# LLM Serving Capacity Planner

> Parse SGLang/vLLM startup logs to explain GPU memory use and request capacity. Use for KV cache budget, mem-fraction-static comparisons, OOM triage, and max-concurrency estimates.

- Skill: `bbuf/llm-serving-capacity-planner` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add bbuf/llm-serving-capacity-planner`
- Raw SKILL.md: https://api.skillmd.com/api/skills/bbuf/llm-serving-capacity-planner/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: bbuf (https://skillmd.com/u/bbuf)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/bbuf/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.

## Confirmation Required

Before running analysis, collect or verify these inputs:

| Item | Why it matters | How to obtain | Default if user skips |
|---|---|---|---|
| Log file path | Primary input; all memory data comes from here | Ask user for the serving startup log | — (required) |
| GPU type | Determines total HBM for decomposition validation | Ask user or infer from log | Auto-detected from log if possible |
| nvidia-smi output | Provides per-rank actual memory for cross-validation | Capture with `nvidia-smi --query-gpu=index,memory.used,memory.free --format=csv,noheader > smi.txt` | — (optional, but recommended) |
| Model config.json | Enables theoretical KV cache byte calculation and replication factor analysis | Ask user for the model's config.json path | — (optional, log data used instead) |
| Request token length | Determines concurrency estimate denominator | Ask user | 4096, 6144, 8192 |

## 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`
- `Memory profiling: available_gpu_memory=XX GB, ...` (newer sglang)
- `SW KV memory calculation: bytes_per_full_token=XX, available_bytes=XX GB, full_token=XX` (SWA models like DeepSeek-V4)
- `Memory pool end. avail mem=XX GB`
- `Capture cuda graph end. ... mem usage=XX GB. avail mem=XX GB.`
- `max_total_num_tokens=XX, ... max_running_requests=XX, ... available_gpu_mem=XX GB`
- `server_args=ServerArgs(...)` (for serving parameters)

Current vLLM V1 logs instead expose:

- `Initial free memory: ...; Requested memory: ...`
- `Model loading took ... GiB`
- `Available KV cache memory: ... GiB`
- `Graph capturing finished ... took ... GiB`
- `GPU KV cache size: ... tokens`
- `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:

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

### Step 3: Run the analyzer

```bash
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):

```bash
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

Controls what fraction of **available GPU memory after weight loading** is reserved for the KV cache pool. Higher values give more KV capacity but less headroom for CUDA graph and other runtime buffers.

- `0.88` (default): aggressive — 88% of post-weight memory goes to KV pool
- `0.60`: conservative — more free memory left for runtime, but significantly less KV capacity

### KV Head Replication

When `num_key_value_heads < tp_size`, KV cache is **replicated** across all TP ranks rather than split. For example, models with `kv_heads=1, tp=8` means each of the 8 cards stores a full copy of the KV cache — 8x the per-card KV memory compared to a split scenario.

### 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

| Limitation | Detail | Workaround |
|---|---|---|
| Framework-specific checkpoints | SGLang and vLLM expose different memory checkpoints, so some fields are not comparable one-to-one | Report unknown fields and compare only shared, explicit evidence |
| SWA compression models | Per-token KV bytes cannot be independently calculated from model config for CSA/HCA attention — the framework's internal SWA window parameters are needed | Use `bytes_per_full_token` from the log directly |
| DeepGEMM JIT memory | The analyzer categorizes DeepGEMM JIT compilation memory as "other" because it is not explicitly reported in the log | Compare with nvidia-smi total for accurate accounting |
| PP (Pipeline Parallelism) | Memory decomposition is per-rank; PP configurations may have uneven memory across stages | Specify `--target-rank` for each PP stage |
| MoE expert buffer | Some frameworks allocate additional buffers for expert routing that are not separately reported | Included in "model weights" or "other" depending on when allocated |

## 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.