[WIP] cuda backend prefetch data blob before .so loading

backends/cuda/runtime/cuda_backend.cpp

@@ -1,3 +1,3 @@
 /*
  * Copyright (c) Meta Platforms, Inc. and affiliates.
  * All rights reserved.
@@ -13,6 +13,11 @@
 #include <executorch/runtime/core/evalue.h>
 #include <executorch/runtime/core/exec_aten/util/scalar_type_util.h>
 #include <executorch/runtime/core/exec_aten/util/tensor_util.h>
+#if !defined(_WIN32)
+#include <sys/mman.h>
+#include <unistd.h>
+#include <cerrno>
+#endif
 #include <cctype>
 #include <cstdio>
 
@@ -275,6 +280,45 @@
         method_name.empty() ? "so_blob" : method_name + "_so_blob";
 
     const NamedDataMap* named_data_map = context.get_named_data_map();
+    ET_CHECK_OR_RETURN_ERROR(
+        named_data_map != nullptr,
+        Internal,
+        "CudaBackend requires a NamedDataMap for weight loading");
+
+    // Prefetch the weights blob — trigger async readahead so pages are
+    // resident by the time update_constants_from_blob memcpy's them.
+    // This overlaps disk I/O with the .so write + dlopen.
+    std::string weights_blob_key =
+        method_name.empty() ? "weights_blob" : method_name + "_weights_blob";
+#if !defined(_WIN32)
+    {
+      auto prefetch_buf = named_data_map->get_data(weights_blob_key.c_str());
+      if (prefetch_buf.ok() && prefetch_buf->data() != nullptr) {
+        uintptr_t addr = reinterpret_cast<uintptr_t>(prefetch_buf->data());
+        size_t page_size = getpagesize();
+        uintptr_t aligned_addr = addr & ~(page_size - 1);
+        size_t aligned_size = prefetch_buf->size() + (addr - aligned_addr);
+        int ret = madvise(
+            reinterpret_cast<void*>(aligned_addr),
+            aligned_size,
+            MADV_WILLNEED);
+        if (ret != 0) {
+          ET_LOG(
+              Info,
+              "CudaBackend::init - madvise(MADV_WILLNEED) failed for %s: %s",
+              weights_blob_key.c_str(),
+              strerror(errno));
+        } else {
+          ET_LOG(
+              Info,
+              "CudaBackend::init - Prefetching %s (%.1f MB)",
+              weights_blob_key.c_str(),
+              prefetch_buf->size() / (1024.0 * 1024.0));
+        }
+      }
+    }
+#endif
+
     auto aoti_dso_buffer = named_data_map->get_data(so_blob_key.c_str());
     ET_CHECK_OR_RETURN_ERROR(
         aoti_dso_buffer.ok(),
@@ -338,9 +382,8 @@
 
     handle->container_handle = container_handle;
 
-    // Look into named data map for constant data
-    std::string weights_blob_key =
-        method_name.empty() ? "weights_blob" : method_name + "_weights_blob";
+    // Look into named data map for constant data (key computed above for
+    // prefetch)
     auto buffer_res = named_data_map->get_data(weights_blob_key.c_str());
     if (buffer_res.ok() && handle->update_constants_from_blob != nullptr) {
       ET_LOG(Info, "Found %s in named data map", weights_blob_key.c_str());