server: coordinate model load with the VRAM arbiter to avoid load-time OOM

Before uploading a model's weights, acquire the shared VRAM token (ring the
doorbell so any resident model releases first). Previously load uploaded weights
to VRAM before the arbiter was active, so loading a large model while another
(e.g. the warm 4B task model) held VRAM could exceed the budget and OOM.

- add vram_arena_open() (idempotent flock/doorbell setup) and
  vram_acquire_for_load(), called from load_model() before
  common_init_from_params().
- a coordinated load now stays warm holding the token and serves its first
  request without a re-warm (drop the init-time go_cold cold-start).

Assisted-by: Claude

tools/server/server-context.cpp

@@ -927,14 +927,17 @@ private:
     std::thread       vram_warden;
     std::atomic<bool> vram_warden_run{false};
 
-    void vram_share_init() {
+    // open the shared arena (flock token + doorbell dir). Idempotent; sets vram_only/vram_flock.
+    void vram_arena_open() {
         if (getenv("LLAMA_SLEEP_VRAM_ONLY") == nullptr) {
             return;
         }
         vram_only = true;
         vram_evict_kv = getenv("LLAMA_SLEEP_EVICT_KV") != nullptr;
-        vram_cold = false; // weights are resident right after load
 #if !defined(_WIN32)
+        if (vram_lock_fd >= 0) {
+            return; // already open
+        }
         const char * arena_env = getenv("LLAMA_VRAM_ARENA");
         const std::string arena = arena_env ? arena_env : "/dev/shm/llama-vram";
         mkdir(arena.c_str(), 0777);
@@ -945,16 +948,49 @@ private:
             vram_pid_str = std::to_string(getpid());
             vram_doorbell_dir = arena + "/doorbell";
             mkdir(vram_doorbell_dir.c_str(), 0777);
+            SRV_INF("VRAM arbiter: cross-process GPU sharing via %s (doorbell %s)\n",
+                    lock_path.c_str(), vram_doorbell_dir.c_str());
+        } else {
+            SRV_WRN("VRAM arbiter: cannot open %s, running VRAM-only sleep without cross-process lock\n", lock_path.c_str());
+        }
+#endif
+    }
+
+    // Acquire the VRAM token BEFORE uploading this model's weights, so any model currently resident
+    // on the GPU releases first and the load uploads into free VRAM instead of racing it (which
+    // could OOM, e.g. the 4B task model holding VRAM while a large model loads). Blocks until free.
+    // Called from load_model() right before common_init_from_params().
+    void vram_acquire_for_load() {
+        vram_arena_open();
+        if (!vram_only) {
+            return;
+        }
+#if !defined(_WIN32)
+        if (vram_flock) {
+            vram_ring_doorbell();         // nudge whoever is resident to release
+            flock(vram_lock_fd, LOCK_EX); // block until the GPU is free
+        }
+#endif
+        vram_cold = false; // we hold the token; weights will be resident after the upload
+    }
+
+    // Start the doorbell warden. Called from init() after the (coordinated) load. The model stays
+    // warm (holding the token acquired in vram_acquire_for_load) and serves its first request
+    // without a re-warm; the warden releases it when another model rings the doorbell.
+    void vram_share_init() {
+        vram_arena_open();
+        if (!vram_only) {
+            return;
+        }
+        vram_cold = false; // resident and holding the token after the coordinated load
+#if !defined(_WIN32)
+        if (vram_flock && vram_inotify_fd < 0) {
             vram_inotify_fd = inotify_init1(IN_NONBLOCK);
             if (vram_inotify_fd >= 0) {
                 inotify_add_watch(vram_inotify_fd, vram_doorbell_dir.c_str(), IN_CLOSE_WRITE);
                 vram_warden_run = true;
                 vram_warden = std::thread([this]{ vram_warden_loop(); });
             }
-            SRV_INF("VRAM arbiter: cross-process GPU sharing via %s (doorbell %s)\n",
-                    lock_path.c_str(), vram_doorbell_dir.c_str());
-        } else {
-            SRV_WRN("VRAM arbiter: cannot open %s, running VRAM-only sleep without cross-process lock\n", lock_path.c_str());
         }
 #endif
     }
@@ -1321,6 +1357,11 @@ private:
             params_base.load_progress_callback_user_data = &load_progress_text;
         }
 
+        // VRAM arbiter: acquire the GPU token before uploading weights, so any resident model (e.g.
+        // the warm 4B task model) releases first and this load uploads into free VRAM instead of
+        // racing it and OOM-ing. No-op unless LLAMA_SLEEP_VRAM_ONLY is set.
+        vram_acquire_for_load();
+
         llama_init = common_init_from_params(params_base);
 
         model_tgt = llama_init->model();
@@ -1588,11 +1629,10 @@ private:
             handle_sleeping_state(sleeping);
         });
 
-        // VRAM arbiter: enable cross-process GPU time-sharing (if LLAMA_SLEEP_VRAM_ONLY is set) and
+        // VRAM arbiter: start the doorbell warden. The load was coordinated (vram_acquire_for_load
-        // start cold - release the just-loaded device weights and drop the shared token, so we only
+        // grabbed the token before uploading), so we stay warm holding the token and serve the first
-        // occupy VRAM while actually serving. The first decode re-acquires the token and restores.
+        // request without a re-warm; the warden releases us when another model rings the doorbell.
         vram_share_init();
-        vram_go_cold();
 
         metrics.init();