doc: add use of mlockall to programmers guide

When I was adding mlockall() to the testpmd application it was
suggested to add a reference to the use case of mlockall(). This patch
adds is.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: John McNamara <john.mcnamara@intel.com>

diff --git a/doc/guides/prog_guide/writing_efficient_code.rst b/doc/guides/prog_guide/writing_efficient_code.rst
index 8223ace..d7ac677 100644 (file)
--- a/doc/guides/prog_guide/writing_efficient_code.rst
+++ b/doc/guides/prog_guide/writing_efficient_code.rst
@@ -105,6 +105,21 @@ meaning that if all memory access operations are done on the first channel only,
 
 By default, the  :ref:`Mempool Library <Mempool_Library>` spreads the addresses of objects among memory channels.
 
+Locking memory pages
+~~~~~~~~~~~~~~~~~~~~
+
+The underlying operating system is allowed to load/unload memory pages at its own discretion.
+These page loads could impact the performance, as the process is on hold when the kernel fetches them.
+
+To avoid these you could pre-load, and lock them into memory with the ``mlockall()`` call.
+
+.. code-block:: c
+
+    if (mlockall(MCL_CURRENT | MCL_FUTURE)) {
+        RTE_LOG(NOTICE, USER1, "mlockall() failed with error \"%s\"\n",
+                strerror(errno));
+    }
+
 Communication Between lcores
 ----------------------------