numa-programming

NUMA Programming

Purpose

Guide agents through NUMA-aware programming: topology detection with numactl and sysfs, libnuma API (numa_alloc_onnode, mbind, set_mempolicy), process binding with numactl, NUMA-aware data structures, remote access diagnosis with perf stat, and lstopo visualization.

When to Use

• Multi-socket server shows poor scaling despite low CPU utilization
• Memory bandwidth saturation on one NUMA node
• Binding database or cache process to local memory
• Designing per-node freelists or sharded allocators
• Measuring remote vs local memory access latency
• Tuning HPC, DPDK, or custom allocator for socket locality

Workflow

1. Topology detection

# Hardware topology summary
numactl --hardware

# Detailed topology with distances
lstopo --of console

# sysfs nodes
ls /sys/devices/system/node/
cat /sys/devices/system/node/node0/meminfo
cat /sys/devices/system/node/node0/cpulist

Typical output:

available: 2 nodes (0-1)
node 0 cpus: 0-15
node 0 size: 65536 MB
node 1 cpus: 16-31
node 1 size: 65536 MB
node distances:
node   0   1
  0:  10  21
  1:  21  10

Distance 10 = local, higher = remote (cross-socket).

2. Process binding with numactl

# Bind to node 0 CPUs and memory
numactl --cpunodebind=0 --membind=0 ./myapp

# Interleave memory across all nodes
numactl --interleave=all ./myapp

# Preferred node (fallback if full)
numactl --preferred=0 ./myapp

# Show process NUMA policy
numactl --show
cat /proc/self/numa_maps

3. libnuma API

#include <numa.h>
#include <numaif.h>
#include <stdio.h>

int main(void) {
    if (numa_available() < 0) {
        fprintf(stderr, "NUMA not available\n");
        return 1;
    }
    int node = numa_node_of_cpu(0);
    printf("CPU 0 on node %d\n", node);

    // Allocate on specific node
    size_t size = 1024 * 1024 * 1024;
    void *mem = numa_alloc_onnode(size, 0);
    if (!mem) return 1;

    // Bind existing memory
    unsigned long nodemask = 1UL << 0;
    mbind(mem, size, MPOL_BIND, &nodemask, sizeof(nodemask) * 8, 0);

    numa_free(mem, size);
    return 0;
}