Queue age to reflect the avg time spent by an item in the dirty queue

Correct the vbucket stat queue_age to reflect the average time spent
by an item in the dirty queue. The following stats would reflect this
change:
- vb_active_queue_age
- vb_replica_queue_age
- vb_pending_queue_age

Change-Id: I01b13177aaff36650024cbfc76476313ef1ac223
Reviewed-on: http://review.couchbase.org/55878
Reviewed-by: Chiyoung Seo <[email protected]>
Tested-by: buildbot <[email protected]>

Author: abhinavdangeti

docs/stats.org

@@ -375,7 +375,7 @@ For introductory information on stats within Couchbase, start with the
 | vb_active_ops_reject          | Number of rejected operations              |
 | vb_active_queue_size          | Active items in disk queue                 |
 | vb_active_queue_memory        | Memory used for disk queue                 |
-| vb_active_queue_age           | Sum of disk queue item age in milliseconds |
+| vb_active_queue_age           | Average disk queue item age in milliseconds|
 | vb_active_queue_pending       | Total bytes of pending writes              |
 | vb_active_queue_fill          | Total enqueued items                       |
 | vb_active_queue_drain         | Total drained items                        |
@@ -401,7 +401,7 @@ For introductory information on stats within Couchbase, start with the
 | vb_replica_ops_reject         | Number of rejected operations              |
 | vb_replica_queue_size         | Replica items in disk queue                |
 | vb_replica_queue_memory       | Memory used for disk queue                 |
-| vb_replica_queue_age          | Sum of disk queue item age in milliseconds |
+| vb_replica_queue_age          | Average disk queue item age in milliseconds|
 | vb_replica_queue_pending      | Total bytes of pending writes              |
 | vb_replica_queue_fill         | Total enqueued items                       |
 | vb_replica_queue_drain        | Total drained items                        |
@@ -427,7 +427,7 @@ For introductory information on stats within Couchbase, start with the
 | vb_pending_ops_reject         | Number of rejected operations              |
 | vb_pending_queue_size         | Pending items in disk queue                |
 | vb_pending_queue_memory       | Memory used for disk queue                 |
-| vb_pending_queue_age          | Sum of disk queue item age in milliseconds |
+| vb_pending_queue_age          | Average disk queue item age in milliseconds|
 | vb_pending_queue_pending      | Total bytes of pending writes              |
 | vb_pending_queue_fill         | Total enqueued items                       |
 | vb_pending_queue_drain        | Total drained items                        |
@@ -458,7 +458,7 @@ The stats below are listed for each vbucket.
 | ops_reject                    | Number of rejected operations              |
 | queue_size                    | Pending items in disk queue                |
 | queue_memory                  | Memory used for disk queue                 |
-| queue_age                     | Sum of disk queue item age in milliseconds |
+| queue_age                     | Average disk queue item age in milliseconds|
 | queue_fill                    | Total enqueued items                       |
 | queue_drain                   | Total drained items                        |
 | pending writes                | Total bytes of pending writes              |

src/ep_engine.h

@@ -166,7 +166,7 @@ class VBucketCountVisitor : public VBucketVisitor {
     size_t getQueueMemory() { return queueMemory; }
     size_t getQueueFill() { return queueFill; }
     size_t getQueueDrain() { return queueDrain; }
-    uint64_t getAge() { return queueAge; }
+    uint64_t getAge() { return (numVbucket > 0 ? queueAge / numVbucket : 0); }
     size_t getPendingWrites() { return pendingWrites; }
     size_t getChkPersistRemaining() { return chkPersistRemaining; }
 

src/vbucket.cc

@@ -206,7 +206,6 @@ void VBucket::doStatsForQueueing(Item& qi, size_t itemBytes)
     ++dirtyQueueSize;
     dirtyQueueMem.fetch_add(sizeof(Item));
     ++dirtyQueueFill;
-    dirtyQueueAge.fetch_add(qi.getQueuedTime());
     dirtyQueuePendingWrites.fetch_add(itemBytes);
 }
 
@@ -220,11 +219,9 @@ void VBucket::doStatsForFlushing(Item& qi, size_t itemBytes)
     }
     ++dirtyQueueDrain;
 
-    if (dirtyQueueAge > qi.getQueuedTime()) {
-        dirtyQueueAge.fetch_sub(qi.getQueuedTime());
-    } else {
-        dirtyQueueAge.store(0);
-    }
+    rel_time_t diff = ep_current_time() - qi.getQueuedTime();
+    dirtyQueueAgeSum.fetch_add(diff > 0 ? diff : 0);
+    dirtyQueueSamples++;
 
     if (dirtyQueuePendingWrites > itemBytes) {
         dirtyQueuePendingWrites.fetch_sub(itemBytes);
@@ -254,7 +251,8 @@ void VBucket::resetStats() {
     dirtyQueueSize.store(0);
     dirtyQueueMem.store(0);
     dirtyQueueFill.store(0);
-    dirtyQueueAge.store(0);
+    dirtyQueueAgeSum.store(0);
+    dirtyQueueSamples.store(0);
     dirtyQueuePendingWrites.store(0);
     dirtyQueueDrain.store(0);
     fileSpaceUsed = 0;

src/vbucket.h

@@ -163,7 +163,8 @@ class VBucket : public RCValue {
         dirtyQueueMem(0),
         dirtyQueueFill(0),
         dirtyQueueDrain(0),
-        dirtyQueueAge(0),
+        dirtyQueueAgeSum(0),
+        dirtyQueueSamples(0),
         dirtyQueuePendingWrites(0),
         metaDataDisk(0),
         numExpiredItems(0),
@@ -304,13 +305,13 @@ class VBucket : public RCValue {
 
     void resetStats();
 
-    // Get age sum in millisecond
+    // Get age average in milliseconds
     uint64_t getQueueAge() {
-        rel_time_t currentAge = ep_current_time() * dirtyQueueSize;
-        if (currentAge < dirtyQueueAge) {
+        if (dirtyQueueSamples == 0) {
             return 0;
+        } else {
+            return (dirtyQueueAgeSum / dirtyQueueSamples) * 1000;
         }
-        return (currentAge - dirtyQueueAge) * 1000;
     }
 
     void fireAllOps(EventuallyPersistentEngine &engine);
@@ -488,7 +489,8 @@ class VBucket : public RCValue {
     AtomicValue<size_t>  dirtyQueueMem;
     AtomicValue<size_t>  dirtyQueueFill;
     AtomicValue<size_t>  dirtyQueueDrain;
-    AtomicValue<uint64_t> dirtyQueueAge;
+    AtomicValue<uint64_t> dirtyQueueAgeSum;
+    AtomicValue<uint64_t> dirtyQueueSamples;
     AtomicValue<size_t>  dirtyQueuePendingWrites;
     AtomicValue<size_t>  metaDataDisk;