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llama-fit-params: lower ctx size for multi GPU #18101

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Merged
JohannesGaessler merged 1 commit into from
Dec 16, 2025
Merged

llama-fit-params: lower ctx size for multi GPU #18101

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42 changes: 29 additions & 13 deletions src/llama.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -180,11 +180,12 @@ static void llama_params_fit_impl(
}
}

int64_t sum_total = 0;
int64_t sum_projected_free = 0;
int64_t min_projected_free = INT64_MAX;
int64_t sum_projected_used = 0;
int64_t sum_projected_ctx = 0;
int64_t sum_total = 0;
int64_t sum_projected_free = 0;
int64_t min_projected_free = INT64_MAX;
int64_t sum_projected_used = 0;
int64_t sum_projected_model = 0;
int64_t sum_projected_ctx = 0;

if (nd > 1) {
LLAMA_LOG_INFO("%s: projected memory use with initial parameters [MiB]:\n", __func__);
Expand All @@ -195,11 +196,12 @@ static void llama_params_fit_impl(
const int64_t projected_used = dmd.mb.total();
const int64_t projected_free = dmd.free - projected_used;

sum_total += dmd.total;
sum_projected_used += projected_used;
sum_projected_free += projected_free;
min_projected_free = std::min(min_projected_free, projected_free);
sum_projected_ctx += dmd.mb.context;
sum_total += dmd.total;
sum_projected_used += projected_used;
sum_projected_free += projected_free;
min_projected_free = std::min(min_projected_free, projected_free);
sum_projected_model += dmd.mb.model;
sum_projected_ctx += dmd.mb.context;

if (nd > 1) {
LLAMA_LOG_INFO("%s: - %s: %6" PRId64 " total, %6" PRId64 " used, %6" PRId64 " %s\n",
Expand Down Expand Up @@ -234,10 +236,24 @@ static void llama_params_fit_impl(
if (cparams->n_ctx == 0) {
if (hp_nct > n_ctx_min) {
const int64_t bytes_per_ctx = sum_projected_ctx / hp_nct;
const uint32_t ctx_reduction = std::min(
uint32_t((-global_surplus + bytes_per_ctx - 1) / bytes_per_ctx), hp_nct - n_ctx_min);

int64_t memory_reduction = -global_surplus;
if (nd > 1) {
// for multiple devices we need to be more conservative in terms of how much context we think can fit:
// - for dense models only whole layers can be assigned to devices
// - for MoE models only whole tensors can be assigned to devices, which we estimate to be <= 1/3 of a layer
// - on average we expect a waste of 0.5 layers/tensors per device
// - use slightly more than the expected average for nd devices to be safe
const int64_t model_per_layer = sum_projected_model / std::min(uint32_t(mparams->n_gpu_layers), hp_ngl);
memory_reduction += (nd + 1) * model_per_layer / (hp_nex == 0 ? 2 : 6);
}

uint32_t ctx_reduction = std::min(uint32_t((memory_reduction + bytes_per_ctx - 1) / bytes_per_ctx), hp_nct - n_ctx_min);
cparams->n_ctx = hp_nct - ctx_reduction;
const int64_t memory_reduction = ctx_reduction * bytes_per_ctx;
cparams->n_ctx = std::max(cparams->n_ctx - cparams->n_ctx % 256, n_ctx_min); // round down context for CUDA backend

ctx_reduction = hp_nct - cparams->n_ctx;
memory_reduction = ctx_reduction * bytes_per_ctx;
global_surplus += memory_reduction;
LLAMA_LOG_INFO("%s: context size reduced from %" PRIu32 " to %" PRIu32 " -> need %" PRId64 " MiB less memory in total\n",
__func__, hp_nct, cparams->n_ctx, memory_reduction/MiB);
Expand Down
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