[dpdk-dev] [PATCH 2/2] doc/linux_gsg: update information on using hugepages

[Anatoly Burakov]

Current information regarding hugepage usage is a little out of date.
Update it to include information on in-memory mode, as well as on
default mountpoints provided by systemd.

Cc: stable at dpdk.org

Signed-off-by: Anatoly Burakov <anatoly.burakov at intel.com>
---
 doc/guides/linux_gsg/sys_reqs.rst | 39 +++++++++++++++++++------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/doc/guides/linux_gsg/sys_reqs.rst b/doc/guides/linux_gsg/sys_reqs.rst
index a124656bcb..2ddd7ed667 100644
--- a/doc/guides/linux_gsg/sys_reqs.rst
+++ b/doc/guides/linux_gsg/sys_reqs.rst
@@ -155,8 +155,12 @@ Without hugepages, high TLB miss rates would occur with the standard 4k page siz
 Reserving Hugepages for DPDK Use
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The allocation of hugepages should be done at boot time or as soon as possible after system boot
-to prevent memory from being fragmented in physical memory.
+The allocation of hugepages can be performed either at run time or at boot time.
+In the general case, reserving hugepages at run time is perfectly fine, but in
+use cases where having lots of physically contiguous memory is required, it is
+preferable to reserve hugepages at boot time, as that will help in preventing
+physical memory from becoming heavily fragmented.
+
 To reserve hugepages at boot time, a parameter is passed to the Linux kernel on the kernel command line.
 
 For 2 MB pages, just pass the hugepages option to the kernel. For example, to reserve 1024 pages of 2 MB, use::
@@ -187,9 +191,9 @@ See the Documentation/admin-guide/kernel-parameters.txt file in your Linux sourc
 
 **Alternative:**
 
-For 2 MB pages, there is also the option of allocating hugepages after the system has booted.
+There is also the option of allocating hugepages after the system has booted.
 This is done by echoing the number of hugepages required to a nr_hugepages file in the ``/sys/devices/`` directory.
-For a single-node system, the command to use is as follows (assuming that 1024 pages are required)::
+For a single-node system, the command to use is as follows (assuming that 1024 of 2MB pages are required)::
 
     echo 1024 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
 
@@ -198,22 +202,27 @@ On a NUMA machine, pages should be allocated explicitly on separate nodes::
     echo 1024 > /sys/devices/system/node/node0/hugepages/hugepages-2048kB/nr_hugepages
     echo 1024 > /sys/devices/system/node/node1/hugepages/hugepages-2048kB/nr_hugepages
 
-.. note::
-
-    For 1G pages, it is not possible to reserve the hugepage memory after the system has booted.
-
 Using Hugepages with the DPDK
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Once the hugepage memory is reserved, to make the memory available for DPDK use, perform the following steps::
+If secondary process support is not required, DPDK is able to use hugepages
+without any configuration by using "in-memory" mode. Please see
+:ref:`linux_eal_parameters` for more details.
+
+If secondary process support is required, mount points for hugepages need to be
+created. On modern Linux distributions, a default mount point for hugepages is provided
+by the system and is located at ``/dev/hugepages``. This mount point will use the
+default hugepage size set by the kernel parameters as described above.
+
+However, in order to use multiple hugepage sizes, it is necessary to manually
+create mount points for hugepage sizes that are not provided by the system
+(e.g. 1GB pages).
+
+To make the hugepages of size 1GB available for DPDK use, perform the following steps::
 
     mkdir /mnt/huge
-    mount -t hugetlbfs nodev /mnt/huge
+    mount -t hugetlbfs pagesize=1GB /mnt/huge
 
 The mount point can be made permanent across reboots, by adding the following line to the ``/etc/fstab`` file::
 
-    nodev /mnt/huge hugetlbfs defaults 0 0
-
-For 1GB pages, the page size must be specified as a mount option::
-
-    nodev /mnt/huge_1GB hugetlbfs pagesize=1GB 0 0
+    nodev /mnt/huge hugetlbfs pagesize=1GB 0 0
-- 
2.17.1