Accelerators and coprocessors are proliferating in the datacenter, and it has been a boon for speeding up certain kinds of workloads and, in many cases, making machine learning or simulation jobs possible at scale for the first time. But ultimately, in a hybrid system, the processors and the accelerators have to share data, and moving it about is a pain in the neck.
Having the memory across these devices operate in a coherent manner – meaning that all devices can address all memory attached to those devices in a single, consistent way – is one of the holy grails of hybrid computing. There are, in fact, a number of ways that this is being done on hybrid systems today, and that is a problem of its own and one that the Coherent Connect Interconnect for Accelerators, or CCIX, consortium seeks to address.
It is a tall order, but one that AMD, ARM Holdings, Huawei Technologies, IBM, Mellanox Technologies, Qualcomm, and Xilinx are taking on, and if they succeed in creating a more open specification to link processors and accelerators (including functions on network cards or non-volatile storage) together with coherent memory, building hybrid systems will be significantly easier than it is today. The question we have, and that the CCIX has not yet answered, is precisely how this might be done.
IBM’s Coherent Processor Accelerator Interface, or CAPI, which was first made available on its Power8 processors, provides a blueprint for how this could be accomplished, and so does Nvidia’s NVLink interconnect.
With CAPI, IBM has created a leaner and meaner coherency protocol that rides atop the PCI-Express peripheral interconnect and that allows accelerators like GPUs, DSPs, and FPGAs to link into the Power8 processor complex and address main memory in the server much like they would their own local memory (if they have any). Nvidia’s NVLink interconnect is another example of an interconnect that is not only used to attach devices to processors with coherency across the memory, but is also used to lash together multiple GPU accelerators so they can share memory in some fashion and more efficiently scale out applications running across those GPUs.
In its mission statement, the CCIX consortium outlined its goals, saying that it wanted to be able to accommodate both offload and bump-in-the-wire accelerators with this memory coherency while at the same time leveraging existing server interconnects and providing this coherency without the need for interrupts or drivers. The idea is also, as with CAPI, to provide higher bandwidth than current coherency protocols running on existing interfaces and at least an order of magnitude of improvement in latencies to make hybrid architectures even more attractive for running various kinds of workloads.
It is not much of a surprise that Intel has not joined the CCIX consortium, but it is a bit of a surprise that Nvidia, which has been gradually evolving the programming model for GPU accelerators to provide increasing levels of coherency between the processing elements in a system over the past eight years. Clearly, Nvidia has a vested interest in supporting coherency, and Intel may have to concede that it is better to play with other accelerators well through an open standard such as that which the CCIX develops rather than allow other processors and accelerators to have advantages that its Xeon processors do not.
The Power and ARM architectures are the two emerging alternatives to the Xeon processor in the datacenter, but neither has gone mainstream for core compute and both camps have aspirations for a sizable chunk of the compute in the coming years. Better leveraging hybrid architectures through a common and efficient coherency protocol that works across multiple interconnects and is more or less invisible to programmers and the applications they create could compel Intel to join up. Intel does not support NVLink or CAPI protocols on its Xeon processors, but there is no reason that it could not do so if enough customers demanded it do so. For now, adoption of CAPI has been fairly low, and NVLink is just getting started with the “Pascal” generation of GPU accelerators from Nvidia and will not ramp in volume until early next year.
If IBM is willing to tweak CAPI to adhere to the CCIX standard, and the CCIX technology is either open sourced or made relatively cheaply available through licensing like the PCI-Express protocol from the PCI-SIG that controls this peripheral interconnect, then there is a good chance that Intel and perhaps other processor manufacturers like Oracle, Fujitsu, and other members of the ARM collective, could get behind it as well. Intel might need a standard way to interface with Altera FPGAs, too, even as it is offering a prototype hybrid Broadwell Xeon-Arria FPGA package and is expected to deliver a true hybrid Xeon-FPGA chip further down the road.
It is not a foregone conclusion that PCI-Express will be the means of creating the coherent fabric on which the CCIX protocol stack will run, Kevin Deierling, vice president of marketing at Mellanox, tells The Next Platform. With the PCI-Express, CAPI, and NVLink protocols, for instance, these protocols are leveraging the same signaling pins, but the way they make use of the pins are very different and CCIX could be as well in a transparent way. It is also reasonable to expect that CCIX will in some sense eventually extend the protocols that link processors to main memory and to each other out across a fabric to other processors and accelerators. This fabric will not replace existing NUMA protocols, but augment them. But this is all conjecture until CCIX releases its first draft of a specification some time before the end of the year.
There is some talk out there that the CCIX effort got its start when Intel acquired Altera last year, but Deierling says that the efforts to have a consistent memory coherency method predate this and even the OpenPower consortium that many of the CCIX players are also a part of.
“The main thing is that we do not reinvent the wheel five or six times,” says Deierling. “There is a need for an industry standard cache coherency interface, and finally everyone agrees the need is clear and the time is now.”
And as for timing for CCIX products, there are those who say that it will take until 2019 or 2020 for the first CCIX products to come to market, but given how there are already precedents in the market with CAPI-enabled FPGAs and network adapter cards, it may not take as long and could be more like the 25G Ethernet standard, which was pushed by the hyperscalers and which moved from idea to product a lot faster than the consortium of Ethernet vendors normally does.
“When we go outside of committees, we can go a lot faster,” says Deierling. This kind of speed is what both hyperscalers and HPC centers both need, and they are the top users of accelerators today and certainly want to be able to plug more types of coprocessors into their systems and not have to radically change their programming models. While hyperscalers and HPC shops have been among the drivers for the CCIX approach, unlike hyperscalers with the 25G Ethernet effort, they are not the main drivers. Suppliers of processors, coprocessors, and networking devices that are aligning and competing against Intel in the datacenter seem to be in agreement that they need to work together.
We happen to think that the generic accelerator interconnect that is derived from NVLink on the Power9 chips, which IBM talked about a bit back in March and which is due in 2018, is in fact going to be supporting CCIX. We also think that ARM server chips need a consistent coherency method that spans more than one socket and that also allows for various accelerators to hook into ARM chips, and CCIX could fill that need nicely if ARM Holdings and its licensees get behind it. Cavium and Applied Micro, which gave their own NUMA interconnects already, are not on board with CCIX yet but Qualcomm, which has server aspirations and no experience with NUMA, is. AMD obviously has done a lot of work on this with the Heterogeneous Systems Architecture that spans its CPU-GPU hybrids, which in theory is expandable to discrete devices and not just its APU chips.
We have reached out to Intel and Nvidia to see what they think about all of this. Stay tuned.
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