This patch introduces a new section in the programmers guide describing the reorder library.
Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy at intel.com> --- doc/guides/prog_guide/index.rst | 1 + doc/guides/prog_guide/reorder_lib.rst | 115 ++++++++++++++++++++++++++++++++++ 2 files changed, 116 insertions(+) create mode 100644 doc/guides/prog_guide/reorder_lib.rst diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst index 8d86dd4..de69682 100644 --- a/doc/guides/prog_guide/index.rst +++ b/doc/guides/prog_guide/index.rst @@ -61,6 +61,7 @@ Programmer's Guide lpm_lib lpm6_lib packet_distrib_lib + reorder_lib ip_fragment_reassembly_lib multi_proc_support kernel_nic_interface diff --git a/doc/guides/prog_guide/reorder_lib.rst b/doc/guides/prog_guide/reorder_lib.rst new file mode 100644 index 0000000..bbd0521 --- /dev/null +++ b/doc/guides/prog_guide/reorder_lib.rst @@ -0,0 +1,115 @@ +.. BSD LICENSE + Copyright(c) 2015 Intel Corporation. All rights reserved. + All rights reserved. + + Redistribution and use in source and binary forms, with or without + modification, are permitted provided that the following conditions + are met: + + * Redistributions of source code must retain the above copyright + notice, this list of conditions and the following disclaimer. + * Redistributions in binary form must reproduce the above copyright + notice, this list of conditions and the following disclaimer in + the documentation and/or other materials provided with the + distribution. + * Neither the name of Intel Corporation nor the names of its + contributors may be used to endorse or promote products derived + from this software without specific prior written permission. + + THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR + A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT + OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, + SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT + LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, + DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY + THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT + (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE + OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + +.. _Reorder_Library: + +Reorder Library +================= + +The Reorder Library provides a mechanism for reordering mbufs based on their +sequence number. + +Operation +---------- + +The reorder library is essentially a buffer that reorders mbufs. +The user inserts out of order mbufs into the reorder buffer and pulls in-order +mbufs from it. + +At a given time, the reorder buffer contains mbufs whose sequence number are +inside the sequence window. The sequence window is determined by the minimum +sequence number and the number of entries that the buffer was configured to hold. +For example, given a reorder buffer with 200 entries and a minimum sequence +number of 350, the sequence window has low and high limits of 350 and 550 +respectively. + +When inserting mbufs, the reorder library differentiates between valid, early +and late mbufs depending on the sequence number of the inserted mbuf: + +* valid: the sequence number is inside the window. +* late: the sequence number is outside the window and less than the low limit. +* early: the sequence number is outside the window and greater than the high + limit. + +The reorder buffer directly returns late mbufs and tries to accommodate early +mbufs. + + +Implementation Details +------------------------- + +The reorder library is implemented as a pair of buffers, which referred to as +the *Order* buffer and the *Ready* buffer. + +On an insert call, valid mbufs are inserted directly into the Order buffer and +late mbufs are returned to the user with an error. + +In the case of early mbufs, the reorder buffer will try to move the window +(incrementing the minimum sequence number) so that the mbuf becomes a valid one. +To that end, mbufs in the Order buffer are moved into the Ready buffer. +Any mbufs that have not arrived yet are ignored and therefore will become +late mbufs. +This means that as long as there is room in the Ready buffer, the window will +be moved to accommodate early mbufs that would otherwise be outside the +reordering window. + +For example, assuming that we have a buffer of 200 entries with a 350 minimum +sequence number, and we need to insert an early mbuf with 565 sequence number. +That means that we would need to move the windows at least 15 positions to +accommodate the mbuf. +The reorder buffer would try to move mbufs from at least the next 15 slots in +the Order buffer to the Ready buffer, as long as there is room in the Ready buffer. +Any gaps in the Order buffer at that point are skipped, and those packet will +be reported as late packets when they arrive. The process of moving packets +to the Ready buffer continues beyond the minimum required until a gap, +i.e. missing mbuf, in the Order buffer is encountered. + +When draining mbufs, the reorder buffer would return mbufs in the Ready +buffer first and then from the Order buffer until a gap is found (mbufs that +have not arrived yet). + +Use Case: Packet Distributor +------------------------------- + +An application using the DPDK packet distributor could make use of the reorder +library to transmit packets in the same order they were received. + +A basic packet distributor use case would consist of a distributor with +multiple workers cores. +The processing of packets by the workers is not guaranteed to be in order, +hence a reorder buffer can be used to order as many packets as possible. + +In such a scenario, the distributor assigns a sequence number to mbufs before +delivering them to the workers. +As the workers finish processing the packets, the distributor inserts those +mbufs into the reorder buffer and finally transmit drained mbufs. + +NOTE: Currently the reorder buffer is not thread safe so the same thread is +responsible for inserting and draining mbufs. -- 1.9.3