release dashmap memory (#1485)

easytier/src/common/mod.rs

@@ -156,6 +156,16 @@ pub fn get_machine_id() -> uuid::Uuid {
     gen_mid
 }
 
+pub fn shrink_dashmap<K: Eq + std::hash::Hash, V>(
+    map: &dashmap::DashMap<K, V>,
+    threshold: Option<usize>,
+) {
+    let threshold = threshold.unwrap_or(16);
+    if map.capacity() - map.len() > threshold {
+        map.shrink_to_fit();
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;