Mooncake PG/EP Troubleshooting

Mooncake PG/EP Troubleshooting#

This page covers common setup, import, runtime, and recovery issues for Mooncake PG and Mooncake EP.

Import fails with a PyTorch version error#

Symptoms:

Mooncake PG was not built against torch==...
Mooncake EP was not built against torch==...

Cause:

mooncake.pg and mooncake.ep load native extension modules whose names include the current PyTorch version. If the wheel or source build does not contain an extension for the active torch.__version__, import fails.

Fixes:

  1. Verify the active PyTorch version:

    python - <<'PY'
import torch
print(torch.__version__)
print(torch.version.cuda)
PY

  2. Install a Mooncake wheel built for that PyTorch/CUDA combination, or rebuild from source with EP/PG enabled:

    cmake .. -DWITH_EP=ON
make -j

  3. Make sure the Python environment used at runtime is the same one used for the build.

dist.get_world_size() differs from the number of active ranks#

Mooncake PG preserves stable in-group rank slots. Consequently:

  • max_group_size is the fixed slot capacity;

  • dist.get_world_size() is the highest active in-group rank plus one;

  • holes below that extent remain visible in the rank space but are skipped by the active mask.

Use pg.get_active_ranks(backend) to inspect the current backend mask.

join_group() hangs or activation times out#

Common causes:

  • No existing rank submitted activate_ranks() / recover_ranks() after the joining process entered join_group().

  • The future active set is not mutually connected, so the activation proposal remains pending until timeout.

EP dispatch/combine timeout marks a rank inactive#

Mooncake EP kernels can update the EP-level active_ranks tensor when a source rank does not make progress before timeout_us.

If this happens unexpectedly:

  • Increase timeout_us to rule out slow startup or transient scheduling delay.

  • Check whether the source rank exited, crashed, or skipped the matching dispatch/combine call.

  • Confirm num_experts % num_ranks == 0 and that every rank uses the same num_experts, top_k, and buffer sizing assumptions.

  • Ensure all ranks call dispatch and combine in the same order.

If timeout detection is not desired for a test, pass timeout_us=-1.

EP falls back instead of using the fast path#

The Python wrapper uses fallback when the native runtime cannot use IBGDA/RDMA or fully accessible P2P.

Possible causes:

  • RDMA devices or drivers are not available inside the container.

  • HCA selection picked the wrong device.

  • GPUDirect RDMA / peer-memory support is missing.

  • CUDA IPC or peer access is unavailable between local GPUs.

  • The environment was built without the required accelerator support.

Debug checklist:

  1. Restrict HCA selection with pg.set_device_filter([...]) or MOONCAKE_PGTEST_DEVICE_FILTERS.

  2. Check whether all intended ranks can see the same accelerator and RDMA devices.

  3. Run PG collectives first; EP metadata exchange depends on a healthy process group.

  4. Run EP tests with fallback enabled and disabled to separate correctness from transport setup.

RDMA connection errors or severe latency from wrong HCA selection#

Symptoms can include connection setup failures, repeated transport timeouts, unexpected fallback, very low bandwidth, or RDMA retry-style errors when the backend auto-selects an unsuitable NIC/HCA. This commonly happens on machines with multiple RDMA devices where some devices are for management traffic, are on different fabrics, or are not reachable from peer ranks.

Fix: set an explicit device filter before initializing Mooncake PG/EP so all ranks use the intended HCA list.

For application code:

from mooncake import pg

pg.set_device_filter(["mlx5_1", "mlx5_2"])
# call dist.init_process_group(...) after setting the filter

For PG tests and benchmarks:

MOONCAKE_PGTEST_DEVICE_FILTERS=mlx5_1,mlx5_2 \
python -m unittest discover -s mooncake-pg/tests -k CUDA -v

Use the same filter on every rank. If the problem disappears after setting the filter, treat the original failure as a topology/device-selection issue rather than an EP kernel or collective correctness problem.

EP output mismatch or buffer overflow symptoms#

Common causes:

  • num_max_dispatch_tokens_per_rank is smaller than the actual per-rank token count.

  • num_experts is not divisible by num_ranks.

  • topk_idx contains expert IDs outside [0, num_experts) except for masked -1 entries.

  • The handle from one dispatch call is reused with an unrelated combine call.

  • zero_copy=True is used without writing expert outputs into get_next_combine_buffer(handle).

Fixes:

  • Size the buffer for peak traffic:

    num_ep_buffer_bytes = Buffer.get_ep_buffer_size_hint(
    max_tokens_per_rank, hidden, world_size, num_experts
)

  • Keep dispatch and combine paired by using the matching handle.

  • Call event.current_stream_wait() or the returned hook() before consuming operation outputs.

P2P send/recv errors#

Mooncake PG’s PyTorch P2P path currently supports single-tensor send/recv through the backend shim. If batch_isend_irecv() fails:

  • verify each P2POp contains one tensor;

  • ensure peer ranks are active;

  • check that all ranks issue matching send/recv operations;

  • test CPU and accelerator backends separately to isolate device-specific transport issues.

Useful smoke commands#

# PG CPU sanity
python -m unittest discover -s mooncake-pg/tests -k CPU -v

# PG CUDA sanity
python -m unittest discover -s mooncake-pg/tests -k CUDA -v

# PG all-reduce benchmark smoke
PYTHONPATH=mooncake-pg \
python mooncake-pg/benchmark/pgbench.py \
  --collective all_reduce --backend mooncake --device cuda -g 2 -b 8 -e 1M -f 2

# EP grid test
python python/tests/ep/test_ep_grid.py

Adapt process counts, device filters, and launchers to the target cluster.

What to include in bug reports#

When reporting PG/EP issues, include:

  • Mooncake commit and installation method;

  • PyTorch version and CUDA/accelerator runtime version;

  • exact backend name (mooncake or mooncake-cpu);

  • world size, max world size, rank IDs, and subgroup layout;

  • active-rank tensor dtype/device/values;

  • HCA/device filters;

  • minimal command or script reproducing the issue;

  • logs from all ranks around initialization, failure detection, and recovery.

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