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

Ananyev, Konstantin konstantin.ananyev at intel.com
Tue Jan 24 02:43:06 CET 2017

> -----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.
> 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.

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