> On Jan 24, 2017, at 3:33 AM, Ananyev, Konstantin > <konstantin.anan...@intel.com> wrote: > > > >> -----Original Message----- >> From: Wiles, Keith >> Sent: Tuesday, January 24, 2017 5:26 AM >> To: Ananyev, Konstantin <konstantin.anan...@intel.com> >> Cc: Stephen Hemminger <step...@networkplumber.org>; Hu, Jiayu >> <jiayu...@intel.com>; dev@dpdk.org; Kinsella, Ray >> <ray.kinse...@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 6:43 PM, Ananyev, Konstantin >>> <konstantin.anan...@intel.com> wrote: >>> >>> >>> >>>> -----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. >> >> 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. > >> 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. > > Konstantin Regards, Keith
