# llama-server slot and unified-KV configuration > `-np` defaults to -1 (auto). With auto slots the server picked 4 slots and `kv_unified = true` in a b9354 log. Parent: [Mac local LLMs: llama.cpp internals](https://llms-explorer.com/tree/mac-local-llms-llama-cpp-internals/) · 1 facets · 26 facts · page: https://llms-explorer.com/tree/llama-server-slot-and-unified-kv-configuration/ ## Facts - `-np` defaults to -1 (auto). With auto slots the server picked 4 slots and `kv_unified = true` in a b9354 log. — source: `asserted` - Under unified KV every slot reports the full pool size (`n_ctx = 32768` for four slots on `--ctx-size 32768`), because any one request can use the whole pool. — source: `asserted` - `--kv-unified-per-slot N` is a newer flag: it sets a per-slot context limit; with no `-c` it sizes the shared pool as `n_parallel * N`. — source: `asserted` - Slot choice for a new task: LRU when nothing matches, otherwise the slot with the best longest-common-prefix similarity above `--slot-prompt-similarity` (0.10). — source: `asserted` - Context checkpoints (`-ctxcp`) are per slot, so checkpoint memory scales with the slot count. — source: `asserted` - 2026-02 (issue 19523): unified KV with several slots found to slow decode on CUDA; Vulkan had a regression fixed within two days. — source: `asserted` - 2026-08 (PR 27496): `--fit` made aware of `n_streams` so non-unified KV with several slots gets full per-slot context; draft/MTP context now follows the target context. — source: `asserted` - With `-kvu --parallel 4`, inactive slots that still hold a populated prompt slowed token generation of the active slot on CUDA; prompt processing was unaffected; slowdown persisted until restart. — source: `asserted` - Under `--fit -no-kvu -np 4` before PR 27496, each slot got `n_ctx_train / 4`; a draft context sized from `n_ctx_train / n_streams` could overflow when a slot filled, returning HTTP 500. — source: `asserted` - Maintainer position (19523): unified KV is poorly supported on CUDA, so either disable it and pay the memory, or run `-np 1`. Router-mode users still run `--parallel 4 --kv-unified` with a per-slot limit (29322). Neither side has Metal measurements. — source: `asserted` - Whether Metal flash attention skips masked blocks under unified KV, i.e. whether the 19523 slowdown exists on Apple GPUs. — source: `asserted` - Whether `--kv-unified-per-slot` changes fit or checkpoint accounting on a Mac. — source: `asserted` - The server README lists `-np, --parallel N` as number of server slots with default -1 meaning auto. — [source](https://github.com/ggml-org/llama.cpp/blob/master/tools/server/README.md) - The server README documents `--kv-unified-per-slot N` (env `LLAMA_ARG_KV_UNIFIED_PER_SLOT`): context limit per parallel slot, default unset, and when set without `-c` the shared KV pool is sized `n_parallel*N`. — [source](https://github.com/ggml-org/llama.cpp/blob/master/tools/server/README.md) - The README states `-ctxcp` is the maximum number of context checkpoints created per slot and also answers to the alias `--swa-checkpoints`. — [source](https://github.com/ggml-org/llama.cpp/blob/master/tools/server/README.md) - A b9354 server log reads `n_parallel is set to auto, using n_parallel = 4 and kv_unified = true`. — [source](https://github.com/ggml-org/llama.cpp/issues/24055) - In that log, all four slots print `new slot, n_ctx = 32768` while `--ctx-size 32768` was the whole pool, so under unified KV each slot reports the pool size. — [source](https://github.com/ggml-org/llama.cpp/issues/24055) - The same log shows slot choice `selected slot by LRU, t_last = -1` for the first request and `selected slot by LCP similarity, sim_best = 0.928 (> 0.100 thold), f_keep = 0.911` for the follow-up. — [source](https://github.com/ggml-org/llama.cpp/issues/24055) - Issue 19523 (b7974, Windows, `-kvu --parallel 4 --ctx-size 196608`, Qwen3-VL-30B-A3B) reports that populated inactive slots (`is_processing: false`) lower token-generation speed of the active slot, that prompt processing is unaffected, and that the slowdown grows over the first four requests as blank slots fill and stays until restart. — [source](https://github.com/ggml-org/llama.cpp/issues/19523) - In 19523 ggerganov answers that the unified KV cache is not well supported by the CUDA backend and offers two options: disable unified KV and accept the memory cost, or use one slot (`-np 1`). — [source](https://github.com/ggml-org/llama.cpp/issues/19523) - In 19523 the Vulkan case (AMD Strix Halo, Windows) was a regression from mask-loading changes (PR 19281) that stopped skipping masked -INF blocks in `-kvu` mode; PR 19582 restored it and the reporter confirmed the fix on 2026-02-14. — [source](https://github.com/ggml-org/llama.cpp/issues/19523) - A 2026-07-23 comment on 19523 says a four-A100 host serving a Qwen3.6-27B GGUF with llama-server hangs and performs poorly under multiple connections. — [source](https://github.com/ggml-org/llama.cpp/issues/19523) - PR 27496 (merged 2026-08-22 as commit 2fb989b) changes `--fit -no-kvu -np 4` so `n_ctx` becomes `n_ctx_train * 4` (524288 in the tinygemma3 test) and each slot gets 131072 instead of 32768. — [source](https://github.com/ggml-org/llama.cpp/pull/27496) - The same commit makes the server's draft context follow the target context: with non-unified KV the target held `n_ctx_train` per sequence while the draft context fell back to `n_ctx_train / n_streams`, and a slot filled past that made the draft batch fail to decode and the server answer 500. — [source](https://github.com/ggml-org/llama.cpp/tree/master/tools/fit-params) - The same commit makes fit measure a draft or MTP model's memory at the largest context the target can take, because that memory grows with context and a fixed byte margin cannot express it. — [source](https://github.com/ggml-org/llama.cpp/tree/master/tools/fit-params) - Inferred: for a single-user agent on a Mac, `--parallel 1` removes slot-selection, per-slot checkpoint multiplication and unified-KV tradeoffs at once; two of the working 24055 configurations use `--parallel 1`. — source: `asserted`