Add pin option to protect models from LRU eviction in router mode

- Add COMMON_ARG_PRESET_PIN define
- Add pin preset option for model protection
- Add pinned field to server_model_meta
- Modify load_models to set pinned flag
- Update unload_lru to skip pinned models

common/arg.cpp

@@ -3524,9 +3524,10 @@ void common_params_add_preset_options(std::vector<common_arg> & args) {
         [](common_params &, int) { /* unused */ }
     ).set_env(COMMON_ARG_PRESET_STOP_TIMEOUT).set_preset_only());
 
-    // args.push_back(common_arg(
-    //     {"pin"},
-    //     "in server router mode, do not unload this model if models_max is exceeded",
-    //     [](common_params &) { /* unused */ }
-    // ).set_preset_only());
+    args.push_back(common_arg(
+        {"pin"},
+        nullptr,
+        "in server router mode, do not unload this model if models_max is exceeded",
+        [](common_params &, const std::string &) { /* unused */ }
+    ).set_env(COMMON_ARG_PRESET_PIN).set_preset_only());
 }

common/arg.h

@@ -11,6 +11,7 @@
 // pseudo-env variable to identify preset-only arguments
 #define COMMON_ARG_PRESET_LOAD_ON_STARTUP "__PRESET_LOAD_ON_STARTUP"
 #define COMMON_ARG_PRESET_STOP_TIMEOUT    "__PRESET_STOP_TIMEOUT"
+#define COMMON_ARG_PRESET_PIN             "__PRESET_PIN"
 
 //
 // CLI argument parsing

tools/server/server-models.cpp

@@ -244,6 +244,14 @@ void server_models::load_models() {
         }
     }
 
+    // handle custom pin option
+    for (auto & [name, inst] : mapping) {
+        std::string val;
+        if (inst.meta.preset.get_option(COMMON_ARG_PRESET_PIN, val)) {
+            inst.meta.pinned = true;
+        }
+    }
+
     // load any autoload models
     std::vector<std::string> models_to_load;
     for (const auto & [name, inst] : mapping) {
@@ -383,7 +391,7 @@ void server_models::unload_lru() {
         for (const auto & m : mapping) {
             if (m.second.meta.is_active()) {
                 count_active++;
-                if (m.second.meta.last_used < lru_last_used) {
+                if (!m.second.meta.pinned && m.second.meta.last_used < lru_last_used) {
                     lru_model_name = m.first;
                     lru_last_used = m.second.meta.last_used;
                 }

tools/server/server-models.h

@@ -58,6 +58,7 @@ struct server_model_meta {
     std::vector<std::string> args; // args passed to the model instance, will be populated by render_args()
     int exit_code = 0; // exit code of the model instance process (only valid if status == FAILED)
     int stop_timeout = 0; // seconds to wait before force-killing the model instance during shutdown
+    bool pinned = false; // if true, this model will not be unloaded by LRU
 
     bool is_active() const {
         return status == SERVER_MODEL_STATUS_LOADED || status == SERVER_MODEL_STATUS_LOADING;