Technical Overview Of The Intel® Xeon® Scalable processor Max Series

Overview

This paper discusses the new features and enhancements available in the Intel Xeon Scalable processor Max Series (formerly codenamed Sapphire Rapids with high bandwidth memory (HBM)) and how developers can take advantage of them. The 10nm enhanced SuperFin processor provides core microarchitecture changes and new technologies including on-die accelerators, Compute Express Link™ (CXL™), and more. Data center workloads can benefit from the increased core count, increased instructions per cycle, increased Intel® UPI GT/s, increased processor cache sizes, increased memory MT/s, and more PCI Express and CXL lanes.

The table below provides a comparison between the third generation and the Intel Xeon Scalable processor Max Series. The new generation builds upon features found in the third generation. New capabilities/changes relative to the previous generation are in Bolded Italic. Note that the table below reflects the top capabilities for each category. There are many models of processors available with different feature combinations.

Table 1. The Next-Generation Intel Xeon Scalable processor Max Series Microarchitecture Overview

High Bandwidth Memory (HBM) 

The Intel® Xeon® Scalable processor Max Series includes high bandwidth memory that is integrated into the processor package with up to 64GB of HBM2e memory per socket. HBM is meant to help with memory bandwidth sensitive workloads such as modeling and simulation, Big Data Analytics, AI workloads, molecular dynamics, life sciences, quantum computing, drug discovery and In-memory databases. It might also improve the total cost of ownership due to a reduced need for DDR memory. The memory operates in three different modes HBM only, Flat and Cache. These modes are defined by the BIOS at boot time. The end user will need to set the mode of operation they prefer within the BIOS.

In HBM only mode, there is no DDR memory installed. HBM only mode is best suited when you have a workload that can fit within the HBM capacity of the socket. On a 64GB HBM2e version it would be beneficial to workloads that can fit within 1GB per core. The Heterogeneous Memory Attribute Table enumeration is utilized by the kernel to discover the HBM attributes and append them to the NUMA node memory target. This will allow you to pin an application through the operating system to a specific NUMA node associated with the HBM and prioritize it. This provides access to the memory region without requiring modification of the application.

The Flat mode of operation combines both the HBM and DDR memory together. Both memory regions are exposed to the software as separate NUMA nodes. The software will need to balance the application within the memory space in order to optimize performance. One way for the software to access the memory regions is through the use of memkind library. Hbm_malloc() allows you to explicitly define what buffers in the HBM that you want your application to use, but this will require modifications of your applications code. Alternatively, you can use OpenMP which will allow you to define what buffers in HBM that you want the application to use. This will also require a change to your applications code.

By segregating the HBM and DDR by NUMA node it allows you to utilize existing NUMA utilities such as numactl. For example, numactl with Flat mode can be used as follows:

• numactl -m 1 ./application # allocations must go to the HBM on NUMA node 1 and cannot exceed the HBM capacity of 64GB
• numactl -p 1 ./application # first allocations must go to the HBM on NUMA node 1, then to the DDR when exceeding 64GB

If you are using Intel MPI, instead of numactl you can use an environment variable to place your application into the HBM.

• Environment variable: I_MPI_HBW_POLICY [hbw_bind | hbw_preferred]

The Cache mode of operation requires no software modifications to use it. Local cacheable request first looks to the HBM memory controller which will determine if the data is within the HBM or whether to look for it within the DDR memory. On a miss in HBM cache the DDR memory controller returns data to both the requesting core and the HBM controller. The HBM controller will write back modified memory lines at fill index back to the DDR controller when necessary.

For all modes of operation, you will likely see maximum benefit when using SNC4 mode (four sub-NUMA clusters) due to extra memory pathways. There is no support for Intel SGX or MKTME with HBM Only or Cache modes. 

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Additional resources include Intel® HPC Technologies along with Enabling High-Bandwidth Memory in Future Intel Processors, which gives a good overview and enabling guidance.

Uniform Memory Access Domain

Figure 1 – Block Diagram Representing Uniform Memory Access Domain (UMA)

Over the course of time the density of processors, memory controllers, interconnects and supporting infrastructure continues to expand on the die, while the geometry gets smaller. This correlates to an increase in the amount of time it takes for data to move from one side of the die to another. A uniform memory access (UMA) domain is where we originally started and is still an option on the system. It provides a single contiguous address space that is interleaved among all the memory controllers. UMA does not provide a way to optimize data flow from the nearest resources. This is normally done through processor affinity which is a way to dictate what processor(s) a given software thread uses. Therefore, all cores have equal access to the last level cache and to memory when they are part of an UMA domain. In this situation you may have a processor accessing a memory controller or a slice of the last level cache that is on the opposite side of the die versus from the nearest.

Sub-NUMA Clustering

Figure 2 – Block Diagram Representing Domains of sub-NUMA With Two Clusters (SNC-2)

Figure 3 – Block Diagram Representing Domains of sub-NUMA With Four Clusters (SNC-4)

To decrease the latency of data movements across the die sub-NUMA clusters (SNC) were introduced. In a two cluster SNC (SNC-2), two localization domains exist within a processor. Each domain has mapped addresses from the local memory controller and local Last Level Cache (LLC) slices. Processors in the local domain will use the local memory controller and local LLC slices. Processors in the local domain will observe lower LLC and memory latency compared to latency on accesses mapped to locations outside of the same SNC domain.

SNC has a unique location for every address in the LLC, and it is never duplicated within the LLC banks. Localization of addresses within the LLC for each SNC domain applies only to addresses mapped to the memory controllers in the same socket. All addresses mapped to memory on remote sockets are uniformly distributed across all LLC banks independent of the SNC mode. Therefore, even in the SNC mode, the entire LLC capacity on the socket is available to each core, and the LLC capacity reported through the CPUID is not affected by the SNC mode.

Figure x represents a two-cluster configuration that consists of SNC Domain 0 and 1 in addition to their associated cores, LLCs, and memory controllers. Each SNC domain contains half of the processors on the socket, half of the LLC banks, and one of the memory controllers with its associated DDR channels. The affinity of cores, LLC, and memory within a domain are expressed via software using the OS NUMA affinity parameters, which can take SNC domains into account in scheduling tasks and allocating memory to a process for optimal performance.

The latest generation of silicon provides either 2 domain or 4 domain versions of SNC. SNC is enabled at the BIOS level and requires that the memory be symmetrically populated, and the software must be optimized to take advantage of it. If these criteria can be met it will provide the best performance out of all the modes of operation.

Hemisphere and Quadrant Modes

Figure 4 – Block Diagram Representing Domains of Hemisphere Mode

Figure 5 – Block Diagram Representing Domains of Quadrant Mode

For those customers who can meet the requirement of having symmetrically populated memory but who do not wish to alter their software as must be done with sub-NUMA clustering there are two new modes available. Hemisphere and Quadrant divide the die into either two or four domains respectively. Each processor has access to all the caching agents and memory controllers. However, based on a hash function each home agent will automatically ensure the caching agent is routed to the closest memory controller within the same domain. Once a processor accesses a caching agent the distance is minimized as it is always going to interact with the closest memory controller. These modes of operation will tend to produce a better performance result as compared to UMA but not as good as compared to sub-NUMA clustering. The settings are found within the BIOS with Quadrant as the default.  The mode used by the BIOS will be dictated by the memory symmetry.  If memory symmetry cannot be met for quadrant mode, then hemisphere mode will be used. If there still isn’t enough symmetrical memory for hemisphere mode, then the system will default to UMA mode.

New Instructions

The latest generation of silicon has a new microarchitecture core that brings with it a variety of enhancements which are covered in greater detail throughout this article. In this section we will touch upon some of the instructions that are associated with these changes. A portion of the new instructions center around Accelerator interfacing Architecture (AiA). These are built into the ISA and are an enhancement on the x86 processors meant to optimize data movement by accelerator devices to host services. The instructions MOVDIR, MOVDIR64B, ENQCMD, ENQCMDS, User Interrupts, User IPIs, Umonitor, Umwait, and PAUSE are focused on the AiA operations. The MOVDIR/MOVDIR64B instructions are to offload data movements to the accelerator. The ENQCMD/ENQCMDS instructions facilitate shared and dedicated work queues allowing the hardware to manage the process. This avoids wasting a core processor resource on a persistent thread. Lastly the user interrupts, IPIs, Umonitor, Umwait, and PAUSE instructions deal with signaling and synchronization aspects of the accelerators.

Instructions to support AI workloads via Intel AMX includes Int8 for inference, Bfloat16 for inference/training, and Xsave to help with the management by the operating systems of the dedicated silicon tiles. While Intel® Advanced Vector Extensions 512 (Intel® AVX-512) includes VP2INTERSECT to improve vectorization and scalar code. Along with FP16 which replicates the FP32(SP) computation instruction using a new FP16 data type to help with support for complex number arithmetic. This can be of value for signal and image processing and specifically with 5G networks.

For virtualization there is Hypervisor Linear Address Translation (HLAT) which can be used by the VMM to enforce guest linear address translation to guest physical addresses. When combined with enhanced page-table capability it helps to ensure VMM-enforced guest translations are not tampered with by untrusted system software.

On the security side Intel® Control-Flow Enforcement Technology (Intel® CET) defends the shadow stack against return-oriented programming by implementing a duplicate shadow stack that is managed by the CPU. The two stacks are compared on a call to make sure there are no unexpected discrepancies.

Lastly Intel TSX has two new commands, XSUSLDTRK (suspend tracking load address) and XRESLDTRK (resume tracking load address). These commands allow programmers to choose which memory accesses do not need to be tracked in the TSX read set. This has the potential to benefit in-memory database by reducing the capacity aborts by allowing the software to exclude certain load addresses from entering the transaction read set.

For additional information see the Intel® Architecture Instruction Set Extensions Programming Reference, Intel® 64 and IA-32 Architectures Optimization Reference Manual and the Intel® Architecture Instruction Set Extensions Programming Reference.

Built-In Accelerators

In the fourth generation of the Intel Xeon Scalable processors Max Series an emphasis has been placed on incorporating multiple accelerators into the processor package. The accelerators bring new functional capabilities to support popular use cases associated with AI, cryptographic, in-memory database analytics, and data movement operations. They also help to optimize the throughput of code simply because they are physically closer to the processor. The accelerators can work individually or in combination with each other to help create a multi-layered approach to improving performance. They include Intel® Data Streaming Accelerator (Intel® DSA) and Intel® Advanced Matrix Extensions (Intel® AMX).

Intel® Data Streaming Accelerator (Intel® DSA)

Intel DSA is a replacement to Intel® Quick Data Technology. It is designed to help offload common data mover operations from the processor cores that might be found in storage applications, hypervisors, persistent memory operations, networking applications, operating system cache flushing, page zeroing and memory moves. More specific uses that Intel DSA may assist with include virtual machine fast checkpoint analysis, vSwitch network virtualization, fast replication across non-transparent bridge (NTB), and applications that focus on analytics, in-memory databases, and messaging.

From a hardware perspective Intel DSA has up to four instances per socket depending on the model, available to offload data from the processor core. It supports the data movement between any component on the platform including all compatible attached memory and IO devices. For example, the data movement between the processor cache to main memory, between the processor cache to an addon card, and between two addon cards is supported.

From a software perspective the data can be shared concurrently across the operating system, virtual machine manager, virtual machines, containers, and application processes. A shared virtual memory space allows the device to operate directly in the applications virtual address space without requiring pinned memory. Each instance of Intel DSA is exposed as a PCI-Express complex integrated endpoint and supports a variety of PCI-SIG* Defined services creating highly scalable configurations.

Intel DSA instances are identified by the operating system via the Intel® Data Accelerator Driver (IDXD) which can be utilized by software libraries and frameworks to manage work queues. Compatible libraries for ease of adoption can be found in the Data Plane Development KIT DPDK, the Storage Performance Development Kit, with a more generic dmadev abstraction that can be used with other copy-offload engines, DMA drivers, and an example application. Additional resources include: Introducing The Intel® Data Streaming Accelerator (Intel® DSA), Intel® DSA Specification, Intel® Data Accelerator Driver GitHub* repository, Intel® Data Accelerator Driver Overview on GitHub.io, Intel® Data Accelerator Control Utility and Library, Shared Virtual Memory, Intel® Scalable I/O Virtualization, Intel® 64 and IA-32 Architectures Software Developer Manuals, Intel® Data Streaming Acceleration Architecture Specification and Intel® Data Streaming Accelerator User Guide.

Intel® Advanced Matrix Extensions (Intel® AMX)

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Intel® Advanced Matrix Extensions (Intel® AMX) is designed primarily to improve deep learning inference & training performance. It is a new 64-bit programming paradigm consisting of two components: a set of 2-dimensional registers (tiles) representing sub-arrays from a larger 2-dimensional memory image, and an accelerator able to operate on tiles, the first implementation is called TMUL (tile matrix multiply unit). Intel AMX provides the programmer with configurable tile sizes for flexibility supplied as metadata. A single instruction is capable of autonomously executing multi-cycles in the tile and accelerator hardware. Since execution is driven through metadata, an existing Intel AMX binary could more easily take advantage of changes over time in the Tile sizes without adjustments to the software coding.

Figure 6 - Conceptual Diagram of The Intel AMX architecture.

An Intel architecture host drives the algorithm, the memory blocking, loop indices and pointer arithmetic. Tile loads and stores and accelerator commands are sent to multi-cycle execution units. Intel AMX instructions are synchronous in the Intel architecture instruction stream and the memory loaded and stored by the tile instructions is coherent with respect to the host’s memory accesses. There are no restrictions on interleaving of Intel architecture and Intel AMX code or restrictions on the resources the host can use in parallel with Intel AMX (e.g., Intel AVX512). There is also no architectural requirement on the Intel architecture compute capability of the Intel architecture host other than it supports 64-bit mode.

Intel AVX512 code that is using the FP32 data type will continue to use the Intel AVX512 code path on the platform. Developers that want to convert their FP32 based deep learning models over to BF16 or INT8 for use with Intel AMX can utilize the Intel® Neural Compressor. While the following article provides you with a code sample tutorial, Code Sample: Intel® Advanced Matrix Extensions (Intel® AMX) - Intrinsics Functions and you can get code samples directly on Github.

Intel AMX supports int8 (all sign combinations) with int32 accumulation as well as Bfloat16 with IEEE SP accumulation. It is supported by various tools and libraries which include Intel® oneAPI Deep Neural Network Library (oneDNN), Intel® oneAPI Math Kernel Library (oneMKL), Intel® oneAPI Collective Communications Library (oneCCL), Intel® oneAPI Data Analytics Library (oneDAL), Intel® Neural Compressor, and Intel OpenVINO Toolkit version. More detailed information can be found in Intel® Architecture Instruction Set Extensions Programming Reference. Support for this new technology can be found in Linux Kernel version 5.18+ and Windows Server version 2022+. Use cases that may benefit from Intel AMX include Natural language processing, media analytics, media processing and delivery, image recognition, machine language translation and reinforcement learning.

Compute Express Link™ (CXL™)

Compute Express Link™ (CXL™) is an open interconnect standard that utilizes the fifth generation PCI express physical layer providing more efficient movement of user and kernel level data between the host processor and the attached devices such as Field Programmable Gate Arrays, Graphics Processor Units, or network controllers. CXL makes it possible for cache coherent accesses from the attached devices to the host processor cache. Along with memory accesses from the host processor to memory on the attached device. The built-in accelerators on the platform such as Intel DSA and Intel AMX utilize CXL to optimize their productivity.

CXL has a flexible processor port attached to the PCI express bus that is capable of auto-negotiating the protocols associated with either PCI express or CXL. A device attached to PCIe has to go through layers and checks in order for the data to reach the processor. While a device using the CXL protocols is able to bypass the PCIe hierarchy and obtain a more direct communication with the processor through address translation and access to the physical address space. This means that by comparison CXL is able to provide increased bandwidth and lower latencies as compared to PCI express.

On our latest generation of the platform, we have what is called a flex bus. The flex bus is capable of providing both standard PCIe interconnects as well as up to four CXL interconnects 16 lanes wide with up to 32 Gigabits per second. The flex bus supports hot swappable PCIe devices, while CXL devices are only recognized and supported by the BIOS at boot time.

There are three protocols utilized by CXL. They are CLX.io, CXL.cache, and CXL.mem.

CXL.io protocol operates in similar to the PCIe protocol and follows the same base PCIe specifications for the transaction and link layers. It is used for load and store operations, discovery, interrupts, enumeration, RAS features, etc. Regardless of the use case CXL.io is a required protocol in order to manage and operate the CXL device. CXL.io does provide some additional enhancements that are specific to the CXL protocol for management of the device, memory address translation services and power events.

CXL.cache is a protocol that utilizes hardware cache coherency with a 64 byte cache line. It is designed to provide coherent access from the attached device to the processor cache. The protocol will snoop the coherent cached state for the attached device that made the request. It is possible to use a subset of transactions to avoid snoop requests if the attached device doesn’t implement a cache. The protocol uses host physical addressing only and most of the requests are unordered.

CXL.mem protocol provides a host processor with ability to perform simple reads and writes to an attached device that has its own private memory. The protocol is transactional which allows it to interface with any type of memory. Therefore, you can use DDR, Optane, or even HBM and it could be in a flat model, far memory or even a mix of near and far memory. The protocol uses host physical addressing only.

Figure 7 - CXL Use Models

The platform supports CXL version 1.1 with two different use models. Type one use model is associated with attached caching devices without private memory while type two use model is associated with an attached device with private memory.

Type one devices might include a smart NIC or an accelerator. The CXL.io protocol can be used to communicate and configure the devices. While the CXL.cache protocol can be used for non-coherent flows to read the latest value of the cache. Alternatively, the entire cache line can be updated without taking ownership by the attached device using push write flows. The CXL.cache protocol can also use coherent flows to take ownership of the cache line to modify it.

Two of the more interesting use cases associated with Type one devices would be PGAS NICs and NIC atomics. Partitioned Global Address Space (PGAS) programming models offer a shared address space model that simplifies programming while exposing data and thread locality to enhance performance. Normally this model uses PCIe that must deal with PCIe’s ordering scheme, which in this situation can require a serialized approach. CXL takes the PGAS model utilized by the NICs into a much more simplified and faster solution primarily because it is able to bypass the PCIe ordering scheme. It is able to do this by prefetching the cache locations and then ordering transactions. The second use case can benefit NICs that support different atomic types. PCI express protocols can limit which atomics end up being useable by the NIC as it typically requires industry acceptance. CXL.cache however doesn’t have the same atomic limitations found with PCI express protocols because you can directly access them within the cache. This opens up the possibility for atomics associated with floating point operations used by NICs on the CXL bus.

Type two devices are when the processor is working with other accelerators that have memory attached. This could include other processors, graphics processing units or field programmable array units. The attached memory on any of these devices appears as additional system memory to the host processor but with a different proximity and latency. The software can determine the proximity and latency for the attached device memory and access it via the CXL.mem protocol. This use case may benefit In-memory databases and AI deep learning applications.

From a programming perspective legacy software that was designed to interact with PCIe devices will view CXL devices as part of a PCIe compliant root complex integrated endpoint. The devices will be capable of standard PCIe interactions via software including multi-function and SR-IOV. In terms of software designed to operate with CXL it starts with the firmware. The firmware will initialize the bus, setup the registers, and communicate the address to the operating system using the CXL early discovery table which is a new type of an ACPI table. By following the CXL 1.1 specification and utilizing the CXL Device Designated Vendor Specific Extended Capability (DVSEC) structure the programmer can create software to interact with the CXL device.

Resources:

CXL Memory Device Software Guide describes how system firmware, OS and UEFI may configure CXL Type 3 devices, much of the information will be applicable to CXL 1.1

CXL-cache/mem Protocol Interface (CPI) specification has been developed to map coherent protocols between an agent and a fabric

Streaming Fabric Interface (SFI) Specification has been developed to map Load/Store protocols (like PCIe) between an agent and a fabric.

Scalability is a necessity as the capabilities of the processor and platform continue to expand over time. Intel Scalable IOV is designed to handle the increased demand for IO operations associated with virtualized environments. Intel Scalable IOV has replaced the pre-existing Single root IO virtualization (SR-IOV) technology by providing a flexible composition of Virtual Functions using software from native hardware interfaces. While SR-IOV implements a complete virtual function interface, Intel Scalable IOV instead uses a lightweight assignable device interface (ADIs) that is optimized for fast data path operations for direct access from the guest. Intel Scalable IOV easily interfaces with PCIe or CXL via Process Address Space IDs (PASIDs). Thereby allowing multiple guest operating systems to natively share PCIe or CXL devices simultaneously. Intel Scalable IOV supports all the accelerators found on the latest generation of silicon as well as any discrete accelerators.

Virtual devices are dynamically composed from fast-path resources and slow-path software emulation, therefore the technology requires support by the operating system and virtual machine manager through device drivers and firmware. For additional insights see the Intel® Scalable I/O Virtualization Technical Specification.

Additional Resources

Intel® Xeon® CPU Max Series Configuration and Tuning Guide

Using Intel® VTune™ Profiler to Optimize Workloads on Intel® Max Series CPUs & GPUs

Technical Overview Of The 4th Gen Intel® Xeon® Scalable processor family

Code Sample: Intel® Advanced Matrix Extensions (Intel® AMX) - Intrinsics Functions

Intel® Performance Counter Monitor (Intel® PCM)

The Author: David Mulnix is a software engineer and has been with Intel Corporation for over 20 years. His areas of focus include technical content development and training, software automation, server performance and power analysis with SPECPower, and he supported the development effort for the Server Efficiency Rating Tool^TM.

Contributors: Arijit Biswas, Ruchira Sasanka, Vinodh Gopal, Wajdi Feghali, Mahesh Wagh, Alberto Villarreal and Dan Zimmerman

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