Compact memory after dropping caches

With this commit we request memory compaction after dropping caches in
order to avoid increasing memory fragmentation when running multiple
benchmarks.

Relates elastic#64

fixtures/ansible/roles/drop-caches/tasks/main.yml

@@ -10,3 +10,8 @@
 - name: Free slab objects and pagecache
   command: echo 3 > /proc/sys/vm/drop_caches
   become: true
+
+# also request compaction to reduce memory fragmentation
+- name: Request memory compaction
+  command: echo 1 > /proc/sys/vm/compact_memory
+  become: true