Subject: Processors | June 15, 2017 - 04:00 PM | Ryan Shrout
Tagged: xeon scalable, xeon, skylake-x, skylake-sp, skylake-ep, ring, mesh, Intel
Though we are just days away from the release of Intel’s Core i9 family based on Skylake-X, and a bit further away from the Xeon Scalable Processor launch using the same fundamental architecture, Intel is sharing a bit of information on how the insides of this processor tick. Literally. One of the most significant changes to the new processor design comes in the form of a new mesh interconnect architecture that handles the communications between the on-chip logical areas.
Since the days of Nehalem-EX, Intel has utilized a ring-bus architecture for processor design. The ring bus operated in a bi-directional, sequential method that cycled through various stops. At each stop, the control logic would determine if data was to be the collected to deposited with that module. These ring bus stops are located at memory controllers, CPU cores / caches, the PCI Express interface, memory controllers, LLCs, etc. This ring bus was fairly simple and easily expandable by simply adding more stops on the ring bus itself.
However, over several generations, the ring bus has become quite large and unwieldly. Compare the ring bus from Nehalem above, to the one for last year’s Xeon E5 v5 platform.
The spike in core counts and other modules caused a ballooning of the ring that eventually turned into multiple rings, complicating the design. As you increase the stops on the ring bus you also increase the physical latency of the messaging and data transfer, for which Intel compensated by increasing bandwidth and clock speed of this interface. The expense of that is power and efficiency.
For an on-die interconnect to remain relevant, it needs to be flexible in bandwidth scaling, reduce latency, and remain energy efficient. With 28-core Xeon processors imminent, and new IO capabilities coming along with it, the time for the ring bus in this space is over.
Starting with the HEDT and Xeon products released this year, Intel will be using a new on-chip design called a mesh that Intel promises will offer higher bandwidth, lower latency, and improved power efficiency. As the name implies, the mesh architecture is one in which each node relays messages through the network between source and destination. Though I cannot share many of the details on performance characteristics just yet, Intel did share the following diagram.
As Intel indicates in its blog on the mesh announcements, this generic diagram “shows a representation of the mesh architecture where cores, on-chip cache banks, memory controllers, and I/O controllers are organized in rows and columns, with wires and switches connecting them at each intersection to allow for turns. By providing a more direct path than the prior ring architectures and many more pathways to eliminate bottlenecks, the mesh can operate at a lower frequency and voltage and can still deliver very high bandwidth and low latency. This results in improved performance and greater energy efficiency similar to a well-designed highway system that lets traffic flow at the optimal speed without congestion.”
The bi-directional mesh design allows a many-core design to offer lower node to node latency than the ring architecture could provide, and by adjusting the width of the interface, Intel can control bandwidth (and by relation frequency). Intel tells us that this can offer lower average latency without increasing power. Though it wasn’t specifically mentioned in this blog, the assumption is that because nothing is free, this has a slight die size cost to implement the more granular mesh network.
Using a mesh architecture offers a couple of capabilities and also requires a few changes to the cache design. By dividing up the IO interfaces (think multiple PCI Express banks, or memory channels), Intel can provide better average access times to each core by intelligently spacing the location of those modules. Intel will also be breaking up the LLC into different segments which will share a “stop” on the network with a processor core. Rather than the previous design of the ring bus where the entirety of the LLC was accessed through a single stop, the LLC will perform as a divided system. However, Intel assures us that performance variability is not a concern:
Negligible latency differences in accessing different cache banks allows software to treat the distributed cache banks as one large unified last level cache. As a result, application developers do not have to worry about variable latency in accessing different cache banks, nor do they need to optimize or recompile code to get a significant performance boosts out of their applications.
There is a lot to dissect when it comes to this new mesh architecture for Xeon Scalable and Core i9 processors, including its overall effect on the LLC cache performance and how it might affect system memory or PCI Express performance. In theory, the integration of a mesh network-style interface could drastically improve the average latency in all cases and increase maximum memory bandwidth by giving more cores access to the memory bus sooner. But, it is also possible this increases maximum latency in some fringe cases.
Further testing awaits for us to find out!