chore: add metrics for heartbeat evictions (#4952)

* evict in heartbeat if expire table is not empty
* add metrics around heartbeat evictions (total evictions and total evicted bytes)

Signed-off-by: kostas <kostas@dragonflydb.io>

Author: kostasrim

src/server/engine_shard.cc

@@ -297,7 +297,7 @@ string EngineShard::TxQueueInfo::Format() const {
 }
 
 EngineShard::Stats& EngineShard::Stats::operator+=(const EngineShard::Stats& o) {
-  static_assert(sizeof(Stats) == 64);
+  static_assert(sizeof(Stats) == 88);
 
 #define ADD(x) x += o.x
 
@@ -309,6 +309,9 @@ EngineShard::Stats& EngineShard::Stats::operator+=(const EngineShard::Stats& o)
   ADD(tx_optimistic_total);
   ADD(tx_batch_schedule_calls_total);
   ADD(tx_batch_scheduled_items_total);
+  ADD(total_heartbeat_expired_keys);
+  ADD(total_heartbeat_expired_bytes);
+  ADD(total_heartbeat_expired_calls);
 
 #undef ADD
   return *this;
@@ -802,12 +805,18 @@ void EngineShard::RetireExpiredAndEvict() {
 
     db_cntx.db_index = i;
     auto [pt, expt] = db_slice.GetTables(i);
-    if (expt->size() > pt->size() / 4) {
+    if (expt->size() > 0) {
       DbSlice::DeleteExpiredStats stats = db_slice.DeleteExpiredStep(db_cntx, ttl_delete_target);
 
       eviction_goal -= std::min(eviction_goal, size_t(stats.deleted_bytes));
       counter_[TTL_TRAVERSE].IncBy(stats.traversed);
       counter_[TTL_DELETE].IncBy(stats.deleted);
+      stats_.total_heartbeat_expired_keys += stats.deleted;
+      stats_.total_heartbeat_expired_bytes += stats.deleted_bytes;
+      ++stats_.total_heartbeat_expired_calls;
+      VLOG(1) << "Heartbeat expired " << stats.deleted << " keys with total bytes "
+              << stats.deleted_bytes << " with total expire flow calls "
+              << stats_.total_heartbeat_expired_calls;
     }
 
     if (eviction_goal) {

src/server/engine_shard.h

@@ -44,6 +44,10 @@ class EngineShard {
     // Number of transactions scheduled via ScheduleBatchInShard.
     uint64_t tx_batch_scheduled_items_total = 0;
 
+    uint64_t total_heartbeat_expired_keys = 0;
+    uint64_t total_heartbeat_expired_bytes = 0;
+    uint64_t total_heartbeat_expired_calls = 0;
+
     Stats& operator+=(const Stats&);
   };
 

src/server/server_family.cc

@@ -2484,6 +2484,9 @@ string ServerFamily::FormatInfoMetrics(const Metrics& m, std::string_view sectio
     append("rejected_connections", -1);
     append("expired_keys", m.events.expired_keys);
     append("evicted_keys", m.events.evicted_keys);
+    append("total_heartbeat_expired_keys", m.shard_stats.total_heartbeat_expired_keys);
+    append("total_heartbeat_expired_bytes", m.shard_stats.total_heartbeat_expired_bytes);
+    append("total_heartbeat_expired_calls", m.shard_stats.total_heartbeat_expired_calls);
     append("hard_evictions", m.events.hard_evictions);
     append("garbage_checked", m.events.garbage_checked);
     append("garbage_collected", m.events.garbage_collected);