Return the actual RSS instead of VSZ

Summary:
We are returning the virtual memory size (VSZ) instead of the resident set size (RSS).
We do want RSS which is approximately the memory used.

Reviewed By: compositor

Differential Revision: D53470567

fbshipit-source-id: f854c253fc484b0e0d56464443908c41bcfc41ac

sapp/operating_system.py

@@ -16,4 +16,4 @@
 def get_rss_in_gb() -> float:
     if psutil is None:
         return 0
-    return psutil.virtual_memory().used / (1000**3)
+    return psutil.Process().memory_info().rss / (1000**3)