[EC][Mooncake] Default TransferEngine protocol to TCP with host buffers

[stmatengss]

Purpose

Mooncake TransferEngine TCP cannot GPUDirect. Registering CUDA receive/staging pools and then calling batch_transfer_sync_write on GPU pointers hangs on machines without RDMA (the EPD 1E+1PD path never becomes ready; the scheduler keeps retrying reserve).

This change makes TCP the default for ECMooncakeConnector and actually uses host transport:

• Default mooncake_protocol is tcp (set rdma when GPUDirect RDMA is available).
• TCP registers pinned host producer staging + consumer receive pools.
• Encoder-cache tensors are copied onto ec_buffer_device after a TCP receive.
• D2H staging copies are synchronized before Mooncake starts the transfer.
• CUDA events are skipped for host pools.

This is a follow-up to the Mooncake EC connector work (vllm-project/vllm#41567); it does not duplicate that PR. Open Mooncake EC PRs (vllm-project#41567, vllm-project#47302) do not default the TransferEngine path to TCP with host buffers.

Test Plan

PYTHONPATH=. python -m pytest tests/v1/ec_connector/unit/test_ec_mooncake_connector.py -v

Result: 45 passed (including new TestECMooncakeTCPTransport cases for default TCP, RDMA still selecting CUDA transport, TCP host pools when ec_buffer_device=cuda, and CPU materialize when the buffer device is CPU).

Integration script run_epd_mooncake_ec_full_pipeline.sh now defaults MOONCAKE_EC_PROTOCOL=tcp.

(Optional) Documentation

• ECTransferConfig.ec_connector docstring notes TCP-by-default.
• Connector extra-config docs describe TCP host pools vs RDMA CUDA pools.

AI assistance was used to implement and test this change.

  

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