MB-49469: Introduce CM::Counter::memFreedByCheckpointRemoval

That accounts the memory released by CheckpointRemoval by a specific CM
instance.

The new quantity will be used in a follow-up for computing and exposing
new vb_{state} aggregates for the same quantity.

Change-Id: Iba966b559f7c7ced03bba234c66f83be906141f2
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/169679
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Richard de Mellow <richard.demellow@couchbase.com>

engines/ep/src/checkpoint_manager.cc

@@ -47,7 +47,8 @@ CheckpointManager::CheckpointManager(EPStats& st,
       maxVisibleSeqno(maxVisibleSeqno),
       flusherCB(std::move(cb)),
       checkpointDisposer(std::move(checkpointDisposer)),
-      memFreedByExpel(stats.memFreedByCheckpointItemExpel) {
+      memFreedByExpel(stats.memFreedByCheckpointItemExpel),
+      memFreedByCheckpointRemoval(stats.memFreedByCheckpointRemoval) {
     std::lock_guard<std::mutex> lh(queueLock);
 
     lastBySeqno.setLabel("CheckpointManager(" + vb.getId().to_string() +
@@ -559,7 +560,7 @@ CheckpointManager::updateStatsForCheckpointRemoval(
     }
     numItems.fetch_sub(numNonMetaItemsRemoved + numMetaItemsRemoved);
     stats.itemsRemovedFromCheckpoints.fetch_add(numNonMetaItemsRemoved);
-    stats.memFreedByCheckpointRemoval += memoryReleased;
+    memFreedByCheckpointRemoval += memoryReleased;
 
     EP_LOG_DEBUG(
             "CheckpointManager::updateStatsForCheckpointRemoval: Removed {} "
@@ -2045,4 +2046,8 @@ CheckpointManager::Counter& CheckpointManager::Counter::operator-=(
 
 size_t CheckpointManager::getMemFreedByItemExpel() const {
     return memFreedByExpel;
+}
+
+size_t CheckpointManager::getMemFreedByCheckpointRemoval() const {
+    return memFreedByCheckpointRemoval;
 }

engines/ep/src/checkpoint_manager.h

@@ -594,6 +594,8 @@ class CheckpointManager {
     // @return the amount of memory (in bytes) released by ItemExpel
     size_t getMemFreedByItemExpel() const;
 
+    size_t getMemFreedByCheckpointRemoval() const;
+
     /**
      * Member std::function variable, to allow us to inject code into
      * removeCursor() for unit MB36146
@@ -990,6 +992,9 @@ class CheckpointManager {
     // Memory released by item expel in this CM
     Counter memFreedByExpel;
 
+    // Memory released by checkpoint removal in this CM
+    Counter memFreedByCheckpointRemoval;
+
     friend std::ostream& operator<<(std::ostream& os, const CheckpointManager& m);
 };
 

engines/ep/src/stats.h

@@ -306,7 +306,7 @@ class EPStats {
     Counter cursorsDropped;
 
     //! Amount of memory we have freed by checkpoint removal
-    std::atomic<size_t> memFreedByCheckpointRemoval;
+    Counter memFreedByCheckpointRemoval;
 
     //! Amount of memory we have freed by checkpoint item expel
     Counter memFreedByCheckpointItemExpel;

engines/ep/tests/module_tests/checkpoint_test.cc

@@ -3738,10 +3738,14 @@ TEST_P(CheckpointMemoryTrackingTest, CheckpointManagerMemUsageAtRemoval) {
     testCheckpointManagerMemUsage();
 
     // confirm that no items have been removed from the checkpoint manager
-    ASSERT_EQ(0, engine->getEpStats().itemsRemovedFromCheckpoints);
+    const auto& stats = engine->getEpStats();
+    ASSERT_EQ(0, stats.itemsRemovedFromCheckpoints);
+    ASSERT_EQ(0, stats.memFreedByCheckpointRemoval);
 
     auto vb = store->getVBuckets().getBucket(vbid);
     auto& manager = static_cast<MockCheckpointManager&>(*vb->checkpointManager);
+    ASSERT_EQ(0, manager.getMemFreedByCheckpointRemoval());
+
     // set the eager checkpoint disposer to allow checkpoints to be destroyed
     // "inline" when removed. This avoids needing to drive background tasks.
     manager.setCheckpointDisposer(ImmediateCkptDisposer);
@@ -3812,6 +3816,10 @@ TEST_P(CheckpointMemoryTrackingTest, CheckpointManagerMemUsageAtRemoval) {
     EXPECT_EQ(queued + index + queueOverhead,
               engine->getEpStats().getCheckpointManagerEstimatedMemUsage());
     EXPECT_EQ(queued + index + queueOverhead, manager.getEstimatedMemUsage());
+
+    EXPECT_GT(manager.getMemFreedByCheckpointRemoval(), 0);
+    EXPECT_EQ(manager.getMemFreedByCheckpointRemoval(),
+              stats.memFreedByCheckpointRemoval);
 }
 
 TEST_P(CheckpointMemoryTrackingTest, BackgroundTaskIsNotified) {