HB-series virtual machines overview


This article references CentOS, a Linux distribution that is nearing End Of Life (EOL) status. Please consider your use and plan accordingly. For more information, see the CentOS End Of Life guidance.

Applies to: ✔️ Linux VMs ✔️ Windows VMs ✔️ Flexible scale sets ✔️ Uniform scale sets

Maximizing high performance compute (HPC) application performance on AMD EPYC requires a thoughtful approach memory locality and process placement. Below we outline the AMD EPYC architecture and our implementation of it on Azure for HPC applications. We will use the term “pNUMA” to refer to a physical NUMA domain, and “vNUMA” to refer to a virtualized NUMA domain.

Physically, an HB-series server is 2 * 32-core EPYC 7551 CPUs for a total of 64 physical cores. These 64 cores are divided into 16 pNUMA domains (8 per socket), each of which is four cores and known as a “CPU Complex” (or “CCX”). Each CCX has its own L3 cache, which is how an OS will see a pNUMA/vNUMA boundary. A pair of adjacent CCXs shares access to two channels of physical DRAM (32 GB of DRAM in HB-series servers).

To provide room for the Azure hypervisor to operate without interfering with the VM, we reserve physical pNUMA domain 0 (the first CCX). We then assign pNUMA domains 1-15 (the remaining CCX units) for the VM. The VM will see:

(15 vNUMA domains) * (4 cores/vNUMA) = 60 cores per VM

The VM, itself, doesn't know that pNUMA 0 wasn't given to it. The VM understands pNUMA 1-15 as vNUMA 0-14, with 7 vNUMA on vSocket 0 and 8 vNUMA on vSocket 1. While this is asymmetrical, your OS should boot and operate normally. Later in this guide, we instruct how best to run MPI applications on this asymmetric NUMA layout.

Process pinning will work on HB-series VMs because we expose the underlying silicon as-is to the guest VM. We strongly recommend process pinning for optimal performance and consistency.

The following diagram shows the segregation of cores reserved for Azure Hypervisor and the HB-series VM.

Segregation of cores reserved for Azure Hypervisor and HB-series VM

Hardware specifications

Hardware Specifications HB-series VM
Cores 60 (SMT disabled)
CPU Frequency (non-AVX) ~2.55 GHz (single + all cores)
Memory 4 GB/core (240 GB total)
Local Disk 700 GB SSD
Infiniband 100 Gb EDR Mellanox ConnectX-5
Network 50 Gb Ethernet (40 Gb usable) Azure second Gen SmartNIC

Software specifications

Software Specifications HB-series VM
Max MPI Job Size 18000 cores (300 VMs in a single virtual machine scale set with singlePlacementGroup=true)
MPI Support HPC-X, Intel MPI, OpenMPI, MVAPICH2, MPICH, Platform MPI
Additional Frameworks UCX, libfabric, PGAS
Azure Storage Support Standard and Premium Disks (maximum 4 disks)
OS Support for SRIOV RDMA CentOS/RHEL 7.6+, Ubuntu 18.04+, SLES 15.4, WinServer 2016+
Orchestrator Support CycleCloud, Batch, AKS; cluster configuration options


This document references a release version of Linux that is nearing or at, End of Life(EOL). Please consider updating to a more current version.

Next steps