# GPU / Vulkan Backend

ggmlR is GPU-first: Vulkan is auto-detected at install time and used by default when available. All APIs (sequential, functional, autograd, ONNX) fall back to CPU transparently when no GPU is present.

    library(ggmlR)

## 1. Check availability

    if (ggml_vulkan_available()) {
      cat("Vulkan is available\n")
      ggml_vulkan_status()              # print device list and properties
    } else {
      cat("No Vulkan GPU — running on CPU\n")
    }

    n <- ggml_vulkan_device_count()
    cat("Vulkan device count:", n, "\n")

## 2. Device enumeration

    # Low-level device registry (all backends including CPU)
    ggml_backend_load_all()

    n_dev <- ggml_backend_dev_count()
    for (i in seq_len(n_dev)) {
      dev  <- ggml_backend_dev_get(i - 1L)   # 0-based
      name <- ggml_backend_dev_name(dev)
      desc <- ggml_backend_dev_description(dev)
      mem  <- ggml_backend_dev_memory(dev)
      cat(sprintf("[%d] %s — %s\n", i, name, desc))
      cat(sprintf("    %.1f GB free / %.1f GB total\n",
                  mem["free"] / 1e9, mem["total"] / 1e9))
    }

Full example: `inst/examples/device_discovery.R`

## 3. Autograd device selection

    # Select GPU (falls back to CPU if unavailable)
    device <- tryCatch({
      ag_device("gpu")
      "gpu"
    }, error = function(e) {
      message("GPU not available, using CPU")
      "cpu"
    })

    cat("Active device:", device, "\n")

After `ag_device("gpu")`, all subsequent `ag_param` and `ag_tensor` calls allocate on the GPU. Switch back with `ag_device("cpu")`.

## 4. Mixed precision (f16 / bf16)

    if (device == "gpu") {
      ag_dtype("f16")     # half-precision on Vulkan GPU
      # ag_dtype("bf16") # bfloat16 — falls back to f16 on Vulkan automatically
    } else {
      ag_dtype("f32")     # full precision on CPU
    }

    cat("Active dtype:", ag_default_dtype(), "\n")

`bf16 → f16` fallback is automatic on Vulkan because bf16 is not natively supported in GLSL shaders.

## 5. Vulkan memory info

    if (ggml_vulkan_available()) {
      mem <- ggml_vulkan_device_memory(0L)
      cat(sprintf("GPU memory: %.1f MB free / %.1f MB total\n",
                  mem$free / 1e6, mem$total / 1e6))
    }

## 6. Multi-GPU

ggmlR supports multiple Vulkan devices via the backend scheduler. `dp_train()` distributes data across replicas automatically:

    n_gpu <- ggml_vulkan_device_count()
    cat(sprintf("Using %d GPU(s)\n", n_gpu))

    # dp_train handles multi-GPU internally — see vignette("data-parallel-training")

For low-level multi-GPU scheduler usage see `inst/examples/multi_gpu_example.R`.

## 7. Sequential / functional API on GPU

The high-level `ggml_fit()` API picks up the Vulkan backend automatically — no extra configuration needed:

    data(iris)
    x_train <- scale(as.matrix(iris[, 1:4]))
    y_train <- model.matrix(~ Species - 1, iris)

    model <- ggml_model_sequential() |>
      ggml_layer_dense(64L, activation = "relu", input_shape = 4L) |>
      ggml_layer_dense(3L,  activation = "softmax") |>
      ggml_compile(optimizer = "adam", loss = "categorical_crossentropy")

    # Training runs on GPU if Vulkan is available
    model <- ggml_fit(model, x_train, y_train, epochs = 50L,
                      batch_size = 32L, verbose = 0L)

## 8. ONNX inference on GPU

    # Weights loaded to GPU once at load time
    model_onnx <- onnx_load("model.onnx", device = "vulkan")

    # Repeated inference — no weight re-transfer
    for (i in seq_len(100L)) {
      out <- onnx_run(model_onnx, list(input = batch[[i]]))
    }

## 9. Build configuration

Vulkan support is compiled in automatically when `libvulkan-dev` and `glslc` are detected at install time. SIMD (AVX2/AVX512) for the CPU fallback requires an explicit flag:

    # Default install (Vulkan auto-detected, CPU fallback without SIMD)
    R CMD INSTALL .

    # With CPU SIMD acceleration
    R CMD INSTALL . --configure-args="--with-simd"

    # Also enable the hard-exit path for multi-GPU scripts (see below)
    R CMD INSTALL . --configure-args="--with-simd --enable-hard-exit"

On Windows R ignores `configure.args`, so set the equivalent environment variables before installing from source: `Sys.setenv(GGML_USE_SIMD = "1")`, `Sys.setenv(GGML_VK_HARD_EXIT = "1")`.

`--enable-hard-exit` is off by default and is only relevant to standalone multi-GPU scripts. It controls whether `ggml_vulkan_shutdown(hard = TRUE)` may call `_exit()` to skip the exit-time Vulkan loader teardown, which can otherwise flakily segfault after such a script has printed its results. The released package omits it because CRAN policy forbids a package from terminating the user’s R session; in a default build `hard = TRUE` tears Vulkan down normally and emits a warning rather than silently doing nothing. See `vignette("multi-gpu")` for the full story.

To check what was compiled in:

    cat(ggml_version(), "\n")
    ggml_vulkan_status()   # shows "Vulkan not available" if not compiled in
    ggml_vulkan_hard_exit_available()   # TRUE only with --enable-hard-exit