# NUMA-aware worker pools for merge

## Problem

The merge command's bottleneck is `compute_degrees` in `obidebruinj`: a random pointer-chase over 20–70 M node hash maps that saturates DRAM bandwidth. When multiple partition workers run concurrently, they contend for the shared memory bus, causing super-linear slowdown (measured: 0.016 µs/node solo → 0.95 µs/node with 4–5 concurrent workers, ×60 degradation).

Modern HPC nodes are multi-socket NUMA machines (observed: 2 sockets × 4 NUMA nodes × 24 cores = 192 cores). Cross-NUMA memory traffic compounds the contention:

- Full 192-core run: ~15 min/partition (×10 worse than M3 Mac)
- `taskset` restricted to 4 NUMA nodes (96 cores): ~90 s/partition
- OAR job on 1 NUMA node (24 cores): ~80 s/partition, same throughput as 96 cores

**Conclusion**: the bottleneck is memory bandwidth per NUMA node, not core count. 24 cores on one NUMA node achieve the same throughput as 96 cores across four.

## Strategy

Run N worker groups in parallel, one per NUMA node, each with its own Rayon thread pool whose threads are pinned to the NUMA node's CPUs. Linux's first-touch policy then places graph allocations on local DRAM automatically — no explicit NUMA allocator needed.

Expected throughput: N × single-NUMA throughput. On the 8-NUMA-node HPC: 8 × ~80 s = 9–10 min total instead of >60 min with the current single-pool approach.

## Rayon thread pool isolation

Rayon provides `ThreadPool::install(|| { ... })`: any Rayon call (`par_iter`, `current_num_threads`, etc.) inside the closure uses *that* pool exclusively. Wrapping `merge_partition` in `pool.install()` redirects all downstream Rayon calls — including those in `debruijn.rs` and `partition.rs` — without touching those crates.

```rust
// worker thread, assigned to NUMA pool `pool`
pool.install(|| {
    dst_partition.merge_partition(i, srcs, mode, n_dst_genomes, block_bits, evidence)
})
```

`rayon::current_num_threads()` inside `merge_partition` will return the pool size (e.g. 24), not the global thread count — which is the right value for buffer sizing.

## Thread pinning

`ThreadPoolBuilder::spawn_handler` provides a hook executed for each thread at creation. Inside, `libc::sched_setaffinity` pins the thread to a CPU set:

```rust
let cpus: Vec<usize> = numa_node_cpus(node); // from /sys/devices/system/node/nodeN/cpulist
rayon::ThreadPoolBuilder::new()
    .num_threads(cpus.len())
    .spawn_handler(move |thread| {
        let mut b = std::thread::Builder::new();
        std::thread::Builder::new().spawn(move || {
            pin_to_cpus(&cpus);   // sched_setaffinity via libc
            thread.run()
        })?;
        Ok(())
    })
    .build()?
```

NUMA topology is read from `/sys/devices/system/node/node*/cpulist` — no `libnuma` dependency required. If the `numa` crate is linked, `numa_available()` / `numa_run_on_node()` are an alternative.

## Memory locality

Linux allocates pages on the NUMA node of the thread that first writes them (first-touch policy). Once Rayon threads are pinned to node N, all graph data built by those threads lands on node N's DRAM. No changes to the allocator, no explicit `numa_alloc_onnode` calls.

## Adaptive spawn criterion

The current criterion uses `std::thread::available_parallelism()` (returns total cores = 192) and `max_workers = n_cores / 2`. With NUMA pools:

- `n_cores` per pool = cores per NUMA node (e.g. 24)
- `max_workers` per pool = pool size / 2 (e.g. 12)
- CPU efficiency is measured per pool, not globally

Each NUMA group runs its own independent adaptive pool. Workers are distributed across NUMA groups round-robin or by workload (partition assignment can be pre-split by NUMA group index).

## Required changes

| File | Change |
|------|--------|
| `obikindex/src/merge.rs` | Detect NUMA topology; build N `ThreadPool`s with pinned threads; assign each pre-spawned worker to a pool; wrap `merge_partition` in `pool.install()` |
| `obikindex/src/merge.rs` | Replace `available_parallelism()` with per-NUMA core count for spawn criterion |
| `obikpartitionner/src/merge_layer.rs` | No change — `merge_partition` already works inside any Rayon context |
| `obidebruinj/src/debruijn.rs` | No change — `par_iter` and `current_num_threads` are pool-context-aware |
| `obikpartitionner/src/partition.rs` | No change — same reason |

## Platform guard

NUMA pinning is Linux-only. The fallback is the current single global pool:

```rust
#[cfg(target_os = "linux")]
fn build_numa_pools() -> Option<Vec<rayon::ThreadPool>> { ... }

#[cfg(not(target_os = "linux"))]
fn build_numa_pools() -> Option<Vec<rayon::ThreadPool>> { None }
```

When `build_numa_pools()` returns `None` (macOS, UMA, or single-socket), `merge.rs` uses the existing code path unchanged.

## Open questions

- **Partition assignment**: split partitions by NUMA group up-front (static) or use a shared queue with per-group workers stealing from a common pool? Static split is simpler; stealing is better for load balance when partitions vary widely in size.
- **Intra-NUMA adaptive criterion**: with 24 cores and ~3–5 effective workers per NUMA node, the current marginal-gain criterion needs re-tuning or can be left as-is with per-pool `n_cores = 24`.
- **I/O**: partition data (unitig files) is on a shared filesystem. With 8 concurrent NUMA groups, I/O concurrency increases 8× — need to verify the filesystem (Lustre or local SSD) can absorb it without becoming the new bottleneck.