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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----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.

> 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 :)

Konstantin