[dpdk-dev] [RFC] Add GRO support in DPDK

Ananyev, Konstantin konstantin.ananyev at intel.com
Tue Jan 24 20:45:39 CET 2017

> -----Original Message-----
> From: Wiles, Keith
> Sent: Tuesday, January 24, 2017 2:49 PM
> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> Cc: Stephen Hemminger <stephen at networkplumber.org>; Hu, Jiayu <jiayu.hu at intel.com>; dev at dpdk.org; Kinsella, Ray
> <ray.kinsella at intel.com>; Gilmore, Walter E <walter.e.gilmore at intel.com>; Venkatesan, Venky <venky.venkatesan at intel.com>;
> yuanhan.liu at linux.intel.com
> Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK
> > On Jan 24, 2017, at 3:33 AM, Ananyev, Konstantin <konstantin.ananyev at intel.com> wrote:
> >
> >
> >
> >> -----Original Message-----
> >> From: Wiles, Keith
> >> Sent: Tuesday, January 24, 2017 5:26 AM
> >> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> >> Cc: Stephen Hemminger <stephen at networkplumber.org>; Hu, Jiayu <jiayu.hu at intel.com>; dev at dpdk.org; Kinsella, Ray
> >> <ray.kinsella at intel.com>; Gilmore, Walter E <walter.e.gilmore at intel.com>; Venkatesan, Venky <venky.venkatesan at intel.com>;
> >> yuanhan.liu at linux.intel.com
> >> Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK
> >>
> >>
> >>> On Jan 23, 2017, at 6:43 PM, Ananyev, Konstantin <konstantin.ananyev at intel.com> wrote:
> >>>
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: Wiles, Keith
> >>>> Sent: Monday, January 23, 2017 9:53 PM
> >>>> To: Stephen Hemminger <stephen at networkplumber.org>
> >>>> Cc: Hu, Jiayu <jiayu.hu at intel.com>; dev at dpdk.org; Kinsella, Ray <ray.kinsella at intel.com>; Ananyev, Konstantin
> >>>> <konstantin.ananyev at intel.com>; Gilmore, Walter E <walter.e.gilmore at intel.com>; Venkatesan, Venky
> >> <venky.venkatesan at intel.com>;
> >>>> yuanhan.liu at linux.intel.com
> >>>> Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK
> >>>>
> >>>>
> >>>>> On Jan 23, 2017, at 10:15 AM, Stephen Hemminger <stephen at networkplumber.org> wrote:
> >>>>>
> >>>>> On Mon, 23 Jan 2017 21:03:12 +0800
> >>>>> Jiayu Hu <jiayu.hu at intel.com> wrote:
> >>>>>
> >>>>>> With the support of hardware segmentation techniques in DPDK, the
> >>>>>> networking stack overheads of send-side of applications, which directly
> >>>>>> leverage DPDK, have been greatly reduced. But for receive-side, numbers of
> >>>>>> segmented packets seriously burden the networking stack of applications.
> >>>>>> Generic Receive Offload (GRO) is a widely used method to solve the
> >>>>>> receive-side issue, which gains performance by reducing the amount of
> >>>>>> packets processed by the networking stack. But currently, DPDK doesn't
> >>>>>> support GRO. Therefore, we propose to add GRO support in DPDK, and this
> >>>>>> RFC is used to explain the basic DPDK GRO design.
> >>>>>>
> >>>>>> DPDK GRO is a SW-based packets assembly library, which provides GRO
> >>>>>> abilities for numbers of protocols. In DPDK GRO, packets are merged
> >>>>>> before returning to applications and after receiving from drivers.
> >>>>>>
> >>>>>> In DPDK, GRO is a capability of NIC drivers. That support GRO or not and
> >>>>>> what GRO types are supported are up to NIC drivers. Different drivers may
> >>>>>> support different GRO types. By default, drivers enable all supported GRO
> >>>>>> types. For applications, they can inquire the supported GRO types by
> >>>>>> each driver, and can control what GRO types are applied. For example,
> >>>>>> ixgbe supports TCP and UDP GRO, but the application just needs TCP GRO.
> >>>>>> The application can disable ixgbe UDP GRO.
> >>>>>>
> >>>>>> To support GRO, a driver should provide a way to tell applications what
> >>>>>> GRO types are supported, and provides a GRO function, which is in charge
> >>>>>> of assembling packets. Since different drivers may support different GRO
> >>>>>> types, their GRO functions may be different. For applications, they don't
> >>>>>> need extra operations to enable GRO. But if there are some GRO types that
> >>>>>> are not needed, applications can use an API, like
> >>>>>> rte_eth_gro_disable_protocols, to disable them. Besides, they can
> >>>>>> re-enable the disabled ones.
> >>>>>>
> >>>>>> The GRO function processes numbers of packets at a time. In each
> >>>>>> invocation, what GRO types are applied depends on applications, and the
> >>>>>> amount of packets to merge depends on the networking status and
> >>>>>> applications. Specifically, applications determine the maximum number of
> >>>>>> packets to be processed by the GRO function, but how many packets are
> >>>>>> actually processed depends on if there are available packets to receive.
> >>>>>> For example, the receive-side application asks the GRO function to
> >>>>>> process 64 packets, but the sender only sends 40 packets. At this time,
> >>>>>> the GRO function returns after processing 40 packets. To reassemble the
> >>>>>> given packets, the GRO function performs an "assembly procedure" on each
> >>>>>> packet. We use an example to demonstrate this procedure. Supposing the
> >>>>>> GRO function is going to process packetX, it will do the following two
> >>>>>> things:
> >>>>>> 	a. Find a L4 assembly function according to the packet type of
> >>>>>> 	packetX. A L4 assembly function is in charge of merging packets of a
> >>>>>> 	specific type. For example, TCPv4 assembly function merges packets
> >>>>>> 	whose L3 IPv4 and L4 is TCP. Each L4 assembly function has a packet
> >>>>>> 	array, which keeps the packets that are unable to assemble.
> >>>>>> 	Initially, the packet array is empty;
> >>>>>> 	b. The L4 assembly function traverses own packet array to find a
> >>>>>> 	mergeable packet (comparing Ethernet, IP and L4 header fields). If
> >>>>>> 	finds, merges it and packetX via chaining them together; if doesn't,
> >>>>>> 	allocates a new array element to store packetX and updates element
> >>>>>> 	number of the array.
> >>>>>> After performing the assembly procedure to all packets, the GRO function
> >>>>>> combines the results of all packet arrays, and returns these packets to
> >>>>>> applications.
> >>>>>>
> >>>>>> There are lots of ways to implement the above design in DPDK. One of the
> >>>>>> ways is:
> >>>>>> 	a. Drivers tell applications what GRO types are supported via
> >>>>>> 	dev->dev_ops->dev_infos_get;
> >>>>>> 	b. When initialize, drivers register own GRO function as a RX
> >>>>>> 	callback, which is invoked inside rte_eth_rx_burst. The name of the
> >>>>>> 	GRO function should be like xxx_gro_receive (e.g. ixgbe_gro_receive).
> >>>>>> 	Currently, the RX callback can only process the packets returned by
> >>>>>> 	dev->rx_pkt_burst each time, and the maximum packet number
> >>>>>> 	dev->rx_pkt_burst returns is determined by each driver, which can't
> >>>>>> 	be interfered by applications. Therefore, to implement the above GRO
> >>>>>> 	design, we have to modify current RX implementation to make driver
> >>>>>> 	return packets as many as possible until the packet number meets the
> >>>>>> 	demand of applications or there are not available packets to receive.
> >>>>>> 	This modification is also proposed in patch:
> >>>>>> 	http://dpdk.org/ml/archives/dev/2017-January/055887.html;
> >>>>>> 	c. The GRO types to apply and the maximum number of packets to merge
> >>>>>> 	are passed by resetting RX callback parameters. It can be achieved by
> >>>>>> 	invoking rte_eth_rx_callback;
> >>>>>> 	d. Simply, we can just store packet addresses into the packet array.
> >>>>>> 	To check one element, we need to fetch the packet via its address.
> >>>>>> 	However, this simple design is not efficient enough. Since whenever
> >>>>>> 	checking one packet, one pointer dereference is generated. And a
> >>>>>> 	pointer dereference always causes a cache line miss. A better way is
> >>>>>> 	to store some rules in each array element. The rules must be the
> >>>>>> 	prerequisites of merging two packets, like the sequence number of TCP
> >>>>>> 	packets. We first compare the rules, then retrieve the packet if the
> >>>>>> 	rules match. If storing the rules causes the packet array structure
> >>>>>> 	is cache-unfriendly, we can store a fixed-length signature of the
> >>>>>> 	rules instead. For example, the signature can be calculated by
> >>>>>> 	performing XOR operation on IP addresses. Both design can avoid
> >>>>>> 	unnecessary pointer dereferences.
> >>>>>
> >>>>>
> >>>>> Since DPDK does burst mode already, GRO is a lot less relevant.
> >>>>> GRO in Linux was invented because there is no burst mode in the receive API.
> >>>>>
> >>>>> If you look at VPP in FD.io you will see they already do aggregration and
> >>>>> steering at the higher level in the stack.
> >>>>>
> >>>>> The point of GRO is that it is generic, no driver changes are necessary.
> >>>>> Your proposal would add a lot of overhead, and cause drivers to have to
> >>>>> be aware of higher level flows.
> >>>>
> >>>> NACK
> >>>>
> >>>> The design is not super clear to me here and we need to understand the impact to DPDK, performance and the  application. I would
> like
> >> to
> >>>> have a clean transparent design to the application and as little impact on performance as possible.
> >>>>
> >>>> Let discuss this as I am not sure my previous concerns were addressed in this RFC.
> >>>>
> >>>
> >>> I would agree that design looks overcomplicated and strange:
> >>> If GRO can (and supposed to be) done fully in SW, why do we need to modify PMDs at all,
> >>> why it can't be just a standalone DPDK library that user can use on his/her convenience?
> >>> I'd suggest to start with some simple and most widespread case (TCP?) and try to implement
> >>> a library for it first: something similar to what we have for ip reassembly.
> >>
> >> The reason this should not be a library the application calls is to allow for a transparent design for HW and SW support of this feature.
> Using
> >> the SW version the application should not need to understand (other then performance) that GRO is being done for this port.
> >>
> >
> > Why is that?
> > Let say we have ip reassembly library that is called explicitly by the application.
> > I think for L4 grouping we can do the same.
> > After all it is a pure SW feature, so to me it makes sense to allow application to decide
> > when/where to call it.
> > Again it would allow people to develop/use it without any modifications in current PMDs.
> I guess I did not make it clear, we need to support HW and this SW version transparently just as we handle other features in HW/SW under a
> generic API for DPDK.

Ok, I probably wasn't very clear too.
What I meant:
Let's try to implement GRO (in SW) as a standalone DPDK library,
with clean & simple interface and see how fast and useful it would be.
We can refer to it as step 1.
When (if) we'll have step 1 in place, then we can start thinking
about adding combined HW/SW solution for it (step 2).
I think at that stage it would be much clearer:
 is there any point in it  at all,
and if yes, how it should be done:
  -changes at rte_ethedev or on PMD layers or both
  - would changes at rte_ethdev API be needed and if yes what particular, etc.

>From my perspective, without step 1 in place,  there is no much point in approaching step 2.

BTW, any particular HW you have in mind?
Currently, as I can see LRO (HW) is supported only by ixgbe and probably by viritual PMDs (virtio/vmxent3).
Though even for ixgbe there are plenty of limitations: SRIOV mode should be off, HW CRC stropping should be off, etc.
So my guess, right now step 1 is much more useful and feasible.
> >
> >> As I was told the Linux kernel hides this features and make it transparent.
> >
> > Yes, but DPDK does a lot things in a different way.
> > So it doesn't look like a compelling reason for me :)
> Just looking at different options here and it is a compelling reason to me as it enforces the design can be transparent to the application.
> Having the application in a NFV deciding on hw or sw or both is not a good place to put that logic IMO.

Actually could you provide an example of linux NIC driver, that uses HW offloads (and which) to implement GRO?
I presume some might use HW generated hashes, but apart from that, when HW performs actual packet grouping?
>From what I've seen Intel ones rely SW implementation for that.
But I am not a linux/GRO expert, so feel free to correct me here.

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