[Benchmark] 1x RTX 5090 + Qwen3.5 9B BF16 — 1280 tok/s peak, then TTFT goes from 0.7s to 18s, ShareGPT, concurrency 16–128

#58
by hexgridcloud - opened

We benchmarked Qwen-3.5 9B BF16 on our custom bench on RTX 5090 [1-GPU] using real world ShareGPT dataset.

TL;DR:

Found a clean ceiling: throughput climbs nicely up to concurrency 64 (~1280 tok/s output) and then just... stops. 128 concurrency level gives basically the same throughput but nearly doubles end-to-end latency and triples time-to-first-token (5.7s → 17.9s p95).

So past 64 concurrency , GPU is not getting more work done — it's just making requests wait longer in the queue.

Model

Model: Qwen/Qwen3.5-9B
HF Path: Qwen/Qwen3.5-9B
Quantization / dtype: BF-16
Context length configured: 4096 max-tokens

Serving

Engine: vllm
Cuda: 13.0.1
Engine flags:{'enable_auto_tool_choice': True, 'exclude_tools_when_tool_choice_none': True, 'tool_call_parser': 'qwen3_coder', 'dtype': 'bfloat16', 'max_model_len': 4098, 'served_model_name': ['Qwen/Qwen3.5-9B'], 'generation_config': 'vllm', 'gpu_memory_utilization': 0.9, 'enable_prefix_caching': True, 'language_model_only': True, 'max_num_batched_tokens': 4096, 'enable_chunked_prefill': True}
Endpoint: /v1/chat/completions

Hardware

GPU: 1x RTX 5090
VRAM: 32GB
CPU: 48 vCPU | 177 GB RAM

Workload

Dataset: ShareGPT sample, [1080 unique prompts] x [4-concurrency settings] => Total 4320 prompts
Conversation shape: Multi-turn response per request
Languages: Multilingual with en/zh/ru/th/ko/fr/pl/ja
max_model_len: 4098
max_tokens per completion: 256
temperature: 0.2

Methodology

Load tool: Custom Harness (currently building but will be public soon)
Concurrency Request levels: 16, 32, 64, 128
Streaming: ON
Metrics

Concurrency Requests Output tok/s E2E p95 TTFT p95
16 1080 444.4 7.48s 0.70s
32 1080 999.9 8.55s 0.99s
64 1080 1279.2 14.59s 5.68s
128 1080 1253.3 27.01s 17.92s
Some charts:

Benchmark started at <<09:41>> in the charts and stopped at <<10.01>>. Benchmark was run first for 16 concurrency, then 32, 64, 128 and the performance flattened out after 64.

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Anybody here was able to achieve a higher output for this and can constructively criticise our deployment?

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