Take snapshots once for all cf flushes (#4934)

Summary:
FlushMemTablesToOutputFiles calls FlushMemTableToOutputFile for each column family. The patch moves the take-snapshot logic to outside FlushMemTableToOutputFile so that it does it once for all the flushes. This also addresses a deadlock issue for resetting the managed snapshot of job_snapshot in the 2nd call to FlushMemTableToOutputFile.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4934

Differential Revision: D13900747

Pulled By: maysamyabandeh

fbshipit-source-id: f3cd650c5fff24cf95c1aaf8a10c149d42bf042c

db/compaction_job.cc

@@ -1135,8 +1135,9 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
     sub_compact->compaction_job_stats.file_prepare_write_nanos +=
         IOSTATS(prepare_write_nanos) - prev_prepare_write_nanos;
     sub_compact->compaction_job_stats.cpu_micros -=
-            (IOSTATS(cpu_write_nanos) - prev_cpu_write_nanos
+        (IOSTATS(cpu_write_nanos) - prev_cpu_write_nanos +
-            + IOSTATS(cpu_read_nanos) - prev_cpu_read_nanos) / 1000;
+         IOSTATS(cpu_read_nanos) - prev_cpu_read_nanos) /
+        1000;
     if (prev_perf_level != PerfLevel::kEnableTimeAndCPUTimeExceptForMutex) {
       SetPerfLevel(prev_perf_level);
     }

db/db_impl.h

@@ -923,11 +923,13 @@ class DBImpl : public DB {
   // Flush the in-memory write buffer to storage.  Switches to a new
   // log-file/memtable and writes a new descriptor iff successful. Then
   // installs a new super version for the column family.
-  Status FlushMemTableToOutputFile(ColumnFamilyData* cfd,
+  Status FlushMemTableToOutputFile(
-                                   const MutableCFOptions& mutable_cf_options,
+      ColumnFamilyData* cfd, const MutableCFOptions& mutable_cf_options,
-                                   bool* madeProgress, JobContext* job_context,
+      bool* madeProgress, JobContext* job_context,
-                                   SuperVersionContext* superversion_context,
+      SuperVersionContext* superversion_context,
-                                   LogBuffer* log_buffer);
+      std::vector<SequenceNumber>& snapshot_seqs,
+      SequenceNumber earliest_write_conflict_snapshot,
+      SnapshotChecker* snapshot_checker, LogBuffer* log_buffer);
 
   // Argument required by background flush thread.
   struct BGFlushArg {

db/db_impl_compaction_flush.cc

@@ -132,16 +132,14 @@ Status DBImpl::SyncClosedLogs(JobContext* job_context) {
 Status DBImpl::FlushMemTableToOutputFile(
     ColumnFamilyData* cfd, const MutableCFOptions& mutable_cf_options,
     bool* made_progress, JobContext* job_context,
-    SuperVersionContext* superversion_context, LogBuffer* log_buffer) {
+    SuperVersionContext* superversion_context,
+    std::vector<SequenceNumber>& snapshot_seqs,
+    SequenceNumber earliest_write_conflict_snapshot,
+    SnapshotChecker* snapshot_checker, LogBuffer* log_buffer) {
   mutex_.AssertHeld();
   assert(cfd->imm()->NumNotFlushed() != 0);
   assert(cfd->imm()->IsFlushPending());
 
-  std::vector<SequenceNumber> snapshot_seqs;
-  SequenceNumber earliest_write_conflict_snapshot;
-  SnapshotChecker* snapshot_checker;
-  GetSnapshotContext(job_context, &snapshot_seqs,
-                     &earliest_write_conflict_snapshot, &snapshot_checker);
 
   FlushJob flush_job(
       dbname_, cfd, immutable_db_options_, mutable_cf_options,
@@ -239,14 +237,20 @@ Status DBImpl::FlushMemTablesToOutputFiles(
     return AtomicFlushMemTablesToOutputFiles(bg_flush_args, made_progress,
                                              job_context, log_buffer);
   }
+  std::vector<SequenceNumber> snapshot_seqs;
+  SequenceNumber earliest_write_conflict_snapshot;
+  SnapshotChecker* snapshot_checker;
+  GetSnapshotContext(job_context, &snapshot_seqs,
+                     &earliest_write_conflict_snapshot, &snapshot_checker);
   Status status;
   for (auto& arg : bg_flush_args) {
     ColumnFamilyData* cfd = arg.cfd_;
     MutableCFOptions mutable_cf_options = *cfd->GetLatestMutableCFOptions();
     SuperVersionContext* superversion_context = arg.superversion_context_;
-    Status s = FlushMemTableToOutputFile(cfd, mutable_cf_options, made_progress,
+    Status s = FlushMemTableToOutputFile(
-                                         job_context, superversion_context,
+        cfd, mutable_cf_options, made_progress, job_context,
-                                         log_buffer);
+        superversion_context, snapshot_seqs, earliest_write_conflict_snapshot,
+        snapshot_checker, log_buffer);
     if (!s.ok()) {
       status = s;
       if (!s.IsShutdownInProgress()) {