NVLink Takes GPU Acceleration To The Next Level

One of the breakthrough moments in computing, which was compelled by necessity, was the advent of symmetric multiprocessor, or SMP, clustering to make two or more processors look and act, as far as the operating system and applications were concerned, as a single, more capacious processor. With NVLink clustering for GPUs and for lashing GPUs to CPUs, Nvidia is bringing something as transformative as SMP was for CPUs to GPU accelerators.

The NVLink interconnect has been in development for years, and is one of the “five miracles” that Nvidia CEO and co-founder Jen-Hsun Huang said at the GPU Technology Conference a month ago had to come together to make the “Pascal” GP100 graphics chip possible.

The other miracles include the Pascal architecture itself, which supports 16-bit half precision floating point math, important for machine learning algorithms and which has lots of tweaks to make use of NVLink.

Another is the shift to 16 nanometer FinFET processes from the 28 nanometer manufacturing techniques used for the “Kepler” and “Maxwell” GPUs; Nvidia skipped the 20 nanometer node at Taiwan Semiconductor Manufacturing Corp, which was perhaps unfortunate because Maxwell could have possibly been a better success for Tesla accelerators had it been available using 20 nanometer processes and filled in some gaps as the market awaits volume shipments of Pascal-based Tesla P100 accelerators in the first quarter of 2017.

The other risk that Nvidia took was for High Bandwidth Memory 2 stacked memory integrated with the Pascal GPU through chip on wafer on substrate, or CoWoS, packaging. The final miracle cited by Huang was improved machine learning algorithms that will be able to make use of up to eight interconnected Pascal GPUs that have shared virtual memory across the CPUs and GPUs in a node thanks to NVLink. That’s not really a miracle of technology, but rather the normal progress of software taking advantage of hardware, which always takes some time.

nvidia-pascal-miracles

This high-speed, shared virtual memory, which will get cache coherency with the NVLink 2.0 that will launch with the “Volta” GV100 GPUs that are expected to launch in 2017 (probably later in the year, but we may get a preview at the GPU Technology Conference that year), is not something that Intel is offering to link multiple “Knights Landing” Xeon Phi processors or coprocessors together.

Intel certainly has the knowledge to allow for either SMP or NUMA linking of chips like Knights Landing and does NUMA links for its Xeon E5 and Xeon E7 processors that are pretty efficient. But as Intel told The Next Platform last year, it cannot off NUMA clustering on Xeon Phi chips because the memory bandwidth on the near and far memory in the chip, which will be in excess of 490 GB/sec combined, is so high that even modest snooping across the NUMA links would swamp any QuickPath Interconnect (QPI) channel linking two Knights Landing chips together through shared memory. We are not saying that Intel can’t – or won’t – come up with something like NVLink for hooking accelerators together, but the idea thus far seems to be to present Xeon Phis as processors in their own right and then to get high bandwidth interconnects – Omni-Path running at 100 Gb/sec and then in 2018 or so at 200 Gb/sec – linking the single-socket nodes together a little more loosely than the NVLink offers.

As we have discussed in our Future Systems series last year, Intel looks like it will be making increasingly smaller compute elements on Knights processors with higher memory capacity and bandwidth, while the OpenPower partners IBM, Nvidia, and Mellanox Technologies are creating hybrid CPU-GPU nodes with much more compute in aggregate and various memories that are shared in a virtual manner. Architecturally, both approaches have their elegance and practicality, and it will be interesting to see what real-world customers can do with them given their existing HPC codes. We think there is a chance that Intel will be able to do some things with Knights processors for machine learning, too, as Nvidia has done, but not having FP16 support in the Xeon Phi is going to be an issue, as we have already discussed.

NVLink Is Not CAPI

The problem with the PCI-Express bus is that it is a bottleneck in a hybrid system that mixes and matches CPUs and other elements, be they GPU, DSP, or FPGA accelerators or even memory devices like flash cards. So even with unified memory and with direct memory access across the bus, PCI-Express just doesn’t have enough bandwidth for unified memory architectures running at the very high bandwidth of devices like the Tesla P100 card, which delivers 720 GB/sec of memory bandwidth into and out of the Pascal GPU in its current iteration.

nvidia-pascal-nvlink-block

With the Tesla GV100 cards based on the future Volta cards, Nvidia has said it is targeting around 1 TB/sec of memory bandwidth. (This was divulged as part of the CORAL supercomputer award with the US Department of Energy for the “Summit” and “Sierra” systems.) This is a reasonable boost in bandwidth, but not as large as the double precision floating point performance will be from Pascal to Volta, we think. In any event, clearly something with more bandwidth than PCI-Express 3.0 or 4.0 is necessary unless someone wanted to gang up PCI-Express ports. (This might have been a simpler solution, and was probably contemplated.)

Even still, both IBM and Nvidia want to retain some of the PCI-Express programming model, whether it is the Coherent Accelerator Processor Interface (CAPI) for the current Power8 and future Power9 processors or Nvidia with its Pascal and Volta GPU accelerator cards because PCI-Express is something people know how to work with. In a sense, both CAPI and NVLink are specialized versions of PCI-Express. IBM leaves the underlying PCI-Express hardware alone and provides abstraction layers in its firmware and its chips to make CAPI devices look like they plug into a “hollow core” on the CPU and act like any other core when it comes to addressing shared memory. Nvidia has provided a much higher bandwidth interconnect than is available with PCI-Express 3.0 and is even better than PCI-Express 4.0 and will widen the gap more, we presume, with its second NVLink iteration.

If Nvidia it didn’t offer a substantial performance advantage, there would be no point to it. Nvidia would just embed a PCI-Express switch on the GPU and be done with it.

There was been some confusion about the bandwidth of the NVLink interconnect, and part of that is on Nvidia, whose presentations in 2014 and 2015 provided bi-directional bandwidth numbers for PCI-Express 3.0 links but only showed the uni-directional bandwidth of NVLink and PCI-Express even though the graphics indicated bi-directional links. Like this one from the Summit and Sierra supercomputer announcements from November 2014:

nvidia-pascal-nvlink-coral

This left many of us scratching our heads about why Nvidia was so excited to have 20 GB/sec of peak bandwidth with NVLink 1.0 besting the 16 GB peak bandwidth of a PCI-Express 3.0 x16 port that would be normally used to support a Tesla accelerator card. Having 25 percent more peak oomph is great, but it is not earth shattering.

It turns out, Nvidia’s first iteration of NVLink is providing 40 GB/sec of bandwidth per link, if you do it bi-directionally, and when I originally wrote this story, I said this was significantly higher than 16 GB/sec of the PCI-Express link and did not understand that PCI-Express 3.0 delivers 32 GB/sec bi-directionally and thought the performance gap between NVLink and PCI-Express was more like a factor of 2.5X instead of 20 percent. In any event, two years ago many of us assumed that the excitement was around being able to gang up as many as four of these ports together to make a virtual uber NVLink pipe between two devices (that gives you the 2.5X in bandwidth that you crave). With PCI-Express 4.0, which debuts with IBM’s Power9 chip next year and which does not appear to be in Intel’s “Skylake” Xeon E5 v5 chip next year, peak bandwidth will be of 32 GB/sec over sixteen lanes each way, for a total of 64 GB/sec. We think that NVLink 2 will provide a bandwidth bump, but got the distinct impression that the number of NVLink ports would not necessarily increase on the GPU cards in the Volta generation. Our guess is to expect a 20 percent delta again between NVLink and PCI with the next generations of those protocols.

With the first generation of NVLink, the Nvidia Highspeed Signaling (NVHS) interconnect sends data between two devices over a differential pair that operates at 20 Gb/sec. Eight of these pairs are glued together for each direction, forming two sublinks that make up the NVLink and provide 40 GB/sec of peak bandwidth between two devices. The multiple links can be ganged up with two, three or four in a bundle, delivering 80 GB/sec, 120 GB/sec, or 160 GB/sec of bandwidth between devices.

With the Power8 chips now being enhanced with NVLink ports – don’t call it a Power8+ chip because it is not that even though IBM’s roadmaps from last year said there would be a Power8+ upgrade – companies interested in examining the potential uses of brawny hybrid CPU-GPU nodes can start to play with their code on such machines. But it does not take a CPU equipped with NVLink ports to make a more efficient GPU processing complex, as Nvidia’s own DGX-1 server illustrates perfectly. Take a look:

nvidia-pascal-nvlink-dgx-1

Each Pascal P100 card has four NVLink ports, and as you can see these can be used to directly link four GPUs to each other using three of the ports with the fourth port being used to link two sets of quads in a hybrid cube mesh architecture. With this approach, a GPU is no more than two hops away from any other GPU, and five eights of the time a GPU is only one hop away from any other GPU. But this hybrid cube mesh does not leave a way for the CPUs to link to the GPUs. Intel Xeon and IBM Power processors, which tend to be used in HPC settings, have their own on-chip PCI-Express controllers, which would enable linking the CPUs directly to specific GPUs in the system, but Nvidia’s approach with the DGX-1 system (and one that we expect other server makers to follow some of the time) is to put four PCI-Express switches from PLX, a division of Broadcom, onto the system board and link one CPU to half of the Tesla P100 complex and the other CPU to the other set of Teslas. (Some diagrams from Nvidia show only two PLX switches in the system, but the machine shows four switches, so we are going with four.) It is interesting to see that the switches are not used to cross couple the CPUs to each other, but this is done over QuickPath Interconnect and NUMA links over Power8 chips.

The NVLink connections at 40 GB/sec are obviously 2.5X faster than the PCI-Express links at 16 GB/sec, and the assumption that Nvidia is making is that connecting the GPUs so they can share memory and data is more important than passing data back and forth between the CPUs and GPUs, which will also happen thanks to the unified memory space implemented in the CUDA 8 environment and assisted by the page faulting hardware in the Pascal GP100 GPU. It is not entirely clear how data will be pinned to specific GPUs and CPU-GPU complexes and what routing will be used by the system to access data from CPU memory and GPU memory; it is also not clear how CAPI-enabled devices will plug into the unified memory between the CPUs and GPUs.

The Power8 chip is being equipped with NVLink ports precisely to give programmers a chance to play around with this hardware, and Nvidia has created its own DGX-1 system and is doling them out to researchers for the same reason. There is lots of automated memory management going on under the covers, and now the industry will find out how to program on top of it.

Intel does not and almost certainly will not add NVLink support to future Xeons, so PCI-Express ports hanging off the server and PCI-Express switching will be the way to link Xeons to Tesla P100s. For the Power8 machines, Nvidia is suggesting that a mix of NVLink and PCI-Express 3.0 ports be used thus:

nvidia-pascal-nvlink-power8

With this setup, a pair of CPUs is linked to each other and sharing main memory over a NUMA link. Each CPU is connected directly to a pair of Tesla P100 accelerators, using one port of NVLink running at 40 GB/sec and one link of PCI-Express running at 16 GB/sec. The GPUs are cross-coupled using the three remaining ports.

Interestingly, the modified Power8 chip has six NVLink ports, which means that, in theory, more complex nodes with different network topologies could be contemplated. It is not clear how the software stack Nvidia has created would deal with such topologies.

We think it would be interesting to show the performance of Tesla P100 accelerators in a system using just PCI-Express 3.0 x16 links and then turn on NVLink so customers can see the difference. Nvidia did not provide such data, but it did provide some performance comparisons against straight CPU systems to show the effect of using NVLink with Pascal accelerators.

In this comparison, Nvidia is showing the performance of a molecular simulation running in AMBER on a system with one, two, and four P100 cards, in the upper left. The scale shows the number of nanoseconds of movement for the molecule that can be simulated per day of system runtime. This is compared to the same AMBER simulation running on the Comet supercomputer at San Diego Supercomputing Center with a rising number of Xeon processors.

nvidia-pascal-nvlink-perf-1

As you can see, the CPU cluster starts to run out of gas at 96 processors, at around 20 nanoseconds per day. A single hybrid node with four P100 accelerators can do 46 nanoseconds per day, and is probably a lot less costly, too. (Maybe on the order of one quarter the price, we speculate, if an average two-socket Xeon server node costs somewhere around $5,000 using the twelve-core Xeon E5-2680 v3 processors that are in the Comet machine.) If those prices are reasonably accurate, then the price/performance of that four GPU system is 8.5X better than the CPU-only cluster just at the raw compute level. Of course, what we want to know is how a node with eight GPUs might perform, and then how multiple nodes linked by a fast InfiniBand or Ethernet interconnect would do beyond that.

Our guess is that an eight GPU system based on Pascal accelerators would start to taper off, and that performance would get even flatter still over a cluster of such machines, but the numbers in the following chart don’t tell us the case for AMBER, which was only tested to four Tesla cards rather than eight. Anyway, take a look at this NVLink scalability chart for increasing numbers of Tesla P100 accelerators on various benchmarks compared to a two-socket Xeon server using the Haswell Xeons mentioned above.

nvidia-pascal-nvlink-perf-2

As you can see, the speedup of the Tesla accelerators varies quite a bit by application, and the performance boost compared to a pair of Tesla K80 accelerators is substantial. This is due to the combination of all of the features of the Pascal architecture, HBM2 memory, and NVLink all working together. Again, it would be interesting to isolate the effect of NVLink by itself, but Nvidia is selling this as a complete package and no one will be buying a P100 and not using NVLink. But down the road, when other Pascal cards are out, they might be looking at price/performance and decide that directly linked GPUs over PCI-Express 3.0 links are fine for their workloads and their budgets. We estimate that the Tesla P100 costs around $10,500 a pop, compared to around $3,200 street price for a Tesla K40 and $4,500 for a Tesla K80.

No matter the workload, what Nvidia and its server partners will be stressing is that the Tesla P100 offers huge benefits in performance and performance per watt over CPU-only clusters. Here is one such comparison, showing how many racks of servers it takes to match a rack of Tesla P100s on various applications and how much electricity those systems would consume:

nvidia-pascal-nvlink-perf-3

Those dozen DGX-1 nodes would cost $1.55 million, and you would be hard pressed to get your hands on them. So this is largely theoretical until early 2017, when the Tesla P100s are shipping in volume. And don’t get the wrong impression from this chart that the applications are scaling across that iron. This is just aggregating the performance of the benchmark results shown in the relative performance chart above. In other words, this is a capacity statement, not a scalability statement, for thinking at the rack level. Most clusters in HPC and in machine learning are run in this fashion – no one expects to scale applications across hundreds of GPUs or thousands of CPUs except in the most exotic settings.

Sign up to our Newsletter

Featuring highlights, analysis, and stories from the week directly from us to your inbox with nothing in between.

Subscribe now

10 Comments

  1. Two comments:

    a) 20nm at TSMC was not build for High Performance chips just for low-power mobile. So it wasn’t a process useful for nVidia nor AMD. Just for Apple + Co.

    b) Having no Half-16 is not such a big deal for XeonPhi unlike nVidia’s GPU you’ll get decent integer performance on x86 so you can easily substitute that anyway given the larger flexibility that’s comes with to go to lower bit nets and the lower memory requirement it is likely that some of the DCNN networks will go that direction anyway which allows to build larger networks that can be ported to lower power platforms (like DSP) for the final in field stage and this will lock out nVidia and their Pascal architecture pretty much of the game!

  2. AMD Is not using TSMC, it’s using a 14nm process node for its Polaris/Vega SKUs! Global foundries(licensed from Samsung) 14nm process! And AMD’s ACE units are getting more and more of some of the functionality that was once only in the domain of the CPU. Adding to AMDs advantage is its future designs using interposer based APUs for some much higher bandwidth parallel CPU core/s complex wiring to a much larger separately fabricated GPU die on the same interposer package! So that Zen/Vega APU on an interposer could have the very same if not greater than the current GPU to HBM bandwidth on a future APU for CPU to GPU communication via an interposer!

    The GPU to HBM2 JEDEC standard effective bandwidth will provide for up to at least 1 TB/second on a 4096 bit wide total connection to 4 HBM die stacks. So what if an APU on an interposer can have an even wider than 4096 bit fabric to wire up the CPU core/s die to GPU die, in addition to the HBM2 with its own 4096 traces. NVLink is great for wiring up separate GPUs and AMD could go with designs that wire up interposer packages to other interposer packages with interposer based bridge chips and create some larger modules for 2, or 4 APUs on an interposer and have even more inter-pocessor bandwidth than even Nvlink could provide.

    Zen is also getting a 14nm finfet process node. Both AMD and Nvidia(NVLink) have their own IP to wire things up off module, but on the interposer module if it is used for an APU on an interposer type of system, AMD has a lot of interposer silicon to work with both on the top and bottom of an interposer and layers if that’s ever needed. Right now both AMD and Nvidia are only using passive interposer designs, but what if they go to active interposer based designs with the interposer hosting more than just traces. Maybe the interposer will get all the circuity to run a fully coherent connection fabric, or just some interface buffer memory, etc.

    A) do not forget that both AMD and Nvidia will be adding more asynchronous compute features to their GPU’s hardware, it’s more than just the VR gaming people that are interested in doing more non graphics gaming compute on the GPU, and not simply gaming graphics! It’s gaming compute that was traditionally done on the CPU, that is now being done on the GPU for gaming or protein folding, other number crunching or simulating on 16/32/64 or more values!

    B) Intel is behind in the number crunching again with P100’s(Pascal) release, and Vega(AMD)/Volta(Nvidia) are on the way! AMD is gearing Polaris more towards the mainstream market, but Vega will be on the HPC/Workstation systems.

    C) GPUs eat CPUs for lunch for graphics workloads, and watch out for those future AMD ACE units they are damn near CPUs in their own right, CPU with rather large ranks and files SIMD/Other specialized units that no CPU can offer.

    D) AMD has a patent application for placing some extra FPGA compute on the HBM die stacks sandwiched between the HBM’s bottom controller logic die, and the HBM memory dies above. Extra FPGA compute that takes up no extra real-estate in the X and Y, and is right there in the HBM stack next to the memory.

    P.S. Intel has to clock those Xeon Knights much higher to get the HIGH flops at the cost of more power wasted than GPUs that have things done in a more massively parallel fashion at lower clocks! Those Exascale systems will come with power budgets, to avoid needing their own nuke plants for power.

  3. AMD is out of HPC / deep learning field because lack of API support, so it’s useless to talk about them here. Last interview of Bryan Catanzaro, lead scientist at Baidu:
    Outside of sheer hardware, software support is a big deal—and is the key reason why Baidu has not looked to AMD GPUs, which offer comparable performance. “We are open to new things, but in the case of AMD, I would have to write so much software to use them that the investment wouldn’t make sense.” (Baidu makes use of CuDNN)

  4. Exascale government grants, for AMD, Intel, Nvidia, and others! As well as AMDs “Boltzmann Initiative” for providing the software tool chains/middleware to port over from CUDA to OpenCL/other abstraction APIs and programming languages from that Nvidia CUDA proprietary code and such. More competition in the HPC/computer markets will save government and enterprise users money in the long run. The Chinese will be using/licensing some of that AMD APU on an interposer designs for their HPC computing needs/market, so the open source based software tools will be popular among the Chinese and any university that may want to avoid vendor lock-in, enterprises too like to be free of single vendor lock-in.

    The EU for one will want to avoid the CUDA dependencies and does embrace the open source methods, to avoid all sorts of vendor specific entanglements! Those purposed AMD exascale APUs on an interposer have some very definite wide parallel power saving interconnect potential relative to the interconnects that rely on the higher clocked and power hungry narrower interconnects currently in use. The APU on an interposer module and the interposer’s ability to be etched with 10s of thousands of very wide connection fabrics has its advantages!

    The Fair market for exascale computing must have the same competitively bid for supply chain of parts that the mobile markets now enjoys! There must be a second source of HPC/exascale GPU accelerator parts and the competitive markets must be maintained for all supply chain costs to be kept under control by the competitive market forces. Nvidia, and Intel, as well as Microsoft/others should not be tacitly made unregulated public utilities by unresponsive government actions towards fostering the most competitive marketplace for the supplies/supply chains of SOC/APU/CPU/GPU/FPGA/other processor parts, as well as OS/API/software parts/supply chain to the computing marketplace.

    Open Source software/OS alternatives must be supported around the world as the OS/APIs needed for the continued computing prosperity are treated more as vital infrastructure under the control of no one single vested interest, just as it has been done for the highway/roads systems in many countries.

    • All fair and well but you forget the extreme concentration on Foundries.

      Especially for performance chips there are only three bleeding edge foundry companies left in the world (and one of them is for internal use only). That’s the largest bottleneck in the future as everyone would eventually fight over stretched capacities

      • Which foundry is for internal use only? And Samsung and GlobalFoundries have been in a technology sharing foundation/association with IBM for a few years now. The available foundry capacity is being expanded. Everybody will be going 14nm/16nm and 28nm will still be around for a few years. IBM’s internal USE power8’s are still a 22 nm SOI process, and the third party power8 licensees are using the same. 14 nanometers will be for the power9’s.
        AMD’s Zen and Polaris, and Vega, will be on a 14 nm process node. Nvidia is using TSMC’s 16 nm node for its GPU SKUs. Maybe Intel should think about making a indipendent Fab division for any third party business if there is a such a shortfall of fab capacity as you imply! They will fight over capacity by paying more for the chip fabs services!

        AMD may be going with much smaller GPU dies that lead to much better yields for usable dies per wafer, when it introduces its Zen/Navi based APUs on an interposer and Navi GPUs on an interposer for the gaming markets. The APU on an interposer designs will allow for smaller modular GPU dies to be all hosted on an interposer package and wired up via the silicon interposer’s substrate etched traces and still allow for AMD to best any single monolithic based GPU Die designs, that are subject to more yield losses dew to their larger size.

        So Navi may usher in the use of smaller modular GPU die units, with the GPU unit able to be scaled, 1 or more GPU dies to an interposer package, and fitted along with some Zen core die complexes of 4/8/16/32 CPU cores and the HBM dies to make for some very scalable designs for these new APU/GPU systems on an interposer. Small modular dies, better wafer/die yields may be just the thing for any of AMDs custom clients, allowing them to order APU systems on an interposer with more scalability for future product improvements, the interposer used in this way will become a silicon based mainboard of sorts for AMD to fit with its modular Zen core dies and smaller modular GPU dies, and HBM/other dies while the silicon interposer will accept those 10s of thousands of traces to wire things up is such a way that no PCB based design ever could.

        Nvidia may be doing the same, considering the wafer losses are much greater for GPUs owing to the GPUs larger size relative to the other processors(CPU, DSP, FPGA, and such). So Nvidia may also be planning some future designs with its Denver/other ARM based CPU based SOCs on an interposer package made with smaller Nvidia Pascal/Volta GPU modular units. The price pressures will continue upward even more so with Moore’s law/observation coming to an end for those continued process node shrinks in the X and Y, and the chip wafer/yield issues are worse at the smaller fabrication nodes for any of the processor die sizes!

        • 14nm process is the one and only tech Samsung and GF will actually share. TSMC and Samsung are both so obsessed with mobile SOC markets in their roadmap ahead which as we’ve seen already at 22nm it wasn’t really suitable for high performance chips. So actually there is only Intel (their 14nm is more taylored to performance chips and is more in line what TSMC 10nm will probably be). As Samsung doesn’t really do High performance at all. So you only have TSMC, which is more and more concentrating on mobile as Apple SoCs are their main tech driver and finally GF which is falling more and more behind. Although their SOI might be an interesting alternative (as this is IBM’s tech) if they get it a longer roadmap for that. GF current 14nm is FinFet not SOI and they haven’t announced a 14nm SOI yet so I highly doubt that Power9 will go there.

  5. Nvidia’s own blog cites the bandwidth as 20 GB/sec per direction, per link. The way they come up with 40 GB/sec is by summing both directions. According to Wikipedia, PCIe 3.0 is 15.75 GB/sec per direction per x16 connection. So, a single link really *is* a small step up from PCIe 3.0 x16 speeds.

    https://devblogs.nvidia.com/parallelforall/inside-pascal/

    Perhaps they had other reasons for creating NVlink, such reducing the number of pins/traces needed to support high bandwidth connections. Maybe it had to do with power, or distance. Maybe they really wanted to reach 80 GB/sec/direction, between a pair of GPUs or a GPU and CPU (from what I can tell, PCIe maxes out at x32). I assume they had *some* good reason for doing it. However, at least on the surface, this does not seem like a huge leap over PCIe 3.0.

    BTW, SMP does *not* make multiple CPUs look like a singe processor. It simply provides each processor with equal access to all resources (thus, the “symmetric” part). By contrast, NUMA privileges some nodes with respect to some resources (e.g. memories, buses, etc.). Most modern multi-chip computers are cache-coherent NUMA, since true SMP doesn’t scale well. Though, internally, most multi-core CPUs are true SMP.

    • NVLinkB, great comments and you are right on the bandwidth comparison of x16 PCIe (32 GB/s bidi) and one link of NVLINK (40GB/s bidi). Also NLINK provides ability to aggregate up four links as one to get up to 160 GB/s of bidi bw across a single interface.

      In addition, NVLINK is more like an SMP bus between CPUs versus PCIe as a general purpose I/O bus. Some of the advantages for NVLINK as designed are that devices on NVLINK have extremely fast memory access to other devices and a shared address space. This provides low latency transfers with low overhead, outstanding support for Unified Memory access, simpler programming model and also advanced support for keeping computation across devices in lock stop using collectives for HPC and Deep Learning (see https://devblogs.nvidia.com/parallelforall/fast-multi-gpu-collectives-nccl/).

      If you haven’t already for a good deep dive check out the Pascal Architecture paper here: http://www.nvidia.com/object/pascal-architecture-whitepaper.html

      • Thanks for the info.

        The benefits of lower latency are clear, and I can imagine ways that a purpose-built tech like NVLink might surpass PCIe, here. Since only the bandwidth numbers have been published, it’s natural to focus on those.

        It might be interesting to see a deep comparison between NVLink and QPI, then.

Leave a Reply

Your email address will not be published.


*


This site uses Akismet to reduce spam. Learn how your comment data is processed.