Correction: the jax-compile-stall is not a compile stall

complete/2026/08/jax-compile-stall-slow-vs-stall-audit.md

@@ -175,3 +175,68 @@ imply the API one is resolved.
   chasing speedups on scripts that are hanging.
 - No script is un-quarantined by this phase — restoring coverage is phase 3,
   and depends on the fix.
+
+## The stack, captured after close-out — it is NOT a compile stall
+
+The two 1800s runs named above finished at 22:38 and both dumped a
+`faulthandler` traceback at 1440s, exactly as PyAutoFit#1518 intended. **The
+stack contradicts the premise this whole epic was filed under.**
+
+`autogalaxy_workspace_test` run 32668061785, `imaging/jax_likelihood/mge_group.py`:
+
+```
+Thread 0x00007fbf1f0e4b80 (most recent call first):
+  File ".../jax/_src/api.py", line 2764 in try_to_block
+  File ".../jax/_src/api.py", line 2781 in block_until_ready
+  File ".../autofit/non_linear/jax_compile.py", line 264 in wrapper
+  File ".../scripts/imaging/jax_likelihood/mge_group.py", line 175 in <module>
+```
+
+`autolens_workspace_test` run 32668067325, `multi_dataset/jax_likelihood/mge.py`
+— different repo, different script, **same stack**:
+
+```
+  File ".../jax/_src/api.py", line 2781 in block_until_ready
+  File ".../autofit/non_linear/jax_compile.py", line 264 in wrapper
+  File ".../scripts/multi_dataset/jax_likelihood/mge.py", line 150 in <module>
+```
+
+`jax_compile.py:264` is the `jax.block_until_ready(result)` call — the **second**
+half of the wrapped first call. The process is not tracing, lowering or
+compiling. `func(*args, **kwargs)` **returned**; what never returns is the wait
+for the result to materialize.
+
+### What this overturns
+
+- Every marker, issue and prompt in this epic calls it an *"intermittent XLA
+  compile stall"*. On this evidence that name is **wrong**, and it has been
+  wrong since the first quarantine on 2026-08-01. Anyone resuming should treat
+  "compile stall" as a label inherited from a guess, not a finding.
+- The heartbeat line reads `JAX jit still compiling ... 1770s elapsed` while the
+  process sits in `block_until_ready`. That wording is a defect in
+  `log_on_first_compile` — the wrapper cannot tell which half it is in and says
+  "compiling" regardless. **Fix the message to name the phase before anyone
+  reads another one of these logs.**
+- Both entries are `AMBIGUOUS` even at the 1800s cap: 2/2 runs capped on both
+  legs, no completion. Combined with the 300s round, that is **20 consecutive
+  cap hits** for `mge_group.py` and `multi_dataset/mge.py` with zero
+  completions — while `mge.py`'s own marker records it finishing in 32s
+  standalone.
+
+### Where this points instead
+
+An execution/materialization hang, not a compiler one. The hypotheses worth
+testing are now different from the ones this epic queued up:
+
+1. **Device transfer or the async dispatch queue deadlocking** on the runner —
+   `try_to_block` is where a never-arriving buffer would park.
+2. **The persistent compilation cache** is still live but for a different
+   reason than assumed: a cache *read* satisfying the compile instantly and
+   leaving execution to hang is consistent with what is seen here.
+3. **The `vmap(jit)` ordering** result (80% -> 30%, p=0.070) needs reinterpreting
+   in this light: the ordering changes the *shape of the executed computation*,
+   not just the compile graph, which is a more plausible route to an execution
+   hang than to a compile one.
+
+Do not restart from "why is XLA slow to compile". Start from "why does
+`block_until_ready` never return".

draft/bug/ci/jax_vmap_jit_compile_stall.md

@@ -137,6 +137,15 @@ what made this diagnosable at all.
 - The SLOW-vs-stall question in step 1 answered in writing, so the Profiling
   Agent is not chasing speedups on scripts that are hanging.
 
+## CORRECTION — the stack says this is NOT a compile stall
+
+Captured after close-out, from both test workspaces independently: the stalled
+process is parked in `jax.block_until_ready` / `try_to_block`, i.e. the
+**execution** half of the first call, not tracing/lowering/compiling. The title
+of this file, and every marker calling this an "intermittent XLA compile
+stall", inherit a guess made before there was any evidence. See the record's
+final section.
+
 ## CLOSED AS PARTIAL — 2026-08-23
 
 Phase 1 shipped. Phases 2 and 3 were taken to a deliberate stopping point and the

draft/research/ci/smoke_timing_and_profiling.md

@@ -37,6 +37,14 @@ The headline for this task: **a SLOW marker is not evidence of slowness.** Every
 2026-07-14 marker reads "flakes at the 1800s cap" and records no timing at all;
 the one entry measured so far was wrong by a factor of ~50.
 
+## Before you start: the "compile stall" is a misnomer
+
+The stalled entries above were captured mid-hang on 2026-08-23 and the stack
+puts them in `jax.block_until_ready`, not in compilation. If any of this work
+touches those entries, do not carry the "XLA compile stall" framing over — it
+predates the evidence. Detail in
+[`../../../complete/2026/08/jax-compile-stall-slow-vs-stall-audit.md`](../../../complete/2026/08/jax-compile-stall-slow-vs-stall-audit.md).
+
 ## The harness exists
 
 `.github/workflows/retime.yml` + `.github/scripts/retime.py` in both test

epics.md

@@ -32,6 +32,9 @@ epic, never picked standalone.
 - title: Intermittent XLA compile stall in the JAX vmap likelihood path
 - ledger: draft/bug/ci/jax_vmap_jit_compile_stall.md
 - status: CLOSED AS PARTIAL 2026-08-23 — record complete/2026/08/jax-compile-stall-slow-vs-stall-audit.md
+- CORRECTION (post-close-out): the captured stack shows the hang is in jax.block_until_ready, NOT in
+  compilation. The epic's name and every marker calling this an "XLA compile stall" are wrong. Resume
+  from "why does block_until_ready never return", not from compiler behaviour.
 - notes: phase 1 (watchdog) shipped in full; phases 2/3 stopped deliberately at a measured-but-not-root-caused state. The stall is instrumented and characterised (>100x bimodality inside one compile step; vmap-of-jit contributory at p=0.070 but NOT causal; the compile-cache hypothesis never tested) and NOTHING was un-quarantined. Resume via draft/research/ci/smoke_timing_and_profiling.md, which is where this gets dug up. Superseded draft/bug/autolens_workspace_test/multi_dataset_jax_likelihood_xla_stall.md.
 
 ## graphical-ep