[Bug]: [Elastic EP]: AssertionError in replicate_experts during DP4 → DP2 scale-down #30660
Description
Your current environment
The output of python collect_env.py
Your output of `python collect_env.py` here
Collecting environment information...
==============================
System Info
==============================
OS : Ubuntu 22.04.5 LTS (x86_64)
GCC version : (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Clang version : Could not collect
CMake version : version 4.1.0
Libc version : glibc-2.35
==============================
PyTorch Info
==============================
PyTorch version : 2.9.0+cu128
Is debug build : False
CUDA used to build PyTorch : 12.8
ROCM used to build PyTorch : N/A
==============================
Python Environment
==============================
Python version : 3.12.11 (main, Jun 4 2025, 08:56:18) [GCC 11.4.0] (64-bit runtime)
Python platform : Linux-6.14.0-36-generic-x86_64-with-glibc2.35
==============================
CUDA / GPU Info
==============================
Is CUDA available : True
CUDA runtime version : 12.8.93
CUDA_MODULE_LOADING set to :
GPU models and configuration :
GPU 0: NVIDIA H20
GPU 1: NVIDIA H20
GPU 2: NVIDIA H20
GPU 3: NVIDIA H20
GPU 4: NVIDIA H20
GPU 5: NVIDIA H20
GPU 6: NVIDIA H20
GPU 7: NVIDIA H20
Nvidia driver version : 570.124.06
cuDNN version : Could not collect
HIP runtime version : N/A
MIOpen runtime version : N/A
Is XNNPACK available : True
==============================
CPU Info
==============================
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Address sizes: 52 bits physical, 57 bits virtual
Byte Order: Little Endian
CPU(s): 192
On-line CPU(s) list: 0-191
Vendor ID: GenuineIntel
BIOS Vendor ID: Intel(R) Corporation
Model name: INTEL(R) XEON(R) PLATINUM 8558P
BIOS Model name: INTEL(R) XEON(R) PLATINUM 8558P
CPU family: 6
Model: 207
Thread(s) per core: 2
Core(s) per socket: 48
Socket(s): 2
Stepping: 2
CPU max MHz: 4000.0000
CPU min MHz: 800.0000
BogoMIPS: 5400.00
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cat_l2 cdp_l3 intel_ppin cdp_l2 ssbd mba ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid cqm rdt_a avx512f avx512dq rdseed adx smap avx512ifma clflushopt clwb intel_pt avx512cd sha_ni avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local split_lock_detect user_shstk avx_vnni avx512_bf16 wbnoinvd dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req hfi vnmi avx512vbmi umip pku ospke waitpkg avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg avx512_vpopcntdq la57 rdpid bus_lock_detect cldemote movdiri movdir64b enqcmd fsrm md_clear serialize tsxldtrk pconfig arch_lbr ibt amx_bf16 avx512_fp16 amx_tile amx_int8 flush_l1d arch_capabilities
Virtualization: VT-x
L1d cache: 4.5 MiB (96 instances)
L1i cache: 3 MiB (96 instances)
L2 cache: 192 MiB (96 instances)
L3 cache: 520 MiB (2 instances)
NUMA node(s): 2
NUMA node0 CPU(s): 0-47,96-143
NUMA node1 CPU(s): 48-95,144-191
Vulnerability Gather data sampling: Not affected
Vulnerability Ghostwrite: Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Not affected
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed: Not affected
Vulnerability Spec rstack overflow: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; Enhanced / Automatic IBRS; IBPB conditional; PBRSB-eIBRS SW sequence; BHI BHI_DIS_S
Vulnerability Srbds: Not affected
Vulnerability Tsa: Not affected
Vulnerability Tsx async abort: Not affected
Vulnerability Vmscape: Mitigation; IBPB before exit to userspace
==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.5.3
[pip3] numpy==2.2.6
[pip3] nvidia-cublas-cu12==12.8.4.1
[pip3] nvidia-cuda-cupti-cu12==12.8.90
[pip3] nvidia-cuda-nvrtc-cu12==12.8.93
[pip3] nvidia-cuda-runtime-cu12==12.8.90
[pip3] nvidia-cudnn-cu12==9.10.2.21
[pip3] nvidia-cudnn-frontend==1.16.0
[pip3] nvidia-cufft-cu12==11.3.3.83
[pip3] nvidia-cufile-cu12==1.13.1.3
[pip3] nvidia-curand-cu12==10.3.9.90
[pip3] nvidia-cusolver-cu12==11.7.3.90
[pip3] nvidia-cusparse-cu12==12.5.8.93
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cutlass-dsl==4.3.2
[pip3] nvidia-ml-py==13.580.82
[pip3] nvidia-nccl-cu12==2.27.5
[pip3] nvidia-nvjitlink-cu12==12.8.93
[pip3] nvidia-nvshmem-cu12==3.3.20
[pip3] nvidia-nvtx-cu12==12.8.90
[pip3] pynccl==0.1.2
[pip3] pyzmq==27.0.1
[pip3] torch==2.9.0
[pip3] torchaudio==2.9.0
[pip3] torchvision==0.24.0
[pip3] transformers==4.57.3
[pip3] triton==3.5.0
[conda] Could not collect
==============================
vLLM Info
==============================
ROCM Version : Could not collect
vLLM Version : 0.1.dev11729+g65125d5f1 (git sha: 65125d5f1)
vLLM Build Flags:
CUDA Archs: Not Set; ROCm: Disabled
GPU Topology:
GPU0 GPU1 GPU2 GPU3 GPU4 GPU5 GPU6 GPU7 NIC0 NIC1 NIC2 NIC3 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X NV18 NV18 NV18 NV18 NV18 NV18 NV18 NODE SYS SYS SYS 0-47,96-143 0 N/A
GPU1 NV18 X NV18 NV18 NV18 NV18 NV18 NV18 NODE SYS SYS SYS 0-47,96-143 0 N/A
GPU2 NV18 NV18 X NV18 NV18 NV18 NV18 NV18 PIX SYS SYS SYS 0-47,96-143 0 N/A
GPU3 NV18 NV18 NV18 X NV18 NV18 NV18 NV18 NODE SYS SYS SYS 0-47,96-143 0 N/A
GPU4 NV18 NV18 NV18 NV18 X NV18 NV18 NV18 SYS PIX NODE NODE 48-95,144-191 1 N/A
GPU5 NV18 NV18 NV18 NV18 NV18 X NV18 NV18 SYS NODE NODE NODE 48-95,144-191 1 N/A
GPU6 NV18 NV18 NV18 NV18 NV18 NV18 X NV18 SYS NODE PIX PIX 48-95,144-191 1 N/A
GPU7 NV18 NV18 NV18 NV18 NV18 NV18 NV18 X SYS NODE NODE NODE 48-95,144-191 1 N/A
NIC0 NODE NODE PIX NODE SYS SYS SYS SYS X SYS SYS SYS
NIC1 SYS SYS SYS SYS PIX NODE NODE NODE SYS X NODE NODE
NIC2 SYS SYS SYS SYS NODE NODE PIX NODE SYS NODE X PIX
NIC3 SYS SYS SYS SYS NODE NODE PIX NODE SYS NODE PIX X
Legend:
X = Self
SYS = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
PHB = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
PXB = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
PIX = Connection traversing at most a single PCIe bridge
NV# = Connection traversing a bonded set of # NVLinks
NIC Legend:
NIC0: mlx5_0
NIC1: mlx5_1
NIC2: mlx5_2
NIC3: mlx5_3
==============================
Environment Variables
==============================
NVIDIA_VISIBLE_DEVICES=all
NVIDIA_REQUIRE_CUDA=cuda>=12.8 brand=unknown,driver>=470,driver<471 brand=grid,driver>=470,driver<471 brand=tesla,driver>=470,driver<471 brand=nvidia,driver>=470,driver<471 brand=quadro,driver>=470,driver<471 brand=quadrortx,driver>=470,driver<471 brand=nvidiartx,driver>=470,driver<471 brand=vapps,driver>=470,driver<471 brand=vpc,driver>=470,driver<471 brand=vcs,driver>=470,driver<471 brand=vws,driver>=470,driver<471 brand=cloudgaming,driver>=470,driver<471 brand=unknown,driver>=535,driver<536 brand=grid,driver>=535,driver<536 brand=tesla,driver>=535,driver<536 brand=nvidia,driver>=535,driver<536 brand=quadro,driver>=535,driver<536 brand=quadrortx,driver>=535,driver<536 brand=nvidiartx,driver>=535,driver<536 brand=vapps,driver>=535,driver<536 brand=vpc,driver>=535,driver<536 brand=vcs,driver>=535,driver<536 brand=vws,driver>=535,driver<536 brand=cloudgaming,driver>=535,driver<536 brand=unknown,driver>=550,driver<551 brand=grid,driver>=550,driver<551 brand=tesla,driver>=550,driver<551 brand=nvidia,driver>=550,driver<551 brand=quadro,driver>=550,driver<551 brand=quadrortx,driver>=550,driver<551 brand=nvidiartx,driver>=550,driver<551 brand=vapps,driver>=550,driver<551 brand=vpc,driver>=550,driver<551 brand=vcs,driver>=550,driver<551 brand=vws,driver>=550,driver<551 brand=cloudgaming,driver>=550,driver<551 brand=unknown,driver>=560,driver<561 brand=grid,driver>=560,driver<561 brand=tesla,driver>=560,driver<561 brand=nvidia,driver>=560,driver<561 brand=quadro,driver>=560,driver<561 brand=quadrortx,driver>=560,driver<561 brand=nvidiartx,driver>=560,driver<561 brand=vapps,driver>=560,driver<561 brand=vpc,driver>=560,driver<561 brand=vcs,driver>=560,driver<561 brand=vws,driver>=560,driver<561 brand=cloudgaming,driver>=560,driver<561 brand=unknown,driver>=565,driver<566 brand=grid,driver>=565,driver<566 brand=tesla,driver>=565,driver<566 brand=nvidia,driver>=565,driver<566 brand=quadro,driver>=565,driver<566 brand=quadrortx,driver>=565,driver<566 brand=nvidiartx,driver>=565,driver<566 brand=vapps,driver>=565,driver<566 brand=vpc,driver>=565,driver<566 brand=vcs,driver>=565,driver<566 brand=vws,driver>=565,driver<566 brand=cloudgaming,driver>=565,driver<566
NCCL_VERSION=2.25.1-1
NVIDIA_DRIVER_CAPABILITIES=compute,utility
NVIDIA_PRODUCT_NAME=CUDA
VLLM_USAGE_SOURCE=production-docker-image
CUDA_DEVICE_ORDER=PCI_BUS_ID
CUDA_VERSION=12.8.1
LD_LIBRARY_PATH=/usr/local/cuda/lib64
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
🐛 Describe the bug
During Elastic EP (Elastic Expert Parallelism) scale-down from data parallelism degree 4 to 2, an AssertionError occurs in the replicate_experts function when calculating expert rebalancing. The assertion assert num_redundant >= 0 fails during the expert redistribution process, indicating a negative redundant expert count calculation.
Sample Code:
vllm serve /ssd3/models/Qwen3-30B-A3B \
--trust-remote-code \
--disable-log-requests \
--tensor-parallel-size 1 \
--gpu-memory-utilization 0.9 \
--max-model-len 16384 \
--no-enable-prefix-caching \
--enable-expert-parallel \
--all2all-backend pplx \
--enable-elastic-ep \
--enable-eplb \
--eplb-config.num_redundant_experts 0 \
--eplb-config.min_replica 1 \
--data-parallel-backend ray \
--data-parallel-size 4 \
--data-parallel-size-local 4 \
--data-parallel-start-rank 0 \
--host 0.0.0.0 \
--port 8006python scale_script.py --host 0.0.0.0 --port 8006 --new-dp-size 2Error Message:
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] EngineCore encountered a fatal error.
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] Traceback (most recent call last):
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/v1/engine/core.py", line 1520, in run
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] self.run_busy_loop()
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/usr/local/lib/python3.12/dist-packages/ray/util/tracing/tracing_helper.py", line 461, in _resume_span
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] return method(self, *_args, **_kwargs)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/v1/engine/core.py", line 1246, in run_busy_loop
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] _ = self.eep_scaling_state.progress()
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/elastic_ep/elastic_state.py", line 131, in progress
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] else self._progress_remaining_engine()
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/elastic_ep/elastic_state.py", line 325, in _progress_remaining_engine
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] self._eplb_reshuffle_before_scale_down()
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/elastic_ep/elastic_state.py", line 477, in _eplb_reshuffle_before_scale_down
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] self.model_executor.collective_rpc(
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/v1/executor/uniproc_executor.py", line 76, in collective_rpc
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] result = run_method(self.driver_worker, method, args, kwargs)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/v1/serial_utils.py", line 479, in run_method
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] return func(*args, **kwargs)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/v1/worker/gpu_worker.py", line 684, in elastic_ep_execute
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] return self.elastic_ep_executor.execute(execute_method, *args, **kwargs)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/elastic_ep/elastic_execute.py", line 142, in execute
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] return method(*args, **kwargs)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/elastic_ep/elastic_execute.py", line 411, in perform_eplb_reshuffle
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] eplb_state.rearrange(rank_mapping=rank_mapping)
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/eplb/eplb_state.py", line 695, in rearrange
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ) = rebalance_experts(
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/eplb/rebalance_algo.py", line 242, in rebalance_experts
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] phy2log, phyrank, logcnt = rebalance_experts_hierarchical(
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/eplb/rebalance_algo.py", line 179, in rebalance_experts_hierarchical
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] phy2mlog, phyrank, mlogcnt = replicate_experts(
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] File "/vllm-workspace/vllm-eep-fork/vllm/distributed/eplb/rebalance_algo.py", line 101, in replicate_experts
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] assert num_redundant >= 0
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] ^^^^^^^^^^^^^^^^^^
(APIServer pid=666598) (DPEngineCoreActor pid=666982) ERROR 12-14 17:43:10 [core.py:1525] AssertionErrorBefore submitting a new issue...
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