Author: Armin Rigo <ar...@tunes.org>
Branch: extradoc
Changeset: r766:c4d464e272a9
Date: 2016-07-14 12:37 +0200
http://bitbucket.org/pypy/pypy.org/changeset/c4d464e272a9/
Log: Document the memory usage behavior of PyPy, because it is not very
clear that memory usage *does* go down in systems with MADV_FREE.
diff --git a/compat.html b/compat.html
--- a/compat.html
+++ b/compat.html
@@ -124,6 +124,15 @@
as on CPython: they run “some time later” in PyPy (or not at all if
the program finishes running in the meantime). See <a class="reference
external"
href="http://pypy.readthedocs.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies";>more
details
here</a>.</p>
+<p>Note that PyPy returns unused memory to the operating system if there
+is a madvise() system call (at least Linux, OS/X, BSD). It is
+important to realize that you may not see this in <tt class="docutils
literal">top</tt>. The unused
+pages are marked with <tt class="docutils literal">MADV_FREE</tt>, which tells
the system “if you
+need more memory at some point, grab this page”. As long as memory is
+plentiful, the <tt class="docutils literal">RES</tt> column in <tt
class="docutils literal">top</tt> remains high. (Exceptions to
+this rule are systems with no <tt class="docutils literal">MADV_FREE</tt>,
where we use
+<tt class="docutils literal">MADV_DONTNEED</tt>, which forcefully lowers the
<tt class="docutils literal">RES</tt>. This includes
+Linux <= 4.4.)</p>
<p>A more complete list of known differences is available at <a
class="reference external"
href="http://pypy.readthedocs.org/en/latest/cpython_differences.html";>our dev
site</a>.</p>
</div>
<div id="sidebar">
diff --git a/source/compat.txt b/source/compat.txt
--- a/source/compat.txt
+++ b/source/compat.txt
@@ -114,6 +114,16 @@
the program finishes running in the meantime). See `more details
here`_.
+Note that PyPy returns unused memory to the operating system if there
+is a madvise() system call (at least Linux, OS/X, BSD). It is
+important to realize that you may not see this in ``top``. The unused
+pages are marked with ``MADV_FREE``, which tells the system "if you
+need more memory at some point, grab this page". As long as memory is
+plentiful, the ``RES`` column in ``top`` remains high. (Exceptions to
+this rule are systems with no ``MADV_FREE``, where we use
+``MADV_DONTNEED``, which forcefully lowers the ``RES``. This includes
+Linux <= 4.4.)
+
A more complete list of known differences is available at `our dev site`_.
.. _`CPython C API`: http://docs.python.org/c-api/
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