{"record":{"id":"9237597f2b8ebd4c","repo":"sgl-project/sglang","slug":"nixl-memory-registration-failed-for-aux-tensors","errorCode":null,"errorMessage":"NIXL memory registration failed for aux tensors","messagePattern":"NIXL memory registration failed for aux tensors","errorType":"exception","errorClass":"Exception","httpStatus":null,"severity":"critical","filePath":"python/sglang/srt/disaggregation/nixl/conn.py","lineNumber":1408,"sourceCode":"                continue\n            kv_descs = self.agent.register_memory(kv_addrs, mem_kind)\n            logger.debug(\n                f\"Register kv tensors, kind={mem_kind}, len(kv_addr)= {len(kv_addrs)}\"\n            )\n            if not kv_descs:\n                raise Exception(\n                    f\"NIXL memory registration failed for {mem_kind} kv tensors\"\n                )\n            self.kv_descs.append(kv_descs)\n        aux_addrs = []\n        for aux_data_ptr, aux_data_len in zip(\n            self.kv_args.aux_data_ptrs, self.kv_args.aux_data_lens\n        ):\n            aux_addrs.append((aux_data_ptr, aux_data_len, 0, \"\"))\n        self.aux_descs = self.agent.register_memory(aux_addrs, \"DRAM\")\n        logger.debug(f\"Register aux tensors, len(aux_addrs)= {len(aux_addrs)}\")\n        if not self.aux_descs:\n            raise Exception(\"NIXL memory registration failed for aux tensors\")\n\n        state_addrs = []\n        for comp_ptrs, comp_lens in zip(\n            self.kv_args.state_data_ptrs or [],\n            self.kv_args.state_data_lens or [],\n        ):\n            for state_data_ptr, state_data_len in zip(comp_ptrs, comp_lens):\n                if state_data_ptr == 0 or state_data_len == 0:\n                    continue\n                state_addrs.append(\n                    (state_data_ptr, state_data_len, self.kv_args.gpu_id, \"\")\n                )\n        if state_addrs:\n            self.state_descs = self.agent.register_memory(state_addrs, \"VRAM\")\n            logger.debug(\n                f\"Register state tensors, len(state_addrs)= {len(state_addrs)}\"\n            )\n            if not self.state_descs:","sourceCodeStart":1390,"sourceCodeEnd":1426,"githubUrl":"https://github.com/sgl-project/sglang/blob/0132848349585cfe6aae51c4941cbae872505f8a/python/sglang/srt/disaggregation/nixl/conn.py#L1390-L1426","documentation":"register_buffer_to_engine registers the aux tensors (aux_data_ptrs/aux_data_lens, metadata buffers used to coordinate transfer completion) as DRAM with the NIXL agent. If aux_addrs is non-empty but agent.register_memory returns a falsy descriptor, this Exception is raised, meaning NIXL failed to register the host aux buffers.","triggerScenarios":"Startup with aux_data_ptrs populated but register_memory(aux_addrs, 'DRAM') returns empty — host memory registration failure due to UCX/NIXL misconfiguration or invalid aux buffer addresses (zero/corrupted pointers).","commonSituations":"Same environmental causes as KV registration failure: broken ucx-py/nixl install, container without pinned-memory permissions or low memlock ulimit, or a model backend returning bad aux buffer pointers.","solutions":["Verify ucx-py/nixl versions and that host-memory (DRAM) registration works on this host","Check that aux_data_ptrs/aux_data_lens are populated and non-zero in KVArgsRegisterInfo","Re-run inside a container with adequate IPC/memory locking limits (ulimit -l) and device exposure","Update SGLang — auxiliary registration behavior has changed across versions"],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":"assert all(p != 0 and l > 0 for p, l in zip(kv_args.aux_data_ptrs, kv_args.aux_data_lens)), 'bad aux buffers'","typeGuard":null,"tryCatchPattern":"try:\n    conn.register_buffer_to_engine()\nexcept Exception as e:\n    if 'aux tensors' in str(e):\n        verify_ucx_dram_registration(); check_ulimit_memlock()","preventionTips":["Validate aux_data_ptrs/lens before init in custom backends","Run a NIXL registration smoke test in CI for the deployment image","Raise ulimit -l / use unlimited memlock in the container runtime"],"tags":["nixl","memory-registration","startup","disaggregation"],"backgroundTag":"memory-registration-failed","analyzedSha":"0132848349585cfe6aae51c4941cbae872505f8a","analyzedAt":"2026-08-28T05:10:05.995Z","schemaVersion":2},"datasetVersion":"2026-08-28T06:17:29.519Z"}