HC-series virtual machine overview


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

Maximizing HPC application performance on Intel Xeon Scalable Processors requires a thoughtful approach to process placement on this new architecture. Here, we outline our implementation of it on Azure HC-series VMs 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. Similarly, we will use the term “pCore” to refer to physical CPU cores, and “vCore” to refer to virtualized CPU cores.

Physically, an HC-series server is 2 * 24-core Intel Xeon Platinum 8168 CPUs for a total of 48 physical cores. Each CPU is a single pNUMA domain, and has unified access to six channels of DRAM. Intel Xeon Platinum CPUs feature a 4x larger L2 cache than in prior generations (256 KB/core -> 1 MB/core), while also reducing the L3 cache compared to prior Intel CPUs (2.5 MB/core -> 1.375 MB/core).

The above topology carries over to the HC-series hypervisor configuration as well. To provide room for the Azure hypervisor to operate without interfering with the VM, we reserve pCores 0-1 and 24-25 (that is, the first 2 pCores on each socket). We then assign pNUMA domains all remaining cores to the VM. Thus, the VM will see:

(2 vNUMA domains) * (22 cores/vNUMA) = 44 cores per VM

The VM has no knowledge that pCores 0-1 and 24-25 weren't given to it. Thus, it exposes each vNUMA as if it natively had 22 cores.

Intel Xeon Platinum, Gold, and Silver CPUs also introduce an on-die 2D mesh network for communication within and external to the CPU socket. We strongly recommend process pinning for optimal performance and consistency. Process pinning will work on HC-series VMs because the underlying silicon is exposed as-is to the guest VM.

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

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

Hardware specifications

Hardware Specifications HC-series VM
Cores 44 (HT disabled)
CPU Intel Xeon Platinum 8168
CPU Frequency (non-AVX) 3.7 GHz (single core), 2.7-3.4 GHz (all cores)
Memory 8 GB/core (352 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 HC-series VM
Max MPI Job Size 13200 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.

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