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# AMD Instinct™ MI250 microarchitecture

The microarchitecture of the AMD Instinct MI250 accelerators is based on the
AMD CDNA 2 architecture that targets compute applications such as HPC,
artificial intelligence (AI), and machine learning (ML) and that run on
everything from individual servers to the world’s largest exascale
supercomputers. The overall system architecture is designed for extreme
scalability and compute performance.

The following image shows the components of a single Graphics Compute Die (GCD) of the CDNA 2 architecture. On the top and the bottom are AMD Infinity Fabric™
interfaces and their physical links that are used to connect the GPU die to the
other system-level components of the node (see also Section 2.2). Both
interfaces can drive four AMD Infinity Fabric links. One of the AMD Infinity
Fabric links of the controller at the bottom can be configured as a PCIe link.
Each of the AMD Infinity Fabric links between GPUs can run at up to 25 GT/sec,
which correlates to a peak transfer bandwidth of 50 GB/sec for a 16-wide link (
two bytes per transaction). Section 2.2 has more details on the number of AMD
Infinity Fabric links and the resulting transfer rates between the system-level
components.

To the left and the right are memory controllers that attach the High Bandwidth
Memory (HBM) modules to the GCD. AMD Instinct MI250 GPUs use HBM2e, which offers
a peak memory bandwidth of 1.6 TB/sec per GCD.

The execution units of the GPU are depicted in the following image as Compute
Units (CU). The MI250 GCD has 104 active CUs. Each compute unit is further
subdivided into four SIMD units that process SIMD instructions of 16 data
elements per instruction (for the FP64 data type). This enables the CU to
process 64 work items (a so-called “wavefront”) at a peak clock frequency of 1.7
GHz. Therefore, the theoretical maximum FP64 peak performance per GCD is 22.6
TFLOPS for vector instructions. This equates to 45.3 TFLOPS for vector instructions for both GCDs together. The MI250 compute units also provide specialized
execution units (also called matrix cores), which are geared toward executing
matrix operations like matrix-matrix multiplications. For FP64, the peak
performance of these units amounts to 90.5 TFLOPS.

![Structure of a single GCD in the AMD Instinct MI250 accelerator.](../../data/conceptual/gpu-arch/image001.png "Structure of a single GCD in the AMD Instinct MI250 accelerator.")

```{list-table} Peak-performance capabilities of the MI250 OAM for different data types.
:header-rows: 1
:name: mi250-perf-table

*
  - Computation and Data Type
  - FLOPS/CLOCK/CU
  - Peak TFLOPS
*
  - Matrix FP64
  - 256
  - 90.5
*
  - Vector FP64
  - 128
  - 45.3
*
  - Matrix FP32
  - 256
  - 90.5
*
  - Packed FP32
  - 256
  - 90.5
*
  - Vector FP32
  - 128
  - 45.3
*
  - Matrix FP16
  - 1024
  - 362.1
*
  - Matrix BF16
  - 1024
  - 362.1
*
  - Matrix INT8
  - 1024
  - 362.1
```

The above table summarizes the aggregated peak performance of the AMD
Instinct MI250 OCP Open Accelerator Modules (OAM, OCP is short for Open Compute
Platform) and its two GCDs for different data types and execution units. The
middle column lists the peak performance (number of data elements processed in a
single instruction) of a single compute unit if a SIMD (or matrix) instruction
is being retired in each clock cycle. The third column lists the theoretical
peak performance of the OAM module. The theoretical aggregated peak memory
bandwidth of the GPU is 3.2 TB/sec (1.6 TB/sec per GCD).

![Dual-GCD architecture of the AMD Instinct MI250 accelerators](../../data/conceptual/gpu-arch/image002.png "Dual-GCD architecture of the AMD Instinct MI250 accelerators")

The following image shows the block diagram of an OAM package that consists
of two GCDs, each of which constitutes one GPU device in the system. The two
GCDs in the package are connected via four AMD Infinity Fabric links running at
a theoretical peak rate of 25 GT/sec, giving 200 GB/sec peak transfer bandwidth
between the two GCDs of an OAM, or a bidirectional peak transfer bandwidth of
400 GB/sec for the same.

## Node-level architecture

The following image shows the node-level architecture of a system that is
based on the AMD Instinct MI250 accelerator. The MI250 OAMs attach to the host
system via PCIe Gen 4 x16 links (yellow lines). Each GCD maintains its own PCIe
x16 link to the host part of the system. Depending on the server platform, the
GCD can attach to the AMD EPYC processor directly or via an optional PCIe switch
. Note that some platforms may offer an x8 interface to the GCDs, which reduces
the available host-to-GPU bandwidth.

![Block diagram of AMD Instinct MI250 Accelerators with 3rd Generation AMD EPYC processor](../../data/conceptual/gpu-arch/image003.png "Block diagram of AMD Instinct MI250 Accelerators with 3rd Generation AMD EPYC processor")

The preceding image shows the node-level architecture of a system with AMD
EPYC processors in a dual-socket configuration and four AMD Instinct MI250
accelerators. The MI250 OAMs attach to the host processors system via PCIe Gen 4
x16 links (yellow lines). Depending on the system design, a PCIe switch may
exist to make more PCIe lanes available for additional components like network
interfaces and/or storage devices. Each GCD maintains its own PCIe x16 link to
the host part of the system or to the PCIe switch. Please note, some platforms
may offer an x8 interface to the GCDs, which will reduce the available
host-to-GPU bandwidth.

Between the OAMs and their respective GCDs, a peer-to-peer (P2P) network allows
for direct data exchange between the GPU dies via AMD Infinity Fabric links (
black, green, and red lines). Each of these 16-wide links connects to one of the
two GPU dies in the MI250 OAM and operates at 25 GT/sec, which corresponds to a
theoretical peak transfer rate of 50 GB/sec per link (or 100 GB/sec
bidirectional peak transfer bandwidth). The GCD pairs 2 and 6 as well as GCDs 0
and 4 connect via two XGMI links, which is indicated by the thicker red line in
the preceding image.
