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

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

> On Jan 24, 2017, at 12:45 PM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
> 
> 
> 
>> -----Original Message-----
>> From: Wiles, Keith
>> Sent: Tuesday, January 24, 2017 2:49 PM
>> 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 24, 2017, at 3:33 AM, Ananyev, Konstantin <konstantin.ananyev@intel.com> wrote:
>>> 
>>> 
>>> 
>>>> -----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.
>> 
>> 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.
> 
> Ok, I probably wasn't very clear too.
> What I meant:
> Let's try to implement GRO (in SW) as a standalone DPDK library,
> with clean & simple interface and see how fast and useful it would be.
> We can refer to it as step 1.
> When (if) we'll have step 1 in place, then we can start thinking
> about adding combined HW/SW solution for it (step 2).
> I think at that stage it would be much clearer:
> is there any point in it  at all,
> and if yes, how it should be done:
>  -changes at rte_ethedev or on PMD layers or both
>  - would changes at rte_ethdev API be needed and if yes what particular, etc.
> 
> From my perspective, without step 1 in place,  there is no much point in approaching step 2.

Currently I believe they have a SW library version of the code, but I think we need to look at the design in that form. At this time the current design or code is not what I would expect needs to be done for the transparent version. To many interactions with the application and a separate Rx/Tx functions were being used (If I remember correctly)

> 
> BTW, any particular HW you have in mind?
> Currently, as I can see LRO (HW) is supported only by ixgbe and probably by viritual PMDs (virtio/vmxent3).
> Though even for ixgbe there are plenty of limitations: SRIOV mode should be off, HW CRC stropping should be off, etc.
> So my guess, right now step 1 is much more useful and feasible.
> 
>> 
>>> 
>>>> 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.
> 
> Actually could you provide an example of linux NIC driver, that uses HW offloads (and which) to implement GRO?
> I presume some might use HW generated hashes, but apart from that, when HW performs actual packet grouping?
> From what I've seen Intel ones rely SW implementation for that.
> But I am not a linux/GRO expert, so feel free to correct me here.
> Konstantin 

Regards,
Keith