doc: update information on using hugepages

[ upstream commit 8397cac ]

Current information regarding hugepage usage is a little out of date.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.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

doc/guides/linux_gsg/sys_reqs.rst

@@ -156,8 +156,36 @@ 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 reservation of hugepages can be performed at run time.
+This is done by echoing the number of hugepages required
+to a ``nr_hugepages`` file in the ``/sys/kernel/`` directory
+corresponding to a specific page size (in Kilobytes).
+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
+
+On a NUMA machine, the above command will usually divide the number of hugepages
+equally across all NUMA nodes (assuming there is enough memory on all NUMA nodes).
+However, pages can also be reserved explicitly on individual NUMA nodes
+using a ``nr_hugepages`` file in the ``/sys/devices/`` directory::
+
+    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::
+
+    Some kernel versions may not allow reserving 1 GB hugepages at run time,
+    so reserving them at boot time may be the only option.
+    Please see below for instructions.
+
+**Alternative:**
+
+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::
@@ -186,35 +214,29 @@ the number of hugepages reserved at boot time is generally divided equally betwe
 
 See the Documentation/admin-guide/kernel-parameters.txt file in your Linux source tree for further details of these and other kernel options.
 
-**Alternative:**
-
-For 2 MB pages, 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)::
-
-    echo 1024 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
-
-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
+Using Hugepages with the DPDK
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-.. note::
+If secondary process support is not required, DPDK is able to use hugepages
+without any configuration by using "in-memory" mode.
+Please see :doc:`linux_eal_parameters` for more details.
 
-    For 1G pages, it is not possible to reserve the hugepage memory after the system has booted.
+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.
 
-Using Hugepages with the DPDK
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+However, in order to use hugepage sizes other than the default, it is necessary
+to manually create mount points for those hugepage sizes (e.g. 1GB pages).
 
-Once the hugepage memory is reserved, to make the memory available for DPDK use, perform the following steps::
+To make the hugepages of size 1GB available for DPDK use,
+following steps must be performed::
 
     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