metal: fix memory unwire if model is freed without any GPU operations (#26082)

* metal: fix memory leak if model is freed without any GPU operations

* metal: run dummy work only if residency sets are used

* metal: wrap function in #if defined

* metal: measure system-wide wired memory in test

* metal: always build regression test

Co-authored-by: YiChen Lv <63285796+forforever73@users.noreply.github.com>

---------

Co-authored-by: YiChen Lv <63285796+forforever73@users.noreply.github.com>

tests/CMakeLists.txt

@@ -278,6 +278,9 @@ set_tests_properties(test-state-restore-fragmented PROPERTIES FIXTURES_REQUIRED
 llama_build_and_test(test-save-load-state.cpp LABEL "model" ARGS -m "${MODEL_DEST}")
 set_tests_properties(test-save-load-state PROPERTIES FIXTURES_REQUIRED test-download-model)
 
+if (APPLE)
+    llama_build(test-rset-release.cpp get-model.cpp)
+endif()
 if (NOT GGML_BACKEND_DL)
     # these tests use the backends directly and cannot be built with dynamic loading
     llama_build_and_test(test-barrier.cpp)

tests/test-rset-release.cpp

@@ -0,0 +1,53 @@
+// ref: https://github.com/ggml-org/llama.cpp/issues/25937
+// only works reliably when run with a large model that occupies 3GB+ of wired memory
+// thus, this test is not run by default
+// example model to run with: google/gemma-4-E4B-it-qat-q4_0-gguf
+
+#include <cstdint>
+#include <mach/mach.h>
+#include <mach/mach_host.h>
+#include <unistd.h>
+
+#include "llama.h"
+#include "get-model.h"
+
+static uint64_t wired_memory() {
+    vm_statistics64_data_t vmstat;
+    mach_msg_type_number_t count = HOST_VM_INFO64_COUNT;
+    if (host_statistics64(mach_host_self(), HOST_VM_INFO64, (host_info64_t)&vmstat, &count) != KERN_SUCCESS) {
+        return UINT64_MAX;
+    }
+    return static_cast<uint64_t>(vmstat.wire_count) * vm_kernel_page_size;
+}
+
+int main(int argc, char ** argv) {
+    auto * model_path = get_model_or_exit(argc, argv);
+
+    llama_backend_init();
+
+    const uint64_t wired_initial = wired_memory();
+
+    llama_model_params params = llama_model_default_params();
+    params.load_mode = LLAMA_LOAD_MODE_NONE;
+    struct llama_model* model = llama_model_load_from_file(model_path, params);
+
+    const uint64_t wired_loaded = wired_memory();
+    const uint64_t wired_delta = wired_loaded - wired_initial;
+    // system memory fluctuates, so we need to allocate enough to reliably detect the release
+    GGML_ASSERT(wired_delta > 2'000'000'000); // 2GB
+
+    llama_model_free(model);
+
+    const uint64_t t_start_ms = ggml_time_ms();
+
+    // expect most of the allocated memory to be released within 10 seconds
+    // we allow for some tolerance due to system-wide memory fluctuations
+    while (wired_memory() > wired_loaded - 0.75 * wired_delta) {
+        GGML_ASSERT(ggml_time_ms() - t_start_ms < 10'000);
+        usleep(100'000); // 100ms
+    }
+
+    llama_backend_free();
+
+    return 0;
+}