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llama.cppの推論エンジンggmlの動作

2023/10/16に公開

LLM

llama

ggml

tech

目的

LLMの推論用リポジトリとして llama.cpp リポジトリが有名。
推論にはTensor library ggml が用いられる。llama.cpp内蔵の ggml の動作を調べた。

調査方法

llama.cppのggml.hを見るとだいたいわかる。詳細はggml.cを確認した。
確認したcommitはd3f202d。

ggmlの使い方

ggml.hのコメントに使い方が書いてある。

ggml_tensor* 型の入力 tensor を定義し、nodeをつないでいって、最終出力 ggml_tensor *f に対して ggml_build_forward(f) で ggml_cgraph を構築する。

ggml_graph_compute_with_ctx(ctx, cgraph, nthread) で実行。

//
// GGML Tensor Library
//
// This documentation is still a work in progress.
// If you wish some specific topics to be covered, feel free to drop a comment:
//
//   https://github.com/ggerganov/whisper.cpp/issues/40
//
// ## Overview
//
// This library implements:
//
//  - a set of tensor operations
//  - automatic differentiation
//  - basic optimization algorithms
//
// The aim of this library is to provide a minimalistic approach for various machine learning tasks. This includes,
// but is not limited to, the following:
//
//  - linear regression
//  - support vector machines
//  - neural networks
//
// The library allows the user to define a certain function using the available tensor operations. This function
// definition is represented internally via a computation graph. Each tensor operation in the function definition
// corresponds to a node in the graph. Having the computation graph defined, the user can choose to compute the
// function's value and/or its gradient with respect to the input variables. Optionally, the function can be optimized
// using one of the available optimization algorithms.
//
// For example, here we define the function: f(x) = a*x^2 + b
//
//   {
//       struct ggml_init_params params = {
//           .mem_size   = 16*1024*1024,
//           .mem_buffer = NULL,
//       };
//
//       // memory allocation happens here
//       struct ggml_context * ctx = ggml_init(params);
//
//       struct ggml_tensor * x = ggml_new_tensor_1d(ctx, GGML_TYPE_F32, 1);
//
//       ggml_set_param(ctx, x); // x is an input variable
//
//       struct ggml_tensor * a  = ggml_new_tensor_1d(ctx, GGML_TYPE_F32, 1);
//       struct ggml_tensor * b  = ggml_new_tensor_1d(ctx, GGML_TYPE_F32, 1);
//       struct ggml_tensor * x2 = ggml_mul(ctx, x, x);
//       struct ggml_tensor * f  = ggml_add(ctx, ggml_mul(ctx, a, x2), b);
//
//       ...
//   }
//
// Notice that the function definition above does not involve any actual computation. The computation is performed only
// when the user explicitly requests it. For example, to compute the function's value at x = 2.0:
//
//   {
//       ...
//
//       struct ggml_cgraph gf = ggml_build_forward(f);
//
//       // set the input variable and parameter values
//       ggml_set_f32(x, 2.0f);
//       ggml_set_f32(a, 3.0f);
//       ggml_set_f32(b, 4.0f);
//
//       ggml_graph_compute_with_ctx(ctx, &gf, n_threads);
//
//       printf("f = %f\n", ggml_get_f32_1d(f, 0));
//
//       ...
//   }
//
// The actual computation is performed in the ggml_graph_compute() function.
//
// The ggml_new_tensor_...() functions create new tensors. They are allocated in the memory buffer provided to the
// ggml_init() function. You have to be careful not to exceed the memory buffer size. Therefore, you have to know
// in advance how much memory you need for your computation. Alternatively, you can allocate a large enough memory
// and after defining the computation graph, call the ggml_used_mem() function to find out how much memory was
// actually needed.
//
// The ggml_set_param() function marks a tensor as an input variable. This is used by the automatic
// differentiation and optimization algorithms.
//
// The described approach allows to define the function graph once and then compute its forward or backward graphs
// multiple times. All computations will use the same memory buffer allocated in the ggml_init() function. This way
// the user can avoid the memory allocation overhead at runtime.
//
// The library supports multi-dimensional tensors - up to 4 dimensions. The FP16 and FP32 data types are first class
// citizens, but in theory the library can be extended to support FP8 and integer data types.
//
// Each tensor operation produces a new tensor. Initially the library was envisioned to support only the use of unary
// and binary operations. Most of the available operations fall into one of these two categories. With time, it became
// clear that the library needs to support more complex operations. The way to support these operations is not clear
// yet, but a few examples are demonstrated in the following operations:
//
//   - ggml_permute()
//   - ggml_conv_1d_1s()
//   - ggml_conv_1d_2s()
//
// For each tensor operator, the library implements a forward and backward computation function. The forward function
// computes the output tensor value given the input tensor values. The backward function computes the adjoint of the
// input tensors given the adjoint of the output tensor. For a detailed explanation of what this means, take a
// calculus class, or watch the following video:
//
//   What is Automatic Differentiation?
//   https://www.youtube.com/watch?v=wG_nF1awSSY
//
//
// ## Tensor data (struct ggml_tensor)
//
// The tensors are stored in memory via the ggml_tensor struct. The structure provides information about the size of
// the tensor, the data type, and the memory buffer where the tensor data is stored. Additionally, it contains
// pointers to the "source" tensors - i.e. the tensors that were used to compute the current tensor. For example:
//
//   {
//       struct ggml_tensor * c = ggml_add(ctx, a, b);
//
//       assert(c->src[0] == a);
//       assert(c->src[1] == b);
//   }
//
// The multi-dimensional tensors are stored in row-major order. The ggml_tensor struct contains fields for the
// number of elements in each dimension ("ne") as well as the number of bytes ("nb", a.k.a. stride). This allows
// to store tensors that are not contiguous in memory, which is useful for operations such as transposition and
// permutation. All tensor operations have to take the stride into account and not assume that the tensor is
// contiguous in memory.
//
// The data of the tensor is accessed via the "data" pointer. For example:
//
//   {
//       const int nx = 2;
//       const int ny = 3;
//
//       struct ggml_tensor * a = ggml_new_tensor_2d(ctx, GGML_TYPE_F32, nx, ny);
//
//       for (int y = 0; y < ny; y++) {
//           for (int x = 0; x < nx; x++) {
//               *(float *) ((char *) a->data + y*a->nb[1] + x*a->nb[0]) = x + y;
//           }
//       }
//
//       ...
//   }
//
// Alternatively, there are helper functions, such as ggml_get_f32_1d() and ggml_set_f32_1d() that can be used.

計算の動作

ggml_graph_compute_with_ctx() を読んでいくとわかる。

マルチスレッド構築まで

ggml_graph_plan() で、各nodeを順にみていき、各nodeを何スレッドで実行するかや、必要なメモリ等を計算
ggml_graph_compute() で nthred分 ggml_graph_compute_thread() を構築し、計算開始

2.移行は、全スレッドを用いて、1nodeずつ実行していく動作（マルチスレッド）。例えば、addの計算は以下のようになっている。

addの計算例

ggml_graph_compute_thread -> ggml_compute_forward() -> ggml_compute_forward_add() -> ggml_compute_forward_add_f32()

nth == 全部でnthスレッドでこのnodeを計算する
ith == 今のcontextが、ith番目のスレッド

src0のnrowをnthスレッドで分割して計算する形になっている。
ggml_tensorでは1次元目がcol、それ以外全部がrow（ggml_nrows()やggml.h参照）

GGML_TENSOR_BINARY_OP_LOCALSは nb0やnb00などを定義するマクロ。

const int64_t ne00 = (src0)->ne[0]; (void)(ne00); const int64_t ne01 = (src0)->ne[1]; (void)(ne01); const int64_t ne02 = (src0)->ne[2]; (void)(ne02); const int64_t ne03 = (src0)->ne[3]; (void)(ne03); const size_t nb00 = (src0)->nb[0]; (void)(nb00); const size_t nb01 = (src0)->nb[1]; (void)(nb01); const size_t nb02 = (src0)->nb[2]; (void)(nb02); const size_t nb03 = (src0)->nb[3]; (void)(nb03); const int64_t ne10 = (src1)->ne[0]; (void)(ne10); const int64_t ne11 = (src1)->ne[1]; (void)(ne11); const int64_t ne12 = (src1)->ne[2]; (void)(ne12); const int64_t ne13 = (src1)->ne[3]; (void)(ne13); const size_t nb10 = (src1)->nb[0]; (void)(nb10); const size_t nb11 = (src1)->nb[1]; (void)(nb11); const size_t nb12 = (src1)->nb[2]; (void)(nb12); const size_t nb13 = (src1)->nb[3]; (void)(nb13); const int64_t ne0 = (dst)->ne[0]; (void)(ne0); const int64_t ne1 = (dst)->ne[1]; (void)(ne1); const int64_t ne2 = (dst)->ne[2]; (void)(ne2); const int64_t ne3 = (dst)->ne[3]; (void)(ne3); const size_t nb0 = (dst)->nb[0]; (void)(nb0); const size_t nb1 = (dst)->nb[1]; (void)(nb1); const size_t nb2 = (dst)->nb[2]; (void)(nb2); const size_t nb3 = (dst)->nb[3]; (void)(nb3);

static void ggml_compute_forward_add_f32(
        const struct ggml_compute_params * params,
        const struct ggml_tensor * src0,
        const struct ggml_tensor * src1,
        struct ggml_tensor * dst) {
    GGML_ASSERT(ggml_can_repeat_rows(src1, src0) && ggml_are_same_shape(src0, dst));

    if (params->type == GGML_TASK_INIT || params->type == GGML_TASK_FINALIZE) {
        return;
    }

    const int ith = params->ith;
    const int nth = params->nth;

    const int nr  = ggml_nrows(src0);

    GGML_TENSOR_BINARY_OP_LOCALS

    GGML_ASSERT( nb0 == sizeof(float));
    GGML_ASSERT(nb00 == sizeof(float));

    // rows per thread
    const int dr = (nr + nth - 1)/nth;

    // row range for this thread
    const int ir0 = dr*ith;
    const int ir1 = MIN(ir0 + dr, nr);

    if (nb10 == sizeof(float)) {
        for (int ir = ir0; ir < ir1; ++ir) {
            // src1 is broadcastable across src0 and dst in i1, i2, i3
            const int64_t i03 = ir/(ne02*ne01);
            const int64_t i02 = (ir - i03*ne02*ne01)/ne01;
            const int64_t i01 = (ir - i03*ne02*ne01 - i02*ne01);

            const int64_t i13 = i03 % ne13;
            const int64_t i12 = i02 % ne12;
            const int64_t i11 = i01 % ne11;

            float * dst_ptr  = (float *) ((char *) dst->data  + i03*nb3  + i02*nb2  + i01*nb1 );
            float * src0_ptr = (float *) ((char *) src0->data + i03*nb03 + i02*nb02 + i01*nb01);
            float * src1_ptr = (float *) ((char *) src1->data + i13*nb13 + i12*nb12 + i11*nb11);

#ifdef GGML_USE_ACCELERATE
            vDSP_vadd(src0_ptr, 1, src1_ptr, 1, dst_ptr, 1, ne00);
#else
            ggml_vec_add_f32(ne00, dst_ptr, src0_ptr, src1_ptr);
#endif
        }
    } else {
        // src1 is not contiguous
（略）

ggml_tensor の注意点

ggml_new_tensor_4d(ctx0, GGML_TYPE_F32, da, db, dc, dd); などで ggml_tensor を定義した時、予想に反して１時限目の側 da がデータ連続している方となる。

ggml_new_tensor_impl()の次元ごとのストライド設定で、１次元目のストライドが最も小さいことからそれがわかる。

    result->nb[0] = ggml_type_size(type);
    result->nb[1] = result->nb[0]*(result->ne[0]/ggml_blck_size(type));
    for (int i = 2; i < GGML_MAX_DIMS; i++) {
        result->nb[i] = result->nb[i - 1]*result->ne[i - 1];
    }

そのためか、たとえば ggml の CNN のサンプルでは、通常とは逆の W,H,C,N の順番で入力tensorの次元を定義している。

所感

ggml.cの行数は22k行と短いが、グラフ構築、各種op、マルチスレッド実行が実装されていて参考になる。

マルチスレッド計算部分は、CUDA kernel の計算に近いものを感じた。

目的