8.10 Release notes for --disable-delayed-os-memory-return [skip ci]

(cherry picked from commit 19680ee533bb95c0c5c42aca5c81197e4b233979)

1 files changed, 19 insertions, 0 deletions

diff --git a/docs/users_guide/8.10.1-notes.rst b/docs/users_guide/8.10.1-notes.rst
index 026e93fa7e..ac2d9180d7 100644
--- a/docs/users_guide/8.10.1-notes.rst
+++ b/docs/users_guide/8.10.1-notes.rst
@@ -266,6 +266,25 @@ Runtime system
   out-of-the-box compatibility with OpenBSD and macOS Catalina (see
   :ghc-ticket:`17353`)
 
+- The RTS API now exposes :ref:`an interface <event_log_output_api>` to
+  configure ``EventLogWriters``, allowing eventlog data to fed to sinks other
+  than ``.eventlog`` files.
+
+- A new ``+RTS`` flag ``--disable-delayed-os-memory-return`` was added to make
+  for accurate resident memory usage of the program as shown in memory
+  usage reporting tools (e.g. the ``RSS`` column in ``top`` and ``htop``).
+
+  This makes it easier to check the real memory usage of Haskell programs.
+
+  Using this new flag is expected to make the program slightly slower.
+
+  Without this flag, the (Linux) RTS returns unused memory "lazily" to the OS.
+  This has making the memory available to other processes while also allowing
+  the RTS to re-use the memory very efficiently (without zeroing pages) in case
+  it needs it again, but common tools will incorrectly show such memory as
+  occupied by the RTS (because they do not process the ``LazyFree`` field in
+  ``/proc/PID/smaps``).
+
 Template Haskell
 ~~~~~~~~~~~~~~~~