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metal : use more command buffers on iOS #427

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Merged
bri-prism merged 1 commit into from
Nov 6, 2026
Merged

metal : use more command buffers on iOS #427

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14 changes: 12 additions & 2 deletions ggml/src/ggml-metal/ggml-metal.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,8 @@
#include "ggml-metal-ops.h"
#include "ggml-metal-tuning.h"

#include <TargetConditionals.h>

#include <mutex>
#include <string>

Expand Down Expand Up @@ -586,6 +588,14 @@ static void ggml_backend_metal_graph_optimize(ggml_backend_t backend, ggml_cgrap
ggml_metal_graph_optimize(ctx, cgraph);
}

// iOS can stop a GPU command buffer that runs for more than a few seconds.
// Use more, shorter command buffers there so large prompt batches stay under that limit.
#if TARGET_OS_IPHONE
#define GGML_METAL_DEFAULT_N_CB 8
#else
#define GGML_METAL_DEFAULT_N_CB 1
#endif

static void ggml_backend_metal_set_n_cb(ggml_backend_t backend, int n_cb) {
GGML_ASSERT(ggml_backend_is_metal(backend));

Expand Down Expand Up @@ -637,7 +647,7 @@ ggml_backend_t ggml_backend_metal_init(void) {
/* .context = */ ctx,
};

ggml_backend_metal_set_n_cb(backend, 1);
ggml_backend_metal_set_n_cb(backend, GGML_METAL_DEFAULT_N_CB);

return backend;
}
Expand Down Expand Up @@ -732,7 +742,7 @@ static ggml_backend_t ggml_backend_metal_device_init_backend(ggml_backend_dev_t
/* .context = */ ctx,
};

ggml_backend_metal_set_n_cb(backend, 1);
ggml_backend_metal_set_n_cb(backend, GGML_METAL_DEFAULT_N_CB);

return backend;

Expand Down
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