how to simulate a Multi-level NUMA in gem5?

[kelark]

I want to simulate a multi-level NUMA architecture in gem5, for example with two NUMA nodes and two sockets for each NUMA node. I didn't find the tutorial in the official documentation. Where should I start?

[ivanaamit]

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Simulating a multi-level NUMA (Non-Uniform Memory Access) architecture in gem5 can be quite complex due to the intricacies of memory access timings and interconnect configurations between different processors and memory nodes. Although detailed tutorials on setting up specific NUMA configurations might not be directly available in the gem5 documentation, you can piece together the required setup from various resources and examples. Here’s a general approach to get you started:

Understanding NUMA in gem5

First, it's essential to understand that NUMA setups involve configuring multiple processors with their own local memory banks, and potentially different latencies to access memory across these nodes. Each socket in a NUMA node can be thought of as a separate processor or CPU cluster with its own local memory, but it can also access memory controlled by other sockets, typically at higher latencies.

Step-by-Step Guide to Setting Up a Multi-Level NUMA Simulation

Step 1: Define the Basic System Architecture
You need to define the CPUs, memory controllers, and the topology of how they are interconnected. This involves setting up multiple system objects in gem5:

• Define CPU Clusters: Create clusters of CPUs for each socket in each NUMA node. In gem5, this could involve setting up instances of DerivO3CPU or TimingSimpleCPU for each CPU.
• Set Up Memory Systems: Assign a memory controller to each NUMA node. You can use objects like DDR3_1600_8x8 or similar for each controller.

Step 2: Configure the Interconnect
The interconnect is crucial for defining the NUMA characteristics. You might use a simple Crossbar or a more complex network like Garnet depending on your simulation's fidelity requirements:

• Crossbar: This can be used for relatively simple interconnects and is easier to configure.
• Mesh Network: For a more realistic simulation of a multi-socket NUMA system, a mesh network might be more appropriate.

Step 3: Set Memory Access Latencies
Define different latencies for local vs. remote memory accesses. This can be done by configuring the latency parameters of the links between CPUs and memory controllers in the system.

Step 4: Implement NUMA-Specific Configuration
Here’s where you specifically set up NUMA characteristics by modifying how each CPU accesses memory:

• Local vs. Remote Memory: Define which memory banks are considered local and which are remote for each CPU.
• Memory Policies: Implement or use existing policies that decide how memory is allocated and accessed across different nodes.

Step 5: Running the Simulation
Set up your simulation script to initialize the system and run your workloads. You’ll need to monitor both performance and how memory is being accessed across different nodes to ensure your NUMA configuration is behaving as expected.

Example Configuration Snippet

Here's a very basic pseudo-setup in Python for gem5 that illustrates some of these concepts:

# Assume necessary imports and basic setup is done
system = System()

# Create clusters
numa_node0 = [TimingSimpleCPU() for _ in range(2)]
numa_node1 = [TimingSimpleCPU() for _ in range(2)]

# Define memory for each NUMA node
mem_ctrl0 = SimpleMemory(latency='50ns', conf_table_reported=False)
mem_ctrl1 = SimpleMemory(latency='50ns', conf_table_reported=False)

# Interconnect setup
system.membus = SystemXBar()

# Connect CPUs to membus and then to respective memory controllers
for cpu in numa_node0:
    cpu.connectBus(system.membus)
    cpu.connectMemPorts(mem_ctrl0.port)

for cpu in numa_node1:
    cpu.connectBus(system.membus)
    cpu.connectMemPorts(mem_ctrl1.port)

# Set up a system-wide memory bus (or other interconnect)
system.system_port = system.membus.slave