feat: system prompt KV caching for SimpleEngine#175
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Thump604 wants to merge 2 commits intowaybarrios:mainfrom
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feat: system prompt KV caching for SimpleEngine#175Thump604 wants to merge 2 commits intowaybarrios:mainfrom
Thump604 wants to merge 2 commits intowaybarrios:mainfrom
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When both --mllm and --enable-mtp are set, SimpleEngine builds a parallel mlx_lm TextModel sharing the VLM backbone weights (zero-copy). Text-only requests route to mlx_lm with MTP speculative decoding; media requests route to the mlx_vlm MLLM path. Key components: - text_model_from_vlm.py: Build mlx_lm TextModel from VLM weights - Per-request routing in stream_chat() via _has_media_content() - _stream_generate_text() for MTP-accelerated text generation - MTP passthrough: --enable-mtp flag through CLI/server/engine/LLM Tested on Qwen3.5-35B-A3B VLM+MTP (8-bit): - Text (MTP): 65.3 tok/s - Vision (MLLM): 63.8 tok/s - Memory: 38.7 GB (zero-copy, same as single model)
Persist backbone KV cache after prefilling system prompt tokens. On subsequent requests with the same system prompt, restore the snapshot and only prefill the suffix (user + history) tokens. For a 10K-token system prompt on the 122B model, this saves ~57s per request by avoiding redundant system prompt prefill. Implementation: - Detect system prefix via ChatML boundary markers - Hash prefix text for cache key validation - On cache miss: prefill system tokens, snapshot backbone KV state - On cache hit: restore snapshot into fresh cache, send suffix only - Token prefix validation ensures correct split at tokenization boundary - Single-entry cache (one system prompt at a time) - Stats exposed via get_stats() → system_kv_cache - Cache cleared on stop(), invalidated on system prompt change
This was referenced Mar 18, 2026
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Superseded — native prefix caching in AsyncEngineCore handles this. System KV cache was a workaround for SimpleEngine's lack of prefix caching. |
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Summary
How it works
<|im_start|>user\n)stream_generatewith pre-populatedprompt_cacheResults (M2 Ultra 128GB, Qwen3.5-122B-A10B-VLM-MTP-5bit)
Scope
_stream_generate_text) — the primary production path for MLLM+MTP modelsstop(), invalidated on system prompt changeget_stats()→system_kv_cache(tokens, hash, memory_mb)Depends on
_stream_generate_textmethod this builds onTest plan