Use concurrent hash map for tracking query memory

core/trino-main/src/main/java/io/trino/memory/MemoryPool.java

@@ -31,6 +31,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
@@ -53,9 +54,9 @@ public class MemoryPool
     @GuardedBy("this")
     private NonCancellableMemoryFuture<Void> future;
 
-    @GuardedBy("this")
     // TODO: It would be better if we just tracked QueryContexts, but their lifecycle is managed by a weak reference, so we can't do that
-    private final Map<QueryId, Long> queryMemoryReservations = new HashMap<>();
+    // It is guarded for updates by this, but can be read without holding a lock
+    private final Map<QueryId, Long> queryMemoryReservations = new ConcurrentHashMap<>();
 
     // This map keeps track of all the tagged allocations, e.g., query-1 -> ['TableScanOperator': 10MB, 'LazyOutputBuffer': 5MB, ...]
     @GuardedBy("this")
@@ -347,7 +348,7 @@ public synchronized long getReservedRevocableBytes()
         return reservedRevocableBytes;
     }
 
-    synchronized long getQueryMemoryReservation(QueryId queryId)
+    long getQueryMemoryReservation(QueryId queryId)
     {
         return queryMemoryReservations.getOrDefault(queryId, 0L);
     }

core/trino-main/src/main/java/io/trino/memory/QueryContext.java

@@ -225,7 +225,7 @@ public synchronized MemoryPool getMemoryPool()
         return memoryPool;
     }
 
-    public synchronized long getUserMemoryReservation()
+    public long getUserMemoryReservation()
     {
         return memoryPool.getQueryMemoryReservation(queryId);
     }