AK: Avoid pagefaults when repeatedly enqueing/dequeing items in a Queue

When repeatedly enqueing and dequeing a single item in a Queue we end
up faulting in all the pages for the underlying Vector. This is a
performance issue - especially where the element type is large.

1 files changed, 7 insertions, 0 deletions

diff --git a/AK/Queue.h b/AK/Queue.h
index d52026f107..c87750a879 100644
--- a/AK/Queue.h
+++ b/AK/Queue.h
@@ -46,6 +46,13 @@ public:
             m_index_into_first = 0;
         }
         --m_size;
+        if (m_size == 0 && !m_segments.is_empty()) {
+            // This is not necessary for correctness but avoids faulting in
+            // all the pages for the underlying Vector in the case where
+            // the caller repeatedly enqueues and then dequeues a single item.
+            m_index_into_first = 0;
+            m_segments.last()->data.clear_with_capacity();
+        }
         return value;
     }