server: add ctx-per-slot (--kv-unified-per-slot) (#24124)

* Add ctx-per-slot argument for unifid KV cache

* Swap out ctx fractions for ctx pool slots

* Formatting cleanup

* Remove ctx-pool-slots, make ctx-per-slot an int

* refactor it

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

common/arg.cpp

@@ -1643,6 +1643,14 @@ common_params_context common_params_parser_init(common_params & params, llama_ex
             }
         }
     ).set_env("LLAMA_ARG_CTX_SIZE"));
+    add_opt(common_arg(
+        { "--kv-unified-per-slot" }, "N",
+        "context limit per parallel slot (default: unset, behavior unchanged).\n"
+        "when set without -c/--ctx-size, the shared KV pool is sized to n_parallel*N",
+        [](common_params & params, int value) {
+            params.kv_unified_per_slot = value;
+        }
+    ).set_env("LLAMA_ARG_KV_UNIFIED_PER_SLOT").set_examples({ LLAMA_EXAMPLE_SERVER }));
     add_opt(common_arg(
         {"-n", "--predict", "--n-predict"}, "N",
         string_format(

common/common.h

@@ -627,6 +627,7 @@ struct common_params {
     bool    cache_prompt        = true;  // whether to enable prompt caching
     bool    cache_idle_slots    = true;  // save and clear idle slots upon starting a new task
     int32_t n_ctx_checkpoints   = 32;    // max number of context checkpoints per slot
+    int32_t kv_unified_per_slot = 0;     // max context per parallel slot; 0 = unset
     int32_t checkpoint_min_step = 8192;  // minimum spacing between context checkpoints
     int32_t cache_ram_mib       = 8192;  // -1 = no limit, 0 - disable, 1 = 1 MiB, etc.