sgl-project/sglang · error · RuntimeError

Mooncake's batch register requires a newer version of moonca

Error message

Mooncake's batch register requires a newer version of mooncake-transfer-engine. Please upgrade Mooncake.

What it means

MooncakeTransferEngine.batch_register_memory registers many buffers at once by calling engine.batch_register_memory. Older mooncake-transfer-engine builds lack this method, so the call raises AttributeError, which is caught, and the code re-raises a RuntimeError telling you to upgrade when the attribute is missing. (If the method exists but fails, ret_value is -1 and it only logs a debug message instead.)

Source

Thrown at python/sglang/srt/distributed/device_communicators/mooncake_transfer_engine.py:173

    def deregister(self, ptr):
        try:
            ret_value = self.engine.unregister_memory(ptr)
        except Exception:
            # Mark deregister as failed
            ret_value = -1

        if ret_value != 0:
            logger.debug("Mooncake memory deregistration %s failed.", ptr)

    def batch_register(self, ptrs: List[int], lengths: List[int]) -> int:
        """Batch register multiple memory regions."""
        try:
            ret_value = self.engine.batch_register_memory(ptrs, lengths)
        except Exception:
            # Mark batch register as failed
            ret_value = -1
            if not hasattr(self.engine, "batch_register_memory"):
                raise RuntimeError(
                    "Mooncake's batch register requires a newer version of "
                    "mooncake-transfer-engine. Please upgrade Mooncake."
                )

        if ret_value != 0:
            logger.debug("Mooncake batch memory registration failed.")
        return ret_value

    def batch_deregister(self, ptrs: List[int]) -> int:
        """Batch deregister multiple memory regions."""
        try:
            ret_value = self.engine.batch_unregister_memory(ptrs)
        except Exception:
            # Mark batch deregister as failed
            ret_value = -1

        if ret_value != 0:
            logger.debug("Mooncake batch memory deregistration failed.")

View on GitHub (pinned to 0132848349)

Solutions

1. pip install mooncake-transfer-engine --upgrade to a version that includes batch_register_memory
2. Check the installed version: pip show mooncake-transfer-engine, and compare with SGLang's required minimum
3. If upgrading is impossible, pin/use an SGLang version compatible with your mooncake build

Example fix

# before
pip show mooncake-transfer-engine  # e.g. 0.2.x -> RuntimeError on batch_register

# after
pip install mooncake-transfer-engine --upgrade
python -c "from mooncake.engine import TransferEngine; print(hasattr(TransferEngine, 'batch_register_memory'))"  # True

from mooncake.engine import TransferEngine
if not hasattr(TransferEngine, 'batch_register_memory'):
    raise RuntimeError('mooncake too old for batch_register; upgrade mooncake-transfer-engine')

def supports_batch_register(engine) -> bool:
    return hasattr(engine.engine, 'batch_register_memory')

try:
    eng.batch_register(...)
except RuntimeError as e:
    if 'upgrade Mooncake' in str(e):
        # fall back to per-buffer register() calls or abort with upgrade hint
        ...
    raise

When it happens

Trigger: Calling MooncakeTransferEngine.batch_register (public API) with an installed mooncake-transfer-engine version that does not expose TransferEngine.batch_register_memory — i.e. an older release than the one SGLang requires for batch registration.

Common situations: Running a new SGLang version (which uses batch registration for KV cache/HiCache) against a stale mooncake-transfer-engine pinned in the image or requirements; downgrading mooncake to work around another bug and hitting this; mixed-version clusters where only some nodes were upgraded.

Related errors

• Mooncake's batch transfer requires mooncake-transfer-engine
• Failed to import 'set_transfer_engine' from 'mooncake.pg'. P
• Serve backend {name!r} uses API version {backend.api_version
• Kimi-K3 DCP with decode_attention_backend='cutedsl_mla' requ
• SGLANG_DISAGG_STAGING_BUFFER with pp_size > 1 is only suppor

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/f4e6d9f465f71469. Report an issue: GitHub.