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sycl: always set the main device after initialization #7909

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sycl: always set the main device after initialization #7909

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sycl: always set the main device after initialization
bashbaug Jun 12, 2024
d38f1ae
check devices for the same SYCL platform rather than the same backend
bashbaug Jun 13, 2024
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18 changes: 7 additions & 11 deletions ggml-sycl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3307,15 +3307,18 @@ class sycl_gpu_mgr {

void detect_sycl_gpu_list_with_max_cu() try {
int device_count = dpct::dev_mgr::instance().device_count();
sycl::backend backend;

for (int id = 0; id < device_count; id++) {
sycl::device device = dpct::dev_mgr::instance().get_device(id);
if (!device.is_gpu())
continue;
dpct::device_info prop;
dpct::get_device_info(prop, device);
if (max_compute_units < prop.get_max_compute_units())
if (max_compute_units < prop.get_max_compute_units()) {
max_compute_units = prop.get_max_compute_units();
backend = device.get_backend();
}
}

for (int id = 0; id < device_count; id++) {
Expand All @@ -3325,7 +3328,7 @@ class sycl_gpu_mgr {
dpct::device_info prop;
dpct::get_device_info(prop, device);
if (max_compute_units == prop.get_max_compute_units() &&
is_ext_oneapi_device(device)) {
backend == device.get_backend()) {
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gpus.push_back(id);
devices.push_back(device);
work_group_size = prop.get_max_work_group_size();
Expand Down Expand Up @@ -3357,15 +3360,6 @@ class sycl_gpu_mgr {
}
GGML_ASSERT(false);
}

bool is_ext_oneapi_device(const sycl::device &dev) {
sycl::backend dev_backend = dev.get_backend();
if (dev_backend == sycl::backend::ext_oneapi_level_zero ||
dev_backend == sycl::backend::ext_oneapi_cuda ||
dev_backend == sycl::backend::ext_oneapi_hip)
return true;
return false;
}
};

static sycl_gpu_mgr *g_sycl_gpu_mgr = NULL;
Expand Down Expand Up @@ -17400,6 +17394,7 @@ GGML_API GGML_CALL void ggml_backend_sycl_set_single_device_mode(int main_gpu_id
g_sycl_gpu_mgr = new sycl_gpu_mgr(main_gpu_id);
g_ggml_sycl_backend_gpu_mode = SYCL_SINGLE_GPU_MODE;
ggml_init_by_gpus(g_sycl_gpu_mgr->get_gpu_count());
ggml_sycl_set_main_device(0);
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In single mode, the main device ID is set by the parameter of cmd line.
So, set it as 0, will disable the parameter: --main-gpu in fact.
So rm it.

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What about ggml_sycl_set_main_device(main_gpu_id)?

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I was confused by this initially also, but I think zero is the only safe and correct initial value. Here's why:

There are two sets of devices we can get and iterate through: The first is the set of devices returned by dpct::dev_mgr::instance().get_device(). This is the set of all devices in the system, and main_gpu_id is an index into this set. The second is the set of devices stored in sycl_gpu_mgr. This is essentially a "filtered" set of devices we've chosen to use, and it can be indexed from zero to sycl_gpu_mgr->get_gpu_count().

In the case where we choose a main GPU on the command line, the filtering will be performed when we create the sycl_gpu_mgr above.

llama.cpp/ggml-sycl.cpp

Line 17392 in 172c825

g_sycl_gpu_mgr = new sycl_gpu_mgr(main_gpu_id);

After the filtering occurs, the only valid index to pass to ggml_sycl_set_main_device() is index zero, because there is only one device in the sycl_gpu_mgr.

g_ggml_backend_sycl_buffer_type_initialized = false;
}

Expand All @@ -17419,6 +17414,7 @@ GGML_API GGML_CALL void ggml_backend_sycl_set_mul_device_mode() {
g_sycl_gpu_mgr = new sycl_gpu_mgr();
g_ggml_sycl_backend_gpu_mode = SYCL_MUL_GPU_MODE;
ggml_init_by_gpus(g_sycl_gpu_mgr->get_gpu_count());
ggml_sycl_set_main_device(0);
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I think this breaks multi-GPU semantics, @NeoZhangJianyu can you try this on a multi-GPU env?

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I convinced myself that this would be OK even in a multi-GPU environment, though admittedly I haven't tested this myself so it'd be great to confirm this works.

My thinking is: we're eventually going to set the main device via some other codepath, such as via ggml_backend_sycl_init (probably through llama_new_context_with_model). We may even set the main device multiple times. This is all fine; we just need some valid initial value, so if we happen to lookup the SYCL queue and hence the SYCL context, say to allocate host USM when loading a model, we have a valid value to perform the lookup.

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In mulitple mode, set main gpu is not needed. #0 gpu is always default main gpu.

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Unfortunately this isn't the case:

llama.cpp/ggml-sycl.cpp

Line 3374 in 172c825

static int g_main_device = -1;

We could change the initial value of g_main_device from -1 to 0, but we'd probably also want to change some other initial values to stay in sync, say for g_main_device_id. It seems safer to me to just call ggml_sycl_set_main_device(0) instead, but let me know what you prefer.

g_ggml_backend_sycl_buffer_type_initialized = false;
}

Expand Down
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