> -----Original Message----- > From: Wiles, Keith > Sent: Monday, January 23, 2017 9:53 PM > To: Stephen Hemminger <step...@networkplumber.org> > Cc: Hu, Jiayu <jiayu...@intel.com>; dev@dpdk.org; Kinsella, Ray > <ray.kinse...@intel.com>; Ananyev, Konstantin > <konstantin.anan...@intel.com>; Gilmore, Walter E > <walter.e.gilm...@intel.com>; Venkatesan, Venky <venky.venkate...@intel.com>; > yuanhan....@linux.intel.com > Subject: Re: [dpdk-dev] [RFC] Add GRO support in DPDK > > > > On Jan 23, 2017, at 10:15 AM, Stephen Hemminger > > <step...@networkplumber.org> wrote: > > > > On Mon, 23 Jan 2017 21:03:12 +0800 > > Jiayu Hu <jiayu...@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. Konstantin
