Resolving Intel Optane PMem NUMA Affinity Issues For Optimal Performance

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#Introduction

In today's data-intensive world, memory performance is more critical than ever. Intel Optane Persistent Memory (PMem) offers a revolutionary approach, bridging the gap between traditional DRAM and storage solutions. However, harnessing its full potential requires careful configuration and understanding of underlying system architectures, especially Non-Uniform Memory Access (NUMA). This article delves into a specific issue encountered with Intel Optane PMem in App Direct mode on a dual-socket server, where incorrect NUMA affinity leads to significant performance problems. We will explore the root cause of the issue, its impact, and potential solutions, focusing on the interplay between ACPI SRAT tables, memory allocation, and operating system behavior.

Understanding NUMA and Memory Affinity

Before diving into the specifics, it's essential to grasp the fundamentals of NUMA. In a NUMA architecture, multiple processors (sockets) have their own local memory. Accessing this local memory is significantly faster than accessing memory attached to a different processor socket. This difference in access latency is the core concept behind NUMA. Memory affinity refers to the proximity of a process or thread to a particular memory region. Ideally, processes should primarily access memory within their local NUMA node to minimize latency and maximize performance. When memory is allocated across NUMA nodes, especially in performance-sensitive applications, it can lead to significant performance degradation due to increased inter-socket memory access.

The ACPI System Resource Affinity Table (SRAT) plays a crucial role in describing the NUMA topology to the operating system. It maps memory regions and processors to specific proximity domains, essentially defining which memory belongs to which NUMA node. An improperly configured SRAT can lead to misallocation of resources, where memory intended for a specific CPU is assigned to a different NUMA node. This misallocation is precisely the issue we'll be addressing in the context of Intel Optane PMem.

The Specific Issue: PMem Assigned to Separate NUMA Domains

The core problem arises when Intel Optane PMem, configured in App Direct mode, is assigned to separate, non-CPU proximity domains within the ACPI SRAT table. For instance, in a dual-socket server, CPUs might be correctly assigned to NUMA domains 0 and 1, but the Optane PMem modules are incorrectly assigned to domains 2 and 3. This means that even though the PMem modules are physically connected to the CPUs, the operating system perceives them as residing in separate NUMA nodes, far away from the CPUs that need to access them. The consequence of this misconfiguration is severe: applications accessing PMem will experience significantly higher latency and reduced bandwidth due to the inter-socket memory access penalty. This defeats the purpose of using high-performance persistent memory like Optane PMem, as its low latency benefits are negated by the NUMA overhead.

The performance impact of this incorrect NUMA assignment can be substantial. Applications that rely on low-latency memory access, such as in-memory databases, caching systems, and high-performance computing workloads, will suffer significantly. The increased latency can lead to slower transaction processing, reduced throughput, and overall system sluggishness. Identifying this issue can be challenging, as the system might appear to be functioning correctly at a basic level. However, performance monitoring tools and careful examination of memory access patterns will reveal the bottleneck caused by the incorrect NUMA affinity.

Root Cause Analysis: ACPI SRAT Table and BIOS Configuration

Understanding the root cause is crucial for implementing effective solutions. The primary culprit in this scenario is often an incorrectly configured ACPI SRAT table. The SRAT table is generated by the system's BIOS during boot and provides the operating system with the system's hardware topology, including NUMA information. If the BIOS has a bug or is misconfigured, it might generate an SRAT table that incorrectly assigns Optane PMem to separate NUMA domains. There are several factors that can contribute to this misconfiguration. One common cause is outdated BIOS firmware. BIOS updates often include fixes for hardware compatibility issues and can resolve problems related to SRAT table generation. Another possibility is incorrect settings within the BIOS related to memory configuration or NUMA settings. For example, if the NUMA configuration is not properly detected or set, it can lead to incorrect SRAT table entries. Additionally, in some cases, specific hardware combinations or motherboard designs might have inherent compatibility issues that result in SRAT table errors.

Diagnosing the problem typically involves examining the SRAT table and comparing it with the physical memory layout. Tools like numactl and dmidecode can be used to gather information about the system's NUMA configuration and memory topology. If the SRAT table indicates that Optane PMem is assigned to different NUMA domains than the CPUs, it confirms the presence of the issue. Further investigation might involve checking the BIOS settings, updating the BIOS firmware, and consulting the motherboard and Optane PMem documentation for any known compatibility issues or configuration guidelines. In some cases, specialized tools or BIOS utilities might be required to directly inspect and modify the SRAT table.

Impact on Operating Systems: Ubuntu and CentOS 7

The issue of incorrect NUMA affinity with Optane PMem can manifest across different operating systems, including Ubuntu and CentOS 7, which were specifically mentioned in the original context. While the underlying problem stems from the hardware configuration and ACPI SRAT table, the way the operating system interprets and utilizes this information can influence the severity and manifestation of the issue. Both Ubuntu and CentOS 7 rely on the ACPI SRAT table to determine the NUMA topology and manage memory allocation. If the SRAT table is incorrect, both operating systems will allocate memory based on the flawed information, leading to the performance degradation discussed earlier.

However, there might be subtle differences in how each operating system handles this situation. For example, the default memory allocation policies or kernel schedulers might behave differently, leading to variations in the observed performance impact. Similarly, the tools and utilities available for diagnosing and addressing NUMA-related issues might differ slightly between Ubuntu and CentOS 7. For instance, while both operating systems offer the numactl tool for managing NUMA affinity, the specific command-line options and output formats might vary. Understanding these nuances is important for effective troubleshooting and optimization. In general, the steps for identifying and resolving the issue, such as examining the SRAT table, checking BIOS settings, and updating firmware, will be similar across operating systems, but the specific commands and procedures might require adjustments based on the distribution.

Solutions and Workarounds

Addressing the NUMA affinity issue with Intel Optane PMem requires a multi-faceted approach, focusing on both hardware and software aspects. The primary goal is to ensure that Optane PMem is correctly assigned to the same NUMA domains as the CPUs that will be accessing it. Here are several solutions and workarounds to consider:

  1. BIOS Update: The first and often most effective solution is to update the system's BIOS to the latest version. BIOS updates frequently include fixes for hardware compatibility issues and can resolve problems related to SRAT table generation. Check the motherboard manufacturer's website for the latest BIOS version and follow the instructions for updating the BIOS. This should be the first step in troubleshooting this issue.

  2. BIOS Configuration: Review the BIOS settings related to memory configuration and NUMA. Ensure that NUMA is properly enabled and that the memory settings are configured correctly for Optane PMem. Some BIOSes might have specific settings for Optane PMem, such as memory mode or persistent memory enablement. Refer to the motherboard and Optane PMem documentation for the recommended BIOS settings. The BIOS must be configured to properly recognize and manage the Optane PMem modules.

  3. SRAT Table Modification: In some cases, it might be necessary to manually modify the SRAT table. This is an advanced solution and should be approached with caution. Modifying the SRAT table typically involves using specialized tools or BIOS utilities to directly edit the table entries. Before attempting this, it's crucial to back up the existing BIOS settings and have a clear understanding of the SRAT table format and the implications of making changes. Incorrect modifications to the SRAT table can lead to system instability or even prevent the system from booting. If manual modification is necessary, consult with the hardware vendor or a qualified expert.

  4. Kernel Boot Parameters: As a temporary workaround, it might be possible to influence the NUMA behavior by using kernel boot parameters. For example, the numa_fake=numaX parameter can be used to force the kernel to treat all memory as belonging to a single NUMA node. This can mitigate the performance impact of incorrect NUMA affinity by effectively disabling NUMA awareness. However, this is a workaround and not a permanent solution, as it negates the benefits of NUMA for other memory regions. It should only be used as a temporary measure while investigating the underlying issue.

  5. Application-Level Affinity: Another approach is to explicitly set the NUMA affinity of the application accessing Optane PMem. This can be done using tools like numactl to bind the application's processes or threads to specific NUMA nodes. By ensuring that the application runs on the same NUMA node as the Optane PMem modules, you can minimize inter-socket memory access. However, this requires careful planning and configuration, and it might not be suitable for all applications.

  6. Memory Interleaving: Some BIOSes offer memory interleaving options. While interleaving can improve overall memory bandwidth, it can also interfere with NUMA awareness. If memory interleaving is enabled, try disabling it to see if it resolves the issue. This might require reconfiguring the memory modules in specific slots. Consult the motherboard manual for details on memory interleaving settings.

  7. Contact Hardware Vendor: If none of the above solutions work, it's recommended to contact the hardware vendor for support. They might have specific knowledge or tools for diagnosing and resolving NUMA-related issues with Optane PMem on their hardware. Provide detailed information about your system configuration, including the motherboard model, BIOS version, Optane PMem modules, and operating system. The vendor might be able to provide a BIOS update or suggest a specific configuration that addresses the issue. Troubleshooting can be much easier with proper vendor support.

Conclusion

The NUMA affinity issue with Intel Optane PMem highlights the importance of proper system configuration and understanding the interplay between hardware and software. When Optane PMem is incorrectly assigned to separate NUMA domains, it can lead to significant performance degradation, negating the benefits of this high-performance memory technology. By carefully examining the ACPI SRAT table, checking BIOS settings, and applying appropriate solutions, such as BIOS updates or SRAT table modifications, it is possible to resolve this issue and unlock the full potential of Optane PMem. Remember that a systematic approach to troubleshooting, starting with the most basic solutions and progressing to more advanced techniques, is key to successfully resolving this complex problem. This detailed exploration ensures that systems leveraging Optane PMem can achieve optimal performance and efficiency.