ADR-0023 D9.7: IPCQ slot-memory latency model (TCM/SRAM/HBM)

Charge per-tier bandwidth + setup overhead at IPCQ slot WRITE
(receiver inbound DMA, in pe_dma._handle_ipcq_inbound) and slot
READ (recv consume, in pe_ipcq._handle_recv). Tier table
(common/ipcq_types.py):
  tcm  : 512 GB/s, 0 ns
  sram : 128 GB/s, 2 ns
  hbm  :  32 GB/s, 6 ns

Before this change, slot read/write was free regardless of
buffer_kind, making memory-tier choice invisible in simulated
latency. After the change, swapping buffer_kind in ccl.yaml
produces measurable per-tier separation in allreduce latency.

Tests:
  test_ipcq_buffer_kind_latency.py — three micro-tests asserting
    tcm < sram < hbm ordering, payload-scaling, and that
    buffer_kind sensitivity grows with payload (credit-only path
    stays fabric-bound).
  test_allreduce_buffer_kind_sweep.py — 12-config parametrized
    sweep emitting buffer_kind_sweep.png (3 lines, torus_2d).

conftest sessionfinish hook generalised to dispatch multiple
sweep aggregators (allreduce + buffer-kind).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/kernbench/common/ipcq_types.py

@@ -31,6 +31,26 @@ class IpcqInvalidDirection(ValueError):
     has no neighbor installed for this PE."""
 
 
+# ── ADR-0023 D9.7: IPCQ slot-memory latency model ───────────────────
+#
+# Per-tier (bw_gbs, overhead_ns) used to charge the slot write (inbound)
+# and slot read (recv consume). Mirrors topology.yaml component values.
+_BUFFER_KIND_BW: dict[str, tuple[float, float]] = {
+    "tcm":  (512.0, 0.0),
+    "sram": (128.0, 2.0),
+    "hbm":  (32.0,  6.0),
+}
+
+
+def slot_io_latency_ns(buffer_kind: str, nbytes: int) -> float:
+    """Per-access latency for one slot read/write of ``nbytes`` against
+    the IPCQ backing memory tier (``buffer_kind``)."""
+    bw_gbs, overhead_ns = _BUFFER_KIND_BW.get(
+        buffer_kind, _BUFFER_KIND_BW["tcm"],
+    )
+    return float(nbytes) / bw_gbs + overhead_ns
+
+
 # ── D2.5: IpcqEndpoint ───────────────────────────────────────────────
 
 

src/kernbench/components/builtin/pe_dma.py

@@ -219,6 +219,16 @@ class PeDmaComponent(PeEngineBase):
 
         token = txn.request
 
+        # ADR-0023 D9.7: charge IPCQ slot-WRITE latency against the
+        # backing-memory tier (tcm/sram/hbm) before the atomic block.
+        # Must come BEFORE the atomic write→IpcqMetaArrival pair (I6).
+        from kernbench.common.ipcq_types import slot_io_latency_ns
+        slot_write_ns = slot_io_latency_ns(
+            token.dst_endpoint.buffer_kind, token.nbytes,
+        )
+        if slot_write_ns > 0:
+            yield env.timeout(slot_write_ns)
+
         # ── ATOMIC: do not introduce yield between these two operations ──
         # 1. Move data via MemoryStore (single-hop DMA write).
         # Prefer the in-flight snapshot stashed by the sender PE_DMA;

src/kernbench/components/builtin/pe_ipcq.py

@@ -329,6 +329,16 @@ class PeIpcqComponent(ComponentBase):
 
         qp["my_tail"] += 1
 
+        # ADR-0023 D9.7: charge IPCQ slot-READ latency against the
+        # backing-memory tier (tcm/sram/hbm). Recv blocks for the
+        # kernel-side slot consume; pe_exec_ns reflects this cost.
+        from kernbench.common.ipcq_types import slot_io_latency_ns
+        slot_read_ns = slot_io_latency_ns(
+            self._buffer_kind, req.result_data.get("nbytes", 0),
+        )
+        if slot_read_ns > 0:
+            yield env.timeout(slot_read_ns)
+
         # Diagnostics trace (D14)
         from kernbench.ccl import diagnostics
         if diagnostics.trace_enabled():