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

[Jiayu Hu <jiayu.hu@intel.com>]

On Wed, Jan 25, 2017 at 05:51:50PM +0800, Ananyev, Konstantin wrote:
> 
> 
> > -----Original Message-----
> > From: Wiles, Keith
> > Sent: Wednesday, January 25, 2017 3:40 AM
> > To: Stephen Hemminger <stephen@networkplumber.org>
> > Cc: Ananyev, Konstantin <konstantin.ananyev@intel.com>; Hu, Jiayu <jiayu.hu@intel.com>; dev@dpdk.org; Kinsella, Ray
> > <ray.kinsella@intel.com>; Gilmore, Walter E <walter.e.gilmore@intel.com>; Venkatesan, Venky <venky.venkatesan@intel.com>;
> > yuanhan.liu@linux.intel.com
> > Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK
> > 
> > 
> > > On Jan 24, 2017, at 2:04 PM, Stephen Hemminger <stephen@networkplumber.org> wrote:
> > >
> > > On Tue, 24 Jan 2017 20:09:07 +0000
> > > "Wiles, Keith" <keith.wiles@intel.com> wrote:
> > >
> > >>> On Jan 24, 2017, at 12:45 PM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
> > >>>
> > >>>
> > >>>
> > >>>> -----Original Message-----
> > >>>> From: Wiles, Keith
> > >>>> Sent: Tuesday, January 24, 2017 2:49 PM
> > >>>> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > >>>> Cc: Stephen Hemminger <stephen@networkplumber.org>; Hu, Jiayu <jiayu.hu@intel.com>; dev@dpdk.org; Kinsella, Ray
> > >>>> <ray.kinsella@intel.com>; Gilmore, Walter E <walter.e.gilmore@intel.com>; Venkatesan, Venky <venky.venkatesan@intel.com>;
> > >>>> yuanhan.liu@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@intel.com> wrote:
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>>> -----Original Message-----
> > >>>>>> From: Wiles, Keith
> > >>>>>> Sent: Tuesday, January 24, 2017 5:26 AM
> > >>>>>> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > >>>>>> Cc: Stephen Hemminger <stephen@networkplumber.org>; Hu, Jiayu <jiayu.hu@intel.com>; dev@dpdk.org; Kinsella, Ray
> > >>>>>> <ray.kinsella@intel.com>; Gilmore, Walter E <walter.e.gilmore@intel.com>; Venkatesan, Venky <venky.venkatesan@intel.com>;
> > >>>>>> yuanhan.liu@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@intel.com> wrote:
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>> -----Original Message-----
> > >>>>>>>> From: Wiles, Keith
> > >>>>>>>> Sent: Monday, January 23, 2017 9:53 PM
> > >>>>>>>> To: Stephen Hemminger <stephen@networkplumber.org>
> > >>>>>>>> Cc: Hu, Jiayu <jiayu.hu@intel.com>; dev@dpdk.org; Kinsella, Ray <ray.kinsella@intel.com>; Ananyev, Konstantin
> > >>>>>>>> <konstantin.ananyev@intel.com>; Gilmore, Walter E <walter.e.gilmore@intel.com>; Venkatesan, Venky
> > >>>>>> <venky.venkatesan@intel.com>;
> > >>>>>>>> yuanhan.liu@linux.intel.com
> > >>>>>>>> Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK
> > >>>>>>>>
> > >>>>>>>>
> > >>>>>>>>> On Jan 23, 2017, at 10:15 AM, Stephen Hemminger <stephen@networkplumber.org> wrote:
> > >>>>>>>>>
> > >>>>>>>>> On Mon, 23 Jan 2017 21:03:12 +0800
> > >>>>>>>>> Jiayu Hu <jiayu.hu@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.
> > >>
> > >> Currently I believe they have a SW library version of the code, but I think we need to look at the design in that form. At this time the
> > current design or code is not what I would expect needs to be done for the transparent version. To many interactions with the application
> > and a separate Rx/Tx functions were being used (If I remember correctly)
> > >>
> > >>>
> > >>> 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.
> > >>> Konstantin
> > >>
> > >> Regards,
> > >> Keith
> > >>
> > >
> > > Linux uses a push (rather than DPDK pull) model for packet receiving.
> > > The Linux driver pushes packets into GRO by calling napi_gro_receive.
> > >
> > > Since DPDK is pull model the API would be simpler.
> > > it could be as simple as:
> > >  nb = rte_eth_rx_burst(port, rx_pkts, N);
> > >  nb = rte_rx_gro(port, rx_pkts, gro_pkts, nb);
> > >
> > > I agree with others, look at ip reassembly library as example.
> > > Also, GRO does not make sense for applications which already do the same vector flow
> > > processing like VPP which is one reason it should be optional.
> > 
> > I agree it should be option, but I worry about making it an example. I would like to see the GRO to be more transparent to the application
> > and supported as a generic feature for DPDK. Maybe the application needs to request the support or it is a config option. The problem with
> > config options is they are hard to test and testing becomes complexed.
> > 
> > Can we not figure out a way to add the feature inline instead of the application needing to call these APIs? It would be nice to have IP
> > fragmentation also a optional feature to the rx/tx ethdev call. It would take it out of the example zone and move it into DPDK as a real
> > feature. Today we expect the application to chain all of these little bits outside of DPDK into something useful, can we help fix that
> > problem?
> 
> If the user would like this feature to be transparent, he/she can
> always setup a RX callback that would call GRO API inside.
> Let say TLDK udpfwd example does the same for ip reassemble.
> Konstantin 

Thanks for all your advices.

After considering all these ideas, I think there are three reasons to
implement GRO as a standalone library first.
    a. Take less efforts since avoid modifying drivers;
    b. Easier to test and estimate performance gains that GRO can bring
    to DPDK based applications;
    c. A standalone library brings more flexibility to applications.
After seeing the benefit, we start to discuss how to combine HW and SW
solutions.

Regards,
Jiayu