Only execute flush from compaction if max_background_flushes = 0

Summary: As title. We shouldn't need to execute flush from compaction if there are dedicated threads doing flushes.

Test Plan: make check

Reviewers: rven, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30579

db/db_impl.cc

@@ -2018,8 +2018,9 @@ Status DBImpl::BackgroundCompaction(bool* madeProgress, JobContext* job_context,
     return Status::OK();
   }
 
-  // FLUSH preempts compaction
+  // If there are no flush threads, then compaction thread needs to execute the
-  // TODO(icanadi) we should only do this if max_background_flushes == 0
+  // flushes
+  if (db_options_.max_background_flushes == 0) {
     // BackgroundFlush() will only execute a single flush. We keep calling it as
     // long as there's more flushes to be done
     while (!flush_queue_.empty()) {
@@ -2029,7 +2030,8 @@ Status DBImpl::BackgroundCompaction(bool* madeProgress, JobContext* job_context,
           "%d, compaction slots available %d",
           db_options_.max_background_flushes - bg_flush_scheduled_,
           db_options_.max_background_compactions - bg_compaction_scheduled_);
-    auto flush_status = BackgroundFlush(madeProgress, job_context, log_buffer);
+      auto flush_status =
+          BackgroundFlush(madeProgress, job_context, log_buffer);
       if (!flush_status.ok()) {
         if (is_manual) {
           manual_compaction_->status = flush_status;
@@ -2040,6 +2042,7 @@ Status DBImpl::BackgroundCompaction(bool* madeProgress, JobContext* job_context,
         return flush_status;
       }
     }
+  }
 
   unique_ptr<Compaction> c;
   InternalKey manual_end_storage;