Pre-calculate whether to slow down for too many level 0 files

Summary: Currently in DBImpl::MakeRoomForWrite(), we do  "versions_->NumLevelFiles(0) >= options_.level0_slowdown_writes_trigger" to check whether the writer thread needs to slow down. However, versions_->NumLevelFiles(0) is slightly more expensive than we expected. By caching the result of the comparison when installing a new version, we can avoid this function call every time.

Test Plan:
make all check
Manually trigger this behavior by applying universal compaction style and make sure inserts are made slow after there are certain number of files.

Reviewers: haobo, kailiu, igor

Reviewed By: kailiu

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D15141

db/db_impl.cc

@@ -3166,8 +3166,7 @@ Status DBImpl::MakeRoomForWrite(bool force,
       break;
     } else if (
         allow_delay &&
-        versions_->NumLevelFiles(0) >=
-          options_.level0_slowdown_writes_trigger) {
+        versions_->NeedSlowdownForNumLevel0Files()) {
       // We are getting close to hitting a hard limit on the number of
       // L0 files.  Rather than delaying a single write by several
       // seconds when we hit the hard limit, start delaying each

db/version_set.cc

@@ -1148,6 +1148,7 @@ VersionSet::VersionSet(const std::string& dbname,
       num_levels_(options_->num_levels),
       dummy_versions_(this),
       current_(nullptr),
+      need_slowdown_for_num_level0_files(false),
       compactions_in_progress_(options_->num_levels),
       current_version_number_(0),
       last_observed_manifest_size_(0),
@@ -1199,6 +1200,9 @@ void VersionSet::AppendVersion(Version* v) {
     current_->Unref();
   }
   current_ = v;
+  need_slowdown_for_num_level0_files =
+      (options_->level0_slowdown_writes_trigger >= 0 && current_ != nullptr &&
+       NumLevelFiles(0) >= options_->level0_slowdown_writes_trigger);
   v->Ref();
 
   // Append to linked list

db/version_set.h

@@ -250,6 +250,12 @@ class VersionSet {
   // Return the current version.
   Version* current() const { return current_; }
 
+  // A Flag indicating whether write needs to slowdown because of there are
+  // too many number of level0 files.
+  bool NeedSlowdownForNumLevel0Files() const {
+    return need_slowdown_for_num_level0_files;
+  }
+
   // Return the current manifest file number
   uint64_t ManifestFileNumber() const { return manifest_file_number_; }
 
@@ -489,6 +495,8 @@ class VersionSet {
   Version dummy_versions_;  // Head of circular doubly-linked list of versions.
   Version* current_;        // == dummy_versions_.prev_
 
+  bool need_slowdown_for_num_level0_files;
+
   // Per-level key at which the next compaction at that level should start.
   // Either an empty string, or a valid InternalKey.
   std::string* compact_pointer_;