diff --git a/docs/compression.rst b/docs/compression.rst
--- a/docs/compression.rst
+++ b/docs/compression.rst
@@ -122,6 +122,12 @@
 
 See ``swh graph compress --help`` for syntax and usage details.
 
+For some steps, Java will to allocate terabytes of memory in sub-gigabyte segments,
+so you should increase the number of allowed allocations first:
+
+.. code:: bash
+
+    sudo sysctl -w vm.max_map_count=1966080
 
 Compression steps
 =================