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Thread management


For the default CPU execution provider, you can use the following knobs in the Python API to control the thread number:

import onnxruntime as rt

sess_options = rt.SessionOptions()

sess_options.intra_op_num_threads = 2
sess_options.execution_mode = rt.ExecutionMode.ORT_SEQUENTIAL
sess_options.graph_optimization_level = rt.GraphOptimizationLevel.ORT_ENABLE_ALL
  • Thread Count

    • sess_options.intra_op_num_threads = 2 controls the number of threads to use to run the model.
  • Sequential vs Parallel Execution

    • sess_options.execution_mode = rt.ExecutionMode.ORT_SEQUENTIAL controls whether the operators in the graph run sequentially or in parallel. Usually when a model has many branches, setting this option to ORT_PARALLEL will provide better performance.
    • When sess_options.execution_mode = rt.ExecutionMode.ORT_PARALLEL, you can set sess_options.inter_op_num_threads to control the number of threads used to parallelize the execution of the graph (across nodes).
  • Graph Optimization Level

    • sess_options.graph_optimization_level = rt.GraphOptimizationLevel.ORT_ENABLE_ALL enables all optimizations which is the default. Please see onnxruntime_c_api.h (enum GraphOptimizationLevel) for the full list of all optimization levels. For details regarding available optimizations and usage, please refer to the Graph Optimizations documentation.

Set number of intra-op threads

Onnxruntime sessions utilize multi-threading to parallelize computation inside each operator. Customer could configure the number of threads like:

sess_opt = SessionOptions()
sess_opt.intra_op_num_threads = 3
sess = ort.InferenceSession('model.onnx', sess_opt)

With above configuration, two threads will be created in the pool, so along with the main calling thread, there will be three threads in total to participate in intra-op computation. By default, each session will create one thread per phyical core (except the 1st core) and attach the thread to that core. However, if customer explicitly set the number of threads like showcased above, there will be no affinity set to any of the created thread.

In addition, Onnxruntime also allow customers to create a global intra-op thread pool to prevent overheated contentions among session thread pools, please find its usage here.

Set number of inter-op threads

A inter-op thread pool is for parallelism between operators, and will only be created when session execution mode set to parallel:

sess_opt = SessionOptions()
sess_opt.execution_mode  = ExecutionMode.ORT_PARALLEL
sess_opt.inter_op_num_threads = 3
sess = ort.InferenceSession('model.onnx', sess_opt)

By default, inter-op thread pool will also have one thread per physical core.

Set intra-op thread affinity

For certain scenarios, it may be beneficial to customize intra-op thread affinities, for example:

  • There are multiple sessions run in parallel, customer might prefer their intra-op thread pools run on separate cores to avoid contention.
  • Customer want to limit a intra-op thread pool to run on only one of the NUMA nodes to reduce overhead of expensive cache miss among nodes.

For session intra-op thread pool, please read the configuration and consume it like:

sess_opt = SessionOptions()
sess_opt.intra_op_num_threads = 3
sess_opt.add_session_config_entry('session.intra_op_thread_affinities', '1;2')
sess = ort.InferenceSession('model.onnx', sess_opt, ...)

For global thread pool, please read the API and usage.

Numa support and performance tuning

Since release 1.14, Onnxruntime thread pool could utilize all physical cores that are available over NUMA nodes. The intra-op thread pool will create a thread on every physical core (except the 1st core). E.g. assume there is a system of 2 NUMA nodes, each has 24 cores. Hence intra-op thread pool will create 47 threads, and set thread affinity to each core.

For NUMA systems, it is recommended to test a few thread settings to explore for best performance, in that threads allocated among NUMA nodes might has higher cache-miss overhead when cooperating with each other. For example, when number of intra-op threads has to be 8, there are different ways to set affinity:

sess_opt = SessionOptions()
sess_opt.intra_op_num_threads = 8
sess_opt.add_session_config_entry('session.intra_op_thread_affinities', '3,4;5,6;7,8;9,10;11,12;13,14;15,16') # set affinities of all 7 threads to cores in the first NUMA node
# sess_opt.add_session_config_entry('session.intra_op_thread_affinities', '3,4;5,6;7,8;9,10;49,50;51,52;53,54') # set affinities for first 4 threads to the first NUMA node, and others to the second
sess = ort.InferenceSession('resnet50.onnx', sess_opt, ...)

Test showed that setting affinities to a single NUMA node has nearly 20 percent performance improvement aginst the other case.

Custom threading callbacks

Occasionally, users may prefer to use their own fine-tuned threads for multithreading. ORT offers thread creation and joining callbacks in the C++ API:

std::vector<std::thread> threads;
void* custom_thread_creation_options = nullptr;
// initialize custom_thread_creation_options

// On thread pool creation, ORT calls CreateThreadCustomized to create a thread
OrtCustomThreadHandle CreateThreadCustomized(void* custom_thread_creation_options, OrtThreadWorkerFn work_loop, void* param) {
    threads.push_back(std::thread(work_loop, param));
    // configure the thread by custom_thread_creation_options
    return reinterpret_cast<OrtCustomThreadHandle>(threads.back().native_handle());

// On thread pool destruction, ORT calls JoinThreadCustomized for each created thread
void JoinThreadCustomized(OrtCustomThreadHandle handle) {
    for (auto& t : threads) {
    if (reinterpret_cast<OrtCustomThreadHandle>(t.native_handle()) == handle) {
        // recycling resources ... 

int main(...) {
    Ort::Env ort_env;
    Ort::SessionOptions session_options;
    Ort::Session session(*ort_env, MODEL_URI, session_options);

For global thread pool:

int main() {
    const OrtApi* g_ort = OrtGetApiBase()->GetApi(ORT_API_VERSION);
    OrtThreadingOptions* tp_options = nullptr;
    g_ort->SetGlobalCustomCreateThreadFn(tp_options, CreateThreadCustomized);
    g_ort->SetGlobalCustomThreadCreationOptions(tp_options, &custom_thread_creation_options);
    g_ort->SetGlobalCustomJoinThreadFn(tp_options, JoinThreadCustomized);
    // disable per-session thread pool, create a session for inferencing

Note that CreateThreadCustomized and JoinThreadCustomized, once set, will be applied to both ORT intra op and inter op thread pools uniformly.