Release netCDF memory-map pages after each frame

package/AUTHORS

@@ -285,6 +285,7 @@ Chronological list of authors
   - Apoorva Verma
   - Aryaman Chaudhri
   - Francesco Siciliani
+  - Erik Fransson
 
 External code
 -------------

package/CHANGELOG

@@ -18,13 +18,18 @@ The rules for this file:
          spyke7, talagayev, tanii1125, BradyAJohnston, hejamu, jeremyleung521,
          harshitgajjela-droid, kunjsinha, aygarwal, jauy123, Dreamstick9,
          ollyfutur, Amarendra22, charity-g, ParthUppal523, apoorva-01, RMeli,
-         raulloiscuns, Aryaman-Chaudhri, sici17
+         raulloiscuns, Aryaman-Chaudhri, sici17, erikfransson
 
  * 2.11.0
 
 Fixes
  * Mass (u) has been added to MDAnalysis base units and clarified that physical
    constants use CODATA 2010 values (Issue #3944, PR #5439)
+ * The NCDF reader now releases the pages of the memory map once a frame has
+   been read into the Timestep. Reading a netCDF trajectory with `mmap=True`
+   (the default for a file name) no longer grows the resident memory of the
+   process towards the size of the whole trajectory file. Requires a platform
+   that provides `MADV_DONTNEED`, i.e. not Windows (Discussion #4792).
  * Added `.gitattributes` to enforce LF (\n) as line endings and renormalized
    existing files to conform (Issue #5315, PR #5446)
  * `AtomGroup.rotate()` and the `rotateby` trajectory transformation now

package/MDAnalysis/coordinates/TRJ.py

@@ -134,6 +134,7 @@
 import warnings
 import errno
 import logging
+import mmap
 from math import isclose
 
 import MDAnalysis
@@ -675,6 +676,10 @@ def _read_frame(self, frame):
                 self.convert_pos_from_native(ts.dimensions[:3])
         ts.frame = frame  # frame labels are 0-based
         self._current_frame = frame
+        # the frame is now copied into ts, so its pages can be dropped again
+        mm = getattr(self.trjfile, "_mm", None)
+        if mm is not None and hasattr(mmap, "MADV_DONTNEED"):
+            mm.madvise(mmap.MADV_DONTNEED)
         return ts
 
     def _reopen(self):