Thread management


For the default CPU execution provider, setting defaults are provided to get fast inference performance. You can customize the performance using the following knobs in the API to control the thread count and other settings:

Python (Defaults):

import onnxruntime as rt

sess_options = rt.SessionOptions()

sess_options.intra_op_num_threads = 0
sess_options.execution_mode = rt.ExecutionMode.ORT_SEQUENTIAL
sess_options.graph_optimization_level = rt.GraphOptimizationLevel.ORT_ENABLE_ALL
sess_options.add_session_config_entry("session.intra_op.allow_spinning", "1")
  • INTRA Thread Count

    • Controls the total number of INTRA threads to use to run the model.
    • INTRA = parallelize computation inside each operator
    • Default: (not specified or 0). sess_options.intra_op_num_threads = 0
      • INTRA Threads Total = Number of physical CPU Cores. Leaving at default also enables some affinitization (explained below)
      • E.g. 6-core machine (with 12 HT logical processors) = 6 total INTRA threads
  • Sequential vs Parallel Execution

    • Controls whether multiple operators in the graph (across nodes) run sequentially or in parallel.
    • Default: sess_options.execution_mode = rt.ExecutionMode.ORT_SEQUENTIAL
    • Usually when a model has many branches, setting this option to ORT_PARALLEL will provide better performance. This could also hurt performance on some models without many branches.
    • 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

    • Default: sess_options.graph_optimization_level = rt.GraphOptimizationLevel.ORT_ENABLE_ALL enables all optimizations.
    • 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.
  • Thread-Pool Spinning Behavior

    • Controls whether additional INTRA or INTER threads spin waiting for work. Provides faster inference but consumes more CPU cycles, resources, and power
    • Default: 1 (Enabled)

Set number of intra-op threads

Onnxruntime sessions utilize multi-threading to parallelize computation inside each operator.

By default with intra_op_num_threads=0 or not set, each session will start with the main thread on the 1st core (not affinitized). Then extra threads per additional physical core are created, and affinitized to that core (1 or 2 logical processors).

Customer could manually configure the total number of threads like:

Python (below) - C/C++ - .NET/C#

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

With the above configuration of 3 total threads, two extra threads will be created in the addtional INTRA pool, so along with the main calling thread, there will be three threads in total to participate in intra-op computation. 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.

Thread spinning behavior

Controls whether additional INTRA or INTER threads spin waiting for work. Provides faster inference but consumes more CPU cycles, resources, and power.

Example disabling spinning so WorkerLoop doesn’t consume extra active cycles spinning waiting or attempting to steal work

Python (below) - C++ - .NET/C# - Keys

sess_opt = SessionOptions()
sess_opt.AddConfigEntry(kOrtSessionOptionsConfigAllowIntraOpSpinning, "0")
sess_opt.AddConfigEntry(kOrtSessionOptionsConfigAllowInterOpSpinning, "0")

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:

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

Python (below) - C/C++ - .NET/C#

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

Set intra-op thread affinity

It is normally best to not set thread affinity and let the OS handle thread assignment for perf and power reasons. However, 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:

Python (below) - C++ - .NET/C# - Keys

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 an extra 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:

Python (below) - C++ - .NET/C#

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.

Usage in custom ops

Since 1.17, custom op developers are entitled to parallelize their cpu code with ort intra-op thread pool.

Please refer to the API, and example for usage.