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

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 24-Aug-20 6:13 PM, Bruce Richardson wrote:
> On Mon, Aug 24, 2020 at 04:45:01PM +0100, Anatoly Burakov wrote:
>> Current information regarding hugepage usage is a little out of date.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@dpdk.org
>>
>> Signed-off-by: Anatoly Burakov <anatoly.burakov@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.
>> +
> 
> Although we are removing the note about 1G pages requiring to be reserved
> at boot time, I think we should still mention here that some older kernel
> versions do not allow 1G reservations post-boot.

Agreed, will fix in v2.

> 
>>   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
> 
> Rather than multiple hugepage sizes, I'd suggest changing this to hugepage
> sizes other than the default.

OK, will fix.

> 
> Do we also want to add a line somewhere explaining that the default size
> can be set a boot using a kernel parameter?

It's already there, right above this :)

> 
>> +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


-- 
Thanks,
Anatoly