[Bug]: Qwen3.5-27B fails to start with CPU KV cache offloading (--kv_offloading_backend native) while Qwen3-32B works fine #36463
Description
Your current environment
The output of python collect_env.py
==============================
System Info
==============================
OS : Ubuntu 22.04.5 LTS (x86_64)
GCC version : (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0
Clang version : Could not collect
CMake version : Could not collect
Libc version : glibc-2.35
==============================
PyTorch Info
==============================
PyTorch version : 2.10.0+cu129
Is debug build : False
CUDA used to build PyTorch : 12.9
ROCM used to build PyTorch : N/A
==============================
Python Environment
==============================
Python version : 3.12.13 (main, Mar 4 2026, 09:23:07) [GCC 11.4.0] (64-bit runtime)
Python platform : Linux-5.10.134-16.3.al8.x86_64-x86_64-with-glibc2.35
==============================
CUDA / GPU Info
==============================
Is CUDA available : True
CUDA runtime version : 12.9.86
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 : 580.82.07
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: 0-91,96-187
Off-line CPU(s) list: 92-95,188-191
Vendor ID: GenuineIntel
Model name: Intel(R) Xeon(R) Platinum 8469C
CPU family: 6
Model: 143
Thread(s) per core: 2
Core(s) per socket: 48
Socket(s): 2
Stepping: 8
CPU max MHz: 3800.0000
CPU min MHz: 800.0000
BogoMIPS: 5200.00
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 monitor 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 invpcid_single intel_ppin cdp_l2 ssbd mba ibrs ibpb stibp ibrs_enhanced tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm 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 avx_vnni avx512_bf16 wbnoinvd dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req hfi avx512vbmi umip pku ospke waitpkg avx512_vbmi2 gfni vaes vpclmulqdq avx512_vnni avx512_bitalg tme avx512_vpopcntdq rdpid bus_lock_detect cldemote movdiri movdir64b enqcmd fsrm uintr md_clear serialize tsxldtrk pconfig arch_lbr 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: 195 MiB (2 instances)
NUMA node(s): 2
NUMA node0 CPU(s): 0-47,96-143
NUMA node1 CPU(s): 48-95,144-191
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Not affected
Vulnerability Retbleed: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl and seccomp
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; Enhanced IBRS, IBPB conditional, RSB filling, PBRSB-eIBRS SW sequence
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected
==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.6.4
[pip3] numpy==2.2.6
[pip3] nvidia-cublas-cu12==12.9.1.4
[pip3] nvidia-cuda-cupti-cu12==12.9.79
[pip3] nvidia-cuda-nvrtc-cu12==12.9.86
[pip3] nvidia-cuda-runtime-cu12==12.9.79
[pip3] nvidia-cudnn-cu12==9.10.2.21
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft-cu12==11.4.1.4
[pip3] nvidia-cufile-cu12==1.14.1.1
[pip3] nvidia-curand-cu12==10.3.10.19
[pip3] nvidia-cusolver-cu12==11.7.5.82
[pip3] nvidia-cusparse-cu12==12.5.10.65
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cutlass-dsl==4.4.1
[pip3] nvidia-cutlass-dsl-libs-base==4.4.1
[pip3] nvidia-ml-py==13.590.48
[pip3] nvidia-nccl-cu12==2.27.5
[pip3] nvidia-nvjitlink-cu12==12.9.86
[pip3] nvidia-nvshmem-cu12==3.4.5
[pip3] nvidia-nvtx-cu12==12.9.79
[pip3] pyzmq==27.1.0
[pip3] torch==2.10.0+cu129
[pip3] torch_c_dlpack_ext==0.1.5
[pip3] torchaudio==2.10.0+cu129
[pip3] torchvision==0.25.0+cu129
[pip3] transformers==4.57.6
[pip3] triton==3.6.0
[conda] Could not collect
==============================
vLLM Info
==============================
ROCM Version : Could not collect
vLLM Version : 0.17.0
vLLM Build Flags:
CUDA Archs: 7.0 7.5 8.0 8.9 9.0 10.0 12.0; ROCm: Disabled
GPU Topology:
GPU0 GPU1 GPU2 GPU3 GPU4 GPU5 GPU6 GPU7 NIC0 NIC1 NIC2 NIC3 NIC4 NIC5 NIC6 NIC7 NIC8 NIC9 NIC10 NIC11 NIC12 NIC13 NIC14 NIC15 NIC16 NIC17 NIC18 NIC19 NIC20 NIC21 NIC22 NIC23 NIC24 NIC25 NIC26 NIC27 NIC28 NIC29 NIC30 NIC31 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X NV18 NV18 NV18 NV18 NV18 NV18 NV18 NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 0-47,96-143 0 N/A
GPU1 NV18 X NV18 NV18 NV18 NV18 NV18 NV18 PIX PIX PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 0-47,96-143 0 N/A
GPU2 NV18 NV18 X NV18 NV18 NV18 NV18 NV18 NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 0-47,96-143 0 N/A
GPU3 NV18 NV18 NV18 X NV18 NV18 NV18 NV18 NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 0-47,96-143 0 N/A
GPU4 NV18 NV18 NV18 NV18 X NV18 NV18 NV18 SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE 48-91,144-187 1 N/A
GPU5 NV18 NV18 NV18 NV18 NV18 X NV18 NV18 SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE 48-91,144-187 1 N/A
GPU6 NV18 NV18 NV18 NV18 NV18 NV18 X NV18 SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX PIX PIX 48-91,144-187 1 N/A
GPU7 NV18 NV18 NV18 NV18 NV18 NV18 NV18 X SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE NODE 48-91,144-187 1 N/A
NIC0 NODE PIX NODE NODE SYS SYS SYS SYS X PIX PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC1 NODE PIX NODE NODE SYS SYS SYS SYS PIX X PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC2 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX X PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC3 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX PIX X PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC4 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX PIX PIX X PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC5 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX PIX PIX PIX X PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC6 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX PIX PIX PIX PIX X PIX NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC7 NODE PIX NODE NODE SYS SYS SYS SYS PIX PIX PIX PIX PIX PIX PIX X NODE NODE NODE NODE NODE NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC8 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE X PIX PIX PIX PIX PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC9 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX X PIX PIX PIX PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC10 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX X PIX PIX PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC11 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX X PIX PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC12 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX X PIX PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC13 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX X PIX PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC14 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX X PIX SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC15 NODE NODE NODE PIX SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX PIX X SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS
NIC16 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS X PIX PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC17 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX X PIX PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC18 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX X PIX PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC19 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX X PIX PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC20 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX PIX X PIX PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC21 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX PIX PIX X PIX PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC22 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX PIX PIX PIX X PIX NODE NODE NODE NODE NODE NODE NODE NODE
NIC23 SYS SYS SYS SYS PIX NODE NODE NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS PIX PIX PIX PIX PIX PIX PIX X NODE NODE NODE NODE NODE NODE NODE NODE
NIC24 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE X PIX PIX PIX PIX PIX PIX PIX
NIC25 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX X PIX PIX PIX PIX PIX PIX
NIC26 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX X PIX PIX PIX PIX PIX
NIC27 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX X PIX PIX PIX PIX
NIC28 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX X PIX PIX PIX
NIC29 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX X PIX PIX
NIC30 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX X PIX
NIC31 SYS SYS SYS SYS NODE NODE PIX NODE SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS NODE NODE NODE NODE NODE NODE NODE NODE PIX PIX PIX PIX PIX PIX 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
NIC4: mlx5_4
NIC5: mlx5_5
NIC6: mlx5_6
NIC7: mlx5_7
NIC8: mlx5_8
NIC9: mlx5_9
NIC10: mlx5_10
NIC11: mlx5_11
NIC12: mlx5_12
NIC13: mlx5_13
NIC14: mlx5_14
NIC15: mlx5_15
NIC16: mlx5_16
NIC17: mlx5_17
NIC18: mlx5_18
NIC19: mlx5_19
NIC20: mlx5_20
NIC21: mlx5_21
NIC22: mlx5_22
NIC23: mlx5_23
NIC24: mlx5_24
NIC25: mlx5_25
NIC26: mlx5_26
NIC27: mlx5_27
NIC28: mlx5_28
NIC29: mlx5_29
NIC30: mlx5_30
NIC31: mlx5_31
==============================
Environment Variables
==============================
NVIDIA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
NVIDIA_REQUIRE_CUDA=cuda>=12.9 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 brand=unknown,driver>=570,driver<571 brand=grid,driver>=570,driver<571 brand=tesla,driver>=570,driver<571 brand=nvidia,driver>=570,driver<571 brand=quadro,driver>=570,driver<571 brand=quadrortx,driver>=570,driver<571 brand=nvidiartx,driver>=570,driver<571 brand=vapps,driver>=570,driver<571 brand=vpc,driver>=570,driver<571 brand=vcs,driver>=570,driver<571 brand=vws,driver>=570,driver<571 brand=cloudgaming,driver>=570,driver<571
TORCH_CUDA_ARCH_LIST=7.0 7.5 8.0 8.9 9.0 10.0 12.0
NVIDIA_DRIVER_CAPABILITIES=all
NCCL_DEBUG=INFO
VLLM_USAGE_SOURCE=production-docker-image
NCCL_IB_GID_INDEX=3
CUDA_VERSION=12.9.1
VLLM_ENABLE_CUDA_COMPATIBILITY=0
LD_LIBRARY_PATH=/usr/local/nvidia/lib64:/usr/local/cuda/lib64:/usr/local/cuda/lib64
OMP_NUM_THREADS=8
NVIDIA_VOL_MNT_PATH=/usr/local/nvidia/
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
TORCHINDUCTOR_CACHE_DIR=/tmp/torchinductor_root
🐛 Describe the bug
Bug Description
Qwen3.5-27B fails to start when CPU KV cache offloading is enabled via --kv_offloading_backend native --kv_offloading_size 500, while Qwen3-32B starts successfully with the exact same offloading configuration in the same environment.
The error occurs during KV cache initialization with:
ValueError: Hybrid KV cache manager is disabled but failed to convert the KV cache specs to one unified type.
It appears that Qwen3.5-27B uses a hybrid attention architecture (likely mixing full attention and sliding window attention layers), and when --disable-hybrid-kv-cache-manager is set, the engine attempts to unify the KV cache specs into a single type but fails because the different attention types produce incompatible KV cache specs. This issue does not occur with Qwen3-32B, which uses a uniform attention architecture.
The root cause seems to be in vllm/v1/core/kv_cache_utils.py at unify_hybrid_kv_cache_specs(), which cannot reconcile the different KV cache spec types when the hybrid KV cache manager is disabled and CPU offloading is enabled simultaneously.
Expected Behavior
Qwen3.5-27B should be able to use CPU KV cache offloading (--kv_offloading_backend native) successfully, similar to Qwen3-32B. Either:
- The hybrid KV cache specs should be handled correctly when CPU offloading is enabled, or
- A clear and actionable error message should be provided explaining the incompatibility and suggesting a workaround.
Environment
- Docker image:
vllm/vllm-openai:0.17.0 - GPU: 8× NVIDIA H20 (single node)
- Model:
Qwen/Qwen3.5-27B
How to Reproduce
✅ Qwen3-32B — Works Successfully
/usr/bin/python /usr/local/bin/vllm serve Qwen/Qwen3-32B \
--gpu-memory-utilization 0.8 \
--served-model-name Qwen/Qwen3-32B \
--host 0.0.0.0 \
--tensor-parallel-size 8 \
--port 8000 \
--enable-prefix-caching \
--language-model-only \
--reasoning-parser qwen3 \
--max-model-len 131072 \
--disable-hybrid-kv-cache-manager \
--kv_offloading_backend native \
--kv_offloading_size 500This starts successfully and provides ~30% prefill speedup.
❌ Qwen3.5-27B — Fails to Start
/usr/bin/python /usr/local/bin/vllm serve Qwen/Qwen3.5-27B \
--enable-auto-tool-choice \
--tool-call-parser qwen3_coder \
--gpu-memory-utilization 0.8 \
--served-model-name Qwen/Qwen3.5-27B \
--host 0.0.0.0 \
--tensor-parallel-size 8 \
--port 8000 \
--max-model-len 262144 \
--enable-prefix-caching \
--language-model-only \
--reasoning-parser qwen3 \
--disable-hybrid-kv-cache-manager \
--kv_offloading_backend native \
--kv_offloading_size 500Full Error Traceback
(Worker pid=38165) (Worker_TP3 pid=38165) INFO 03-09 06:04:07 [backends.py:350] Cache the graph of compile range (52, 8192) for later use
(Worker pid=38162) (Worker_TP0 pid=38162) INFO 03-09 06:04:12 [backends.py:366] Compiling a graph for compile range (52, 8192) takes 5.06 s
(Worker pid=38162) (Worker_TP0 pid=38162) INFO 03-09 06:04:13 [gpu_worker.py:424] Available KV cache memory: 68.49 GiB
(EngineCore_DP0 pid=38138) WARNING 03-09 06:04:13 [kv_cache_utils.py:1170] Hybrid KV cache manager is disabled for this hybrid model, This means we do not enable any optimizations for saving KV cache memory (e.g., dropping the KV cache outside the sliding window). The compute of layers like sliding window is still saved.
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] EngineCore failed to start.
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] Traceback (most recent call last):
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/engine/core.py", line 1090, in run_engine_core
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] engine_core = EngineCoreProc(*args, engine_index=dp_rank, **kwargs)
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/tracing/otel.py", line 178, in sync_wrapper
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] return func(*args, **kwargs)
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/engine/core.py", line 834, in __init__
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] super().__init__(
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/engine/core.py", line 120, in __init__
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] num_gpu_blocks, num_cpu_blocks, kv_cache_config = self._initialize_kv_caches(
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/tracing/otel.py", line 178, in sync_wrapper
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] return func(*args, **kwargs)
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/engine/core.py", line 263, in _initialize_kv_caches
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] kv_cache_configs = get_kv_cache_configs(
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/core/kv_cache_utils.py", line 1553, in get_kv_cache_configs
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] global_kv_cache_groups = get_kv_cache_groups(vllm_config, merged_kv_cache_specs)
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/core/kv_cache_utils.py", line 1231, in get_kv_cache_groups
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] unify_hybrid_kv_cache_specs(kv_cache_spec)
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] File "/usr/local/lib/python3.12/dist-packages/vllm/v1/core/kv_cache_utils.py", line 1211, in unify_hybrid_kv_cache_specs
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] raise ValueError(
(EngineCore_DP0 pid=38138) ERROR 03-09 06:04:13 [core.py:1100] ValueError: Hybrid KV cache manager is disabled but failed to convert the KV cache specs to one unified type.
(Worker pid=38162) (Worker_TP0 pid=38162) WARNING 03-09 06:04:13 [multiproc_executor.py:814] WorkerProc was terminated
Before submitting a new issue...
- Make sure you already searched for relevant issues, and asked the chatbot living at the bottom right corner of the documentation page, which can answer lots of frequently asked questions.