release db mutex when calling ApproximateSize (#4630)

Summary:
`GenSubcompactionBoundaries` calls `VersionSet::ApproximateSize` which gets BlockBasedTableReader for every file and seeks in its index block to find `key`'s offset. If the table or index block aren't in memory already, this involves I/O. This can be improved by releasing DB mutex when calling ApproximateSize.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4630

Differential Revision: D13052653

Pulled By: miasantreble

fbshipit-source-id: cae31d46d10d0860fa8a26b8d5154b2d17d1685f

db/compaction_job.cc

@@ -511,7 +511,10 @@ void CompactionJob::GenSubcompactionBoundaries() {
   // size of data covered by keys in that range
   uint64_t sum = 0;
   std::vector<RangeWithSize> ranges;
-  auto* v = cfd->current();
+  // Get input version from CompactionState since it's already referenced
+  // earlier in SetInputVersioCompaction::SetInputVersion and will not change
+  // when db_mutex_ is released below
+  auto* v = compact_->compaction->input_version();
   for (auto it = bounds.begin();;) {
     const Slice a = *it;
     it++;
@@ -521,7 +524,13 @@ void CompactionJob::GenSubcompactionBoundaries() {
     }
 
     const Slice b = *it;
+
+    // ApproximateSize could potentially create table reader iterator to seek
+    // to the index block and may incur I/O cost in the process. Unlock db
+    // mutex to reduce contention
+    db_mutex_->Unlock();
     uint64_t size = versions_->ApproximateSize(v, a, b, start_lvl, out_lvl + 1);
+    db_mutex_->Lock();
     ranges.emplace_back(a, b, size);
     sum += size;
   }