It reported "ready on cpu" while holding 9756 MiB of VRAM, 33 GB of host RAM
and 1100% CPU. The cause was not the model and not the GPU: /cache had reached
11 MB free of 141 GB -- 15 GB of it pip's download cache from building these
venvs, plus two 6 GB venvs put there by this code -- and HF_HOME is on that
partition, so the shard download died with

    OSError: I/O error: IO Error: No space left on device (os error 28)

the GPU load aborted part-way, and the fallback loaded the model again on the
CPU. Everything about that was invisible:

* The child's stderr was read only if it exited silently, so the reason sat in
  a pipe nobody drained -- which is also a pipe that fills, at which point the
  child blocks writing to it and looks like a model that hangs. It is now
  pumped into the engine log for the child's whole life.
* "ready on cpu" said nothing about whether the CPU was asked for or fallen
  into, and the difference is 30x and tens of GB. The fallback now carries its
  reason (gpu_error) and the parent logs it loudly.
* The aborted GPU load kept what it had already allocated for the life of the
  process: 9756 MiB that the manager's ledger could not see and nothing else
  could use. The card is now released (gc + empty_cache + ipc_collect) before
  the CPU retry.

And the placement that caused it is fixed: a NEW venv goes on the roomiest
candidate root, not the first one named. The engines' own CODERAI_CACHE_DIR is
/AI/guffcache with 197 GB free while the default cache root had 15 GB; six GB
has to go somewhere, and it should not be the fullest disk. An existing venv
still wins over a roomier empty root, so nothing is built twice.

One of this file's own tests was also writing a pointer file into the REAL
config root, naming a pytest tmpdir, which a later process would then have
resolved the venv from. Removed, and a test now fails if any test here calls
ensure_installed() without redirecting _pointer_file.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZozi1Ehk96NG1WesgC