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

["Wiles, Keith" <keith.wiles@intel.com>]

> 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.

Regards,
Keith