Allow setting number of GC threads from command line

Author: pranavk

src/common/init.cpp

@@ -56,6 +56,7 @@ void PelotonInit::Initialize() {
   concurrency::EpochManagerFactory::GetInstance().StartEpoch();
 
   // start GC.
+  gc::GCManagerFactory::Configure(settings::SettingsManager::GetInt(settings::SettingId::gc_num_threads));
   gc::GCManagerFactory::GetInstance().StartGC();
 
   // start index tuner

src/include/settings/settings.h

@@ -99,7 +99,13 @@ SETTING_int(monoqueue_worker_pool_size,
 // Number of connection threads used by peloton
 SETTING_int(connection_thread_count,
             "Number of connection threads (default: std::hardware_concurrency())",
-            std::thread::hardware_concurrency(), false, false)
+            std::thread::hardware_concurrency(),
+	    false, false)
+
+SETTING_int(gc_num_threads,
+            "The number of Garbage collection threads to run",
+            1,
+            true, true)
 
 //===----------------------------------------------------------------------===//
 // WRITE AHEAD LOG

src/settings/settings_manager.cpp

@@ -116,6 +116,8 @@ const std::string SettingsManager::GetInfo() const {
                          GetBool(SettingId::codegen) ? "enabled" : "disabled"));
   info.append(StringUtil::Format("%28s:   %-28s\n", "Optimization Timeout",
                                  GetInt(SettingId::task_execution_timeout)));
+  info.append(StringUtil::Format("%28s:   %-28s\n", "Number of GC threads", 
+                                 GetInt(SettingId::gc_num_threads)));
   return StringBoxUtil::Box(info);
 }