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sched : copy only the used experts when offloading prompt processing (#15346)

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This commit is contained in:
Diego Devesa
2025-08-20 16:35:28 -07:00
committed by GitHub
parent 1bc664a26a
commit 5682a3745f
1 changed files with 87 additions and 9 deletions
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82
ggml/src/ggml-backend.cpp
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@@ -19,9 +19,8 @@
#include <stdio.h> #include <stdio.h>
#include <stdlib.h> #include <stdlib.h>
#include <string.h> #include <string.h>
#include <string>
#include <vector>
#include <algorithm> #include <algorithm>
#include <vector>
#ifdef __APPLE__ #ifdef __APPLE__
#include <sys/types.h> #include <sys/types.h>
@@ -1352,6 +1351,10 @@ static bool ggml_backend_sched_alloc_splits(ggml_backend_sched_t sched) {
static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t sched) { static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t sched) {
struct ggml_backend_sched_split * splits = sched->splits; struct ggml_backend_sched_split * splits = sched->splits;
ggml_tensor * prev_ids_tensor = nullptr;
std::vector<int32_t> ids;
std::vector<ggml_bitset_t> used_ids;
for (int i = 0; i < sched->n_splits; i++) { for (int i = 0; i < sched->n_splits; i++) {
struct ggml_backend_sched_split * split = &splits[i]; struct ggml_backend_sched_split * split = &splits[i];
int split_backend_id = split->backend_id; int split_backend_id = split->backend_id;
@@ -1378,6 +1381,80 @@ static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t s
} else { } else {
ggml_backend_synchronize(split_backend); ggml_backend_synchronize(split_backend);
} }
// when offloading MoE weights, we can reduce the amount of data copied by copying only the experts that are used
ggml_tensor * node = split->graph.nodes[0];
if (split->graph.n_nodes > 0 &&
ggml_backend_buffer_get_usage(input->buffer) == GGML_BACKEND_BUFFER_USAGE_WEIGHTS &&
ggml_backend_buffer_is_host(input->buffer) && (
(node->src[0] == input_cpy && node->op == GGML_OP_MUL_MAT_ID)
//|| (node->src[1] == input_cpy && node->op == GGML_OP_ADD_ID) /* GGML_OP_ADD_ID weights are small and not worth splitting */
)) {
const int64_t n_expert = node->op == GGML_OP_MUL_MAT_ID ? input->ne[2] : input->ne[1];
const size_t expert_size = node->op == GGML_OP_MUL_MAT_ID ? input->nb[2] : input->nb[1];
ggml_backend_synchronize(input_backend);
// get the ids
ggml_tensor * ids_tensor = node->src[2];
if (ids_tensor != prev_ids_tensor) {
ids.resize(ggml_nbytes(ids_tensor) / sizeof(int32_t));
ggml_backend_tensor_get_async(split_backend, ids_tensor, ids.data(), 0, ggml_nbytes(ids_tensor));
ggml_backend_synchronize(split_backend);
// find the used experts
used_ids.clear();
used_ids.resize(ggml_bitset_size(n_expert));
for (int64_t i1 = 0; i1 < ids_tensor->ne[1]; i1++) {
for (int64_t i0 = 0; i0 < ids_tensor->ne[0]; i0++) {
int32_t id = ids[i1 * ids_tensor->nb[1]/sizeof(int32_t) + i0 * ids_tensor->nb[0]/sizeof(int32_t)];
ggml_bitset_set(used_ids.data(), id);
}
}
prev_ids_tensor = ids_tensor;
}
// group consecutive experts and copy them together
auto copy_experts = [&](int32_t first_id, int32_t last_id) {
const size_t expert_offset = first_id * expert_size;
const size_t expert_size_copy = (last_id - first_id + 1) * expert_size;
const size_t padding = std::min<size_t>(expert_size, 512);
const size_t padding_end = last_id < n_expert - 1 ? padding : 0;
ggml_backend_tensor_set_async(split_backend,
input_cpy,
(const uint8_t *)input->data + expert_offset, expert_offset,
// copy a bit extra at the to ensure there are no NaNs in the padding of the last expert
// this is necessary for MMQ in the CUDA backend
expert_size_copy + padding_end);
};
int id = 0;
while (!ggml_bitset_get(used_ids.data(), id)) {
id++;
}
int32_t first_id = id;
int32_t last_id = first_id;
for (++id; id < n_expert; ++id) {
if (!ggml_bitset_get(used_ids.data(), id)) {
continue;
}
if (id == last_id + 1) {
last_id = id;
continue;
}
copy_experts(first_id, last_id);
first_id = id;
last_id = id;
}
copy_experts(first_id, last_id);
} else {
// try async copy, but if not possible, we can still use a sync copy without synchronizing the dst backend, since we handle the synchronization here with multiple copies and events // try async copy, but if not possible, we can still use a sync copy without synchronizing the dst backend, since we handle the synchronization here with multiple copies and events
// TODO: add public function to facilitate this, since applications do not have direct access to the backend interface // TODO: add public function to facilitate this, since applications do not have direct access to the backend interface
if (!split_backend->iface.cpy_tensor_async || !split_backend->iface.cpy_tensor_async(input_backend, split_backend, input, input_cpy)) { if (!split_backend->iface.cpy_tensor_async || !split_backend->iface.cpy_tensor_async(input_backend, split_backend, input, input_cpy)) {
@@ -1391,6 +1468,7 @@ static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t s
} }
} }
} }
}
if (!sched->callback_eval) { if (!sched->callback_eval) {
enum ggml_status ec = ggml_backend_graph_compute_async(split_backend, &split->graph); enum ggml_status ec = ggml_backend_graph_compute_async(split_backend, &split->graph);