server: Collect page defrag results

Signed-off-by: Abhijat Malviya <abhijat@dragonflydb.io>

Author: abhijat

src/server/engine_shard.cc

@@ -386,7 +386,8 @@ bool EngineShard::DefragTaskState::CheckRequired() {
   return false;
 }
 
-bool EngineShard::DoDefrag(CollectPageStats collect_page_stats, const float threshold) {
+std::optional<CollectedPageStats> EngineShard::DoDefrag(CollectPageStats collect_page_stats,
+                                                        const float threshold) {
   // --------------------------------------------------------------------------
   // NOTE: This task is running with exclusive access to the shard.
   // i.e. - Since we are using shared nothing access here, and all access
@@ -406,7 +407,7 @@ bool EngineShard::DoDefrag(CollectPageStats collect_page_stats, const float thre
   // If we found no valid db, we finished traversing and start from scratch next time
   if (!slice.IsDbValid(defrag_state_.dbid)) {
     defrag_state_.ResetScanState();
-    return false;
+    return std::nullopt;
   }
 
   DCHECK(slice.IsDbValid(defrag_state_.dbid));
@@ -447,7 +448,7 @@ bool EngineShard::DoDefrag(CollectPageStats collect_page_stats, const float thre
   stats_.defrag_task_invocation_total++;
   stats_.defrag_attempt_total += attempts;
 
-  return true;
+  return page_usage.CollectedStats();
 }
 
 // the memory defragmentation task is as follow:

src/server/engine_shard.h

@@ -206,7 +206,7 @@ class EngineShard {
   // threshold can be passed, which will be used to determine if defragmentation should be
   // performed.
   // Returns true if defragmentation was performed.
-  bool DoDefrag(CollectPageStats collect_page_stats, float threshold);
+  std::optional<CollectedPageStats> DoDefrag(CollectPageStats collect_page_stats, float threshold);
 
  private:
   struct DefragTaskState {

src/server/memory_cmd.cc

@@ -177,9 +177,15 @@ void MemoryCmd::Run(CmdArgList args) {
     static const float default_threshold =
         absl::GetFlag(FLAGS_mem_defrag_page_utilization_threshold);
     const float threshold = parser.NextOrDefault(default_threshold);
-    shard_set->pool()->AwaitFiberOnAll([threshold](util::ProactorBase*) {
-      if (auto* shard = EngineShard::tlocal(); shard)
-        shard->DoDefrag(CollectPageStats::YES, threshold);
+
+    CollectedPageStats results[shard_set->size()];
+    shard_set->pool()->AwaitFiberOnAll([threshold, &results](util::ProactorBase*) {
+      if (auto* shard = EngineShard::tlocal(); shard) {
+        if (auto shard_res = shard->DoDefrag(CollectPageStats::YES, threshold);
+            shard_res.has_value()) {
+          results[shard->shard_id()] = std::move(shard_res.value());
+        }
+      }
     });
     return builder_->SendSimpleString("OK");
   }