servediff

Same flags, different server. servediff shows what your vLLM or SGLang server actually ran with — and what changed between two runs.

Benchmark result files only record the client side. After an upgrade, the server can silently pick a different MoE or attention backend, change its CUDA graph capture, fall back to another code path, or run against different torch / triton / flashinfer builds. Today you find that by reading startup logs and bisecting by hand. servediff does the reading for you.

• Who it is for: anyone comparing two vLLM / SGLang runs — after an upgrade, a flag change, or between machines — who needs to know what differs on the server side.
• What it reads: the server's startup log, and optionally a snapshot of the server's Python environment. Offline, no GPU, no dependencies.
• What it does not do: it does not say which difference caused a performance change. That still needs an A/B run. It tells you what to A/B.

Try it in one minute (no GPU)

pip install servediff
git clone https://github.com/apex-mochen/servediff && cd servediff
servediff diff examples/issue-excerpts/vllm-47047-v0.23-default.log examples/issue-excerpts/vllm-47047-v0.23-marlin.log

fact                             baseline                                 candidate                                status
backend.moe                      VLLM_CUTLASS (NvFp4)                     MARLIN (NvFp4)                           changed

1 difference(s); 0 fact(s) identical. Absent means the log did not print it, not that it was off.

That is the difference the reporter of vllm#47047 found by hand after a v0.21 → v0.23 throughput drop. The example files are the log lines posted in that issue (see tests/fixtures/README.md).

Real example: same flags, vLLM 0.28.0 vs 0.30.0

Two startup logs from one RTX 4060 Laptop under WSL2, identical flags (Qwen2.5-0.5B-Instruct, fp16, --max-model-len 2048, --gpu-memory-utilization 0.45):

servediff diff vllm-0.28.0.log vllm-0.30.0.log

warning: baseline server failed to start: RuntimeError: UVA is not available; facts after the failure are absent because startup stopped, not because they were off

fact                             baseline                                 candidate                                status
backend.attention                <absent>                                 FLASH_ATTN                               only_candidate
backend.flash_attn_version       <absent>                                 2                                        only_candidate
backend.sampling_flashinfer      <absent>                                 disabled                                 only_candidate
cuda_graph.captures              <absent>                                 ["PIECEWISE x51", "FULL x2", "FULL x35"] only_candidate
cuda_graph.pool_gib              <absent>                                 0.17                                     only_candidate
engine.version                   0.28.0                                   0.30.0                                   changed
startup                          failed                                   ready                                    changed
startup.error                    ["RuntimeError: UVA is not available", " <absent>                                 only_baseline
uva_buffers                      <absent>                                 fallback: device memory                  only_candidate

9 difference(s); 8 fact(s) identical. Absent means the log did not print it, not that it was off.

0.28.0 does not start on WSL2 with default settings; 0.30.0 does, because it falls back to device memory when pinned memory is unavailable. Upstream fixed this in vllm#56908, first released in v0.30.0. Forcing the V1 model runner on 0.28.0 (VLLM_USE_V2_MODEL_RUNNER=0) makes it start, and the remaining server-side differences are:

fact                             baseline                                 candidate                                status
cuda_graph.captures              ["mixed prefill-decode, PIECEWISE x51",  ["PIECEWISE x51", "FULL x2", "FULL x35"] changed
cuda_graph.pool_gib              0.36                                     0.17                                     changed
engine.version                   0.28.0                                   0.30.0                                   changed
model_runner                     <absent>                                 V2                                       only_candidate
uva_buffers                      <absent>                                 fallback: device memory                  only_candidate

5 difference(s); 11 fact(s) identical. Absent means the log did not print it, not that it was off.

Dependency versions: servediff env

Logs do not print torch / triton / flashinfer versions, and upgrades of those change server behaviour too. Snapshot each server environment with its own interpreter — nothing needs to be installed into it:

servediff env --python /path/to/old-venv/bin/python -o old.json
servediff env --python /path/to/new-venv/bin/python -o new.json
servediff diff old.json new.json

Excerpt from the same two environments (20 inference-relevant rows; 74 other packages hidden unless you pass --all):

pkg.flashinfer-python            0.6.16.post3                             0.6.18.post1                             changed
pkg.nvidia-cuda-nvcc             13.3.73                                  13.0.88                                  changed
pkg.nvidia-cutlass-dsl           4.6.2                                    4.7.1                                    changed
pkg.torch                        2.13.0                                   2.13.0+cu130                             changed
pkg.transformers                 5.16.1                                   5.17.0                                   changed
pkg.vllm                         0.28.0                                   0.30.0                                   changed

Note the older vLLM environment carries newer CUDA toolkit wheels (13.3 vs 13.0). Whether that matters for performance is not known — that is what an A/B run is for. Versions are read from package metadata, so the same torch build can show as 2.13.0 in one environment and 2.13.0+cu130 in another.

The snapshot records Python, GPU name and driver (via nvidia-smi), all installed package versions, and env vars starting with VLLM_, SGLANG_, SGL_, CUDA_, NCCL_, TORCH_, PYTORCH_, FLASHINFER_, TRITON_. Values of variables whose name contains TOKEN, KEY, SECRET, PASSWORD, CREDENTIAL or AUTH are replaced with <redacted>. Snapshots and logs contain local paths: review them before sharing.

What it extracts

Every fact keeps the log line it came from (servediff show <log> or --json). A fact the log did not print is <absent>: servediff never fills in defaults. Options shared by both engines (dtype, context length, memory fraction, prefix cache, CUDA graph, TP size) are mapped to common names, so a vLLM log and an SGLang log can be compared, though the result is noisier than a same-engine diff.

Pasting a report into a GitHub issue

servediff diff old.log new.log --markdown

Renders a GitHub-flavored table with a collapsible Evidence section quoting the exact log line behind each fact — ready to paste into a regression report.

Capturing a run in one step

servediff capture -o run1.log --python /path/to/server/bin/python -- vllm serve <model>

Runs the server command, tees stdout+stderr to run1.log, and snapshots the environment to run1.log.env.json before the server starts. After two runs:

servediff diff run1.log run2.log
servediff diff run1.log.env.json run2.log.env.json

Exit codes: 0 no differences, 1 differences, 2 nothing recognised.

Limits

• Tested on: log excerpts posted in vllm#47047 (v0.23) and vllm#53472 (v0.23, v0.26); full startup logs of vLLM 0.28.0 and 0.30.0 and one SGLang log, all single-GPU with a dense model. TP > 1, MoE models in full logs, and other versions are not yet verified.
• Parsing is regex-based on log text, which changes between versions. A missed line shows up as <absent>, never as a wrong value — please report it.

License

Apache-2.0.