clear();
 
-  Status s = versions_->LogAndApply(compact->compaction->edit());
+  Status s = versions_->LogAndApply(compact->compaction->edit(), &mutex_);
   if (s.ok()) {
     compact->compaction->ReleaseInputs();
     DeleteObsoleteFiles();
@@ -1004,9 +1014,9 @@ Status DBImpl::Get(const ReadOptions& options,
     mutex_.Unlock();
     // First look in the memtable, then in the immutable memtable (if any).
     LookupKey lkey(key, snapshot);
-    if (mem_->Get(lkey, value, &s)) {
+    if (mem->Get(lkey, value, &s)) {
       // Done
-    } else if (imm_ != NULL && imm_->Get(lkey, value, &s)) {
+    } else if (imm != NULL && imm->Get(lkey, value, &s)) {
       // Done
     } else {
       s = current->Get(options, lkey, value, &stats);
@@ -1053,34 +1063,65 @@ Status DBImpl::Delete(const WriteOptions& options, const Slice& key) {
   return DB::Delete(options, key);
 }
 
+// There is at most one thread that is the current logger.  This call
+// waits until preceding logger(s) have finished and becomes the
+// current logger.
+void DBImpl::AcquireLoggingResponsibility(LoggerId* self) {
+  while (logger_ != NULL) {
+    logger_cv_.Wait();
+  }
+  logger_ = self;
+}
+
+void DBImpl::ReleaseLoggingResponsibility(LoggerId* self) {
+  assert(logger_ == self);
+  logger_ = NULL;
+  logger_cv_.SignalAll();
+}
+
 Status DBImpl::Write(const WriteOptions& options, WriteBatch* updates) {
   Status status;
   MutexLock l(&mutex_);
+  LoggerId self;
+  AcquireLoggingResponsibility(&self);
   status = MakeRoomForWrite(false);  // May temporarily release lock and wait
   uint64_t last_sequence = versions_->LastSequence();
   if (status.ok()) {
     WriteBatchInternal::SetSequence(updates, last_sequence + 1);
     last_sequence += WriteBatchInternal::Count(updates);
-    versions_->SetLastSequence(last_sequence);
 
-    // Add to log and apply to memtable
-    status = log_->AddRecord(WriteBatchInternal::Contents(updates));
-    if (status.ok() && options.sync) {
-      status = logfile_->Sync();
-    }
-    if (status.ok()) {
-      status = WriteBatchInternal::InsertInto(updates, mem_);
+    // Add to log and apply to memtable.  We can release the lock during
+    // this phase since the "logger_" flag protects against concurrent
+    // loggers and concurrent writes into mem_.
+    {
+      assert(logger_ == &self);
+      mutex_.Unlock();
+      status = log_->AddRecord(WriteBatchInternal::Contents(updates));
+      if (status.ok() && options.sync) {
+        status = logfile_->Sync();
+      }
+      if (status.ok()) {
+        status = WriteBatchInternal::InsertInto(updates, mem_);
+      }
+      mutex_.Lock();
+      assert(logger_ == &self);
     }
+
+    versions_->SetLastSequence(last_sequence);
   }
   if (options.post_write_snapshot != NULL) {
     *options.post_write_snapshot =
         status.ok() ? snapshots_.New(last_sequence) : NULL;
   }
+  ReleaseLoggingResponsibility(&self);
   return status;
 }
 
+// REQUIRES: mutex_ is held
+// REQUIRES: this thread is the current logger
 Status DBImpl::MakeRoomForWrite(bool force) {
   mutex_.AssertHeld();
+  assert(logger_ != NULL);
   bool allow_delay = !force;
   Status s;
   while (true) {
@@ -1249,7 +1290,7 @@ Status DB::Open(const Options& options, const std::string& dbname,
       impl->logfile_ = lfile;
       impl->logfile_number_ = new_log_number;
       impl->log_ = new log::Writer(lfile);
-      s = impl->versions_->LogAndApply(&edit);
+      s = impl->versions_->LogAndApply(&edit, &impl->mutex_);
     }
     if (s.ok()) {
       impl->DeleteObsoleteFiles();