Llm Serving Capacity Planner

LLM Serving Capacity Planner

Overview

Use this when a serving log has enough memory lines to explain where GPU HBM went. The analyzer reads SGLang/vLLM startup logs, extracts weight load, KV pool, CUDA graph, framework overhead, and token-capacity lines, then estimates concurrent requests for common token lengths.

For DeepSeek-V4.1, read the KV layout and page-allocation contract before using 528 B/288 B in an estimate. These are per-stored-token payload and scale sizes, not allocation after page padding or total model KV bytes. Verify actual pool layout, compression ratios and sharing from the pinned source and logs.

Inputs

Before running analysis, collect or verify these inputs:

Workflow

Step 1: Collect the serving log

The user should provide the startup log from an SGLang or vLLM serving instance. Key log lines that the analyzer needs:

• Load weight begin. avail mem=XX GB
• server_args={'model_path': '...', 'tp_size': 8, ...} (resolved parameters)
• Load weight end. ... avail mem=X GB, mem usage=X GB. (weight footprint)
• DSV4 memory calculation: ... bytes_per_full_token=X, available_bytes=X GB, ... full_token=N
• [label] KV Cache is allocated. dtype: ..., #tokens: N, K size: X GB, V size: Y GB (or KV size)
• Mamba Cache is allocated. ... conv_state size: X GB, ssm_state size: Y GB
• Memory pool end. avail mem=XX GB
• Capture target|draft decode|verify|prefill CUDA graph end. elapsed=X s, mem usage=X GB, avail mem=X GB.
• Post-capture KV sizing: KV cache allocated. ... KV size: X GB, avail mem=X GB (opt-in)
• max_total_num_tokens=XX, ... chunked_prefill_size=-1|N, ... available_gpu_mem=XX GB

Legacy ServerArgs(...), Memory profiling: and Capture cuda graph end. remain accepted for older/private branch logs; they are not current main emitters.

Current vLLM V1 logs instead expose:

• Initial free memory: ...; Requested memory: ... (DEBUG only)
• Model loading took ... GiB
• Available KV cache memory: ... GiB
• Graph capturing finished ... took ... GiB
• <device> KV cache size: ... tokens, Maximum concurrency for ... tokens per request: ...x
• Free memory on device (F/T GiB) ... Current kv cache memory in use is K GiB (INFO)
• CUDA graph pool memory: A GiB (actual), E GiB (estimated), ...
• Initial free memory X GiB, reserved Y GiB memory for KV Cache ... (explicit KV bytes)
• Maximum concurrency for ... tokens per request: ...x

If the log is from a running instance, capture it by redirecting stdout/stderr to a file at launch time.

Step 2: Optionally capture nvidia-smi data

For per-rank memory comparison:

bashCopy code
docker exec <container> nvidia-smi --query-gpu=index,memory.used,memory.free --format=csv,noheader > smi.txt

Step 3: Run the analyzer

bashCopy code
python3 skills/llm-serving-capacity-planner/scripts/capacity_analyzer.py \
  --log-file /path/to/sglang.log \
  --nvidia-smi-file /path/to/smi.txt \
  --gpu h200 \
  --config-json /path/to/config.json

For JSON output (automation):

bashCopy code
python3 skills/llm-serving-capacity-planner/scripts/capacity_analyzer.py \
  --log-file /path/to/sglang.log \
  --format json

Step 4: Review and interpret results

The analyzer prints:

1. Memory breakdown table: each category (weights, KV pool, CUDA graph, framework, other) with GiB, MiB, percentage, and derivation
2. Per-rank comparison: nvidia-smi data across all TP ranks
3. KV pool detail: pool configuration, KV dtype, replication factor, per-token byte calculation
4. Concurrency estimate: max concurrent requests for different token lengths
5. Tuning notes: configuration changes that may increase capacity

For vLLM, the report also includes a vLLM Startup Evidence section. Values that are not present in the log stay unknown; do not translate them into SGLang-only fields.

When To Use It

• After launching an LLM serving instance, to understand how GPU memory is distributed
• When comparing different --mem-fraction-static values and their impact on KV pool capacity
• When planning deployment capacity: how many concurrent requests can a given GPU configuration support
• When investigating OOM issues: identifying which memory category is consuming the most
• When evaluating whether fp8 KV cache or EP can improve concurrency

Key Concepts

mem-fraction-static

The inspected SGLang source reserves runtime headroom from pre-model-load free memory: headroom = pre_model_load_memory * (1 - mem_fraction_static). The KV budget starts from currently free memory minus this headroom and other model-specific reservations. It is not post_weight_free * fraction. Post-capture sizing may update the pool again. Use final logged capacity and rank-local evidence; see source contracts.

The default is auto-resolved from the hardware and configuration. Read the resolved value from server_args={...}. A lower value increases runtime headroom at the cost of KV capacity. Do not infer 0.88 when it is absent.

KV Head Replication

For standard GQA/MHA, each rank stores max(1, num_key_value_heads // tp_size) KV heads. When TP exceeds the head count, each head is replicated across a subgroup of ranks, not the whole head set on every rank. For kv_heads=4, tp=8, each rank stores one head; for kv_heads=1, tp=8, all eight ranks store that single head. MLA stores a replicated latent plus RoPE key instead. The theoretical calculation assumes all configured layers reside on the rank; use rank-local layer counts for pipeline parallelism.

SWA (Sliding Window Attention) Compression

Models like DeepSeek-V4 use CSA (Compressed Sliding Attention) and HCA (Hierarchical Context Attention) with sliding windows. This reduces per-token KV cache bytes compared to the theoretical full-attention calculation. The bytes_per_full_token reported in the log already accounts for this compression.

Reporting Checklist

Include:

1. Serving configuration: model, GPU, TP/PP/EP, mem-fraction-static, kv-cache-dtype
2. Memory breakdown table: category / GiB / MiB / percentage / derivation source
3. Per-rank nvidia-smi comparison: used and free memory per TP rank
4. KV pool detail: pool size, bytes_per_full_token, KV dtype, replication factor, theoretical per-token KV calculation (when config.json provided)
5. Concurrency estimate table: request token length / token-limit / request-limit / max concurrent
6. Tuning notes based on free memory and configuration

2026-08-23 Runtime Notes

SGLang v0.5.18 consolidates compiled-kernel caches under SGLANG_CACHE_DIR. The first launch after an upgrade can look like extra "other" memory and a longer startup because Triton / FlashInfer / DeepGEMM / Inductor recompile once. Overlapped checkpoint staging (--startup-weight-load-mode overlap) also changes the startup memory curve: weight pages and CUDA-graph capture can overlap, so do not treat a shorter load-weight interval as a smaller weight footprint.

Known Limitations

References

• references/log-patterns.md: log line patterns and their semantics for memory analysis.
• references/gpu-specs.json: GPU HBM specifications for h20, h100, h200, and b200 aliases.
• scripts/capacity_analyzer.py: the core analysis script.

Memory capacity sources

The bundled hardware table records vendor GB maxima, not usable GiB. Prefer nvidia-smi used+free or vLLM's logged total. B200 retains the DGX 180GB SKU; other Blackwell SKUs may differ. DGX Spark's 128GB is unified system memory, not dedicated HBM. L20 is the official 48GB SKU; do not infer a capacity for l20n from the name. GB200 records the physical 192GB package maximum; its deployed SKU and custom SKUs require measured totals. Official source URLs are stored with each newly added hardware entry.