CPU Device#
The CPU plugin is a part of the Intel® Distribution of OpenVINO™ toolkit. It is developed to achieve high performance inference of neural networks on Intel® x86-64 and Arm® CPUs. The newer 11th generation and later Intel® CPUs provide even further performance boost, especially with INT8 models. For an in-depth description of CPU plugin, see:
Note
The scope of the CPU plugin features and optimizations on Arm® may differ from Intel® x86-64. If the limitation is not mentioned explicitly, the feature is supported for all CPU architectures. CPU inference on ARM64 is not supported for Windows.
Device Name#
The CPU
device name is used for the CPU plugin. Even though there can be more than one
physical socket on a platform, only one device of this kind is listed by OpenVINO.
On multi-socket platforms, load balancing and memory usage distribution between NUMA nodes are
handled automatically. In order to use CPU for inference, the device name should be passed to
the ov::Core::compile_model()
method:
import openvino as ov
core = ov.Core()
compiled_model = core.compile_model(model, "CPU")
ov::Core core;
auto model = core.read_model("model.xml");
auto compiled_model = core.compile_model(model, "CPU");
Supported Model Precision#
CPU plugin supports the following data types as inference precision of internal primitives:
Floating-point data types:
FP32
(Intel® x86-64, Arm®)BF16
(Intel® x86-64)FP16
(Intel® x86-64, Arm®):ref:`MXFP4 <mxfp4_support>`
(Intel® x86-64)
Integer data types:
INT32
(Intel® x86-64, Arm®)
Quantized data types:
uINT8
(Intel® x86-64)INT8
(Intel® x86-64)uINT1
(Intel® x86-64)
Hello Query Device C++ Sample can be used to print out supported data types for all detected devices.
Quantized Data Types Specifics#
Selected precision of each primitive depends on the operation precision in IR, quantization primitives, and available hardware capabilities.
The u1/u8/i8
data types are used for quantized operations only, i.e., those are not selected automatically for non-quantized operations.
For more details on how to get a quantized model see the low-precision optimization guide.
Note
Arm® platforms execute quantized models in simulation mode: the whole model (including quantization operations) is executed in floating-point precision.
Floating Point Data Types Specifics#
CPU plugin supports the following floating-point data types as inference precision of internal primitives:
f32
(Intel® x86-64, Arm®)bf16
(Intel® x86-64)f16
(Intel® x86-64, Arm®)
The default floating-point precision of a CPU primitive is f32
. To support the f16
OpenVINO IR on platforms that do not natively support float16
, the plugin internally converts
all the f16
values to f32
, and all calculations are performed using the native precision of f32
.
On platforms that natively support half-precision calculations (bfloat16
or float16
), the half-precision type (bf16
or f16
) is automatically used instead
of f32
to achieve better performance (see the Execution Mode Hint).
Thus, no special steps are required to run a model with bf16
or f16
inference precision.
Important
The bf16
floating-point precision appears to have some limitations that impact the
inference accuracy in LLM models. For more details, refer to this article.
Using the half-precision provides the following performance benefits:
bfloat16
andfloat16
data types enable Intel® Advanced Matrix Extension (AMX) on 4+ generation Intel® Xeon® Scalable Processors, resulting in significantly faster computations on the corresponding hardware compared to AVX512 or AVX2 instructions in many deep learning operation implementations.float16
data type enables thearmv8.2-a+fp16
extension on ARM64 CPUs, which significantly improves performance due to the doubled vector capacity.Memory footprint is reduced since most weight and activation tensors are stored in half-precision.
For more details about the bfloat16
format, see
the BFLOAT16 – Hardware Numerics Definition white paper.
To check if the CPU device can support the half-precision data type, use the query device properties interface
to query ov::device::capabilities
property, which should contain FP16
or BF16
in the list of CPU capabilities:
core = ov.Core()
cpu_optimization_capabilities = core.get_property("CPU", device.capabilities)
ov::Core core;
auto cpuOptimizationCapabilities = core.get_property("CPU", ov::device::capabilities);
Inference Precision Hint#
If the model has been converted to half-precision (bf16
or f16
), the ov::hint::inference_precision
is set to ov::element::f16
or ov::element::bf16
and can be checked via
the ov::CompiledModel::get_property
call. The code below demonstrates how to get the element type:
core = ov.Core()
compiled_model = core.compile_model(model, "CPU")
inference_precision = core.get_property("CPU", hints.inference_precision)
ov::Core core;
auto network = core.read_model("sample.xml");
auto exec_network = core.compile_model(network, "CPU");
auto inference_precision = exec_network.get_property(ov::hint::inference_precision);
To infer the model in f32
precision instead of half-precision (bf16
or f16
) on targets with native half-precision support, set the ov::hint::inference_precision
to ov::element::f32
.
core = ov.Core()
core.set_property("CPU", {hints.inference_precision: ov.Type.f32})
ov::Core core;
core.set_property("CPU", ov::hint::inference_precision(ov::element::f32));
The Bfloat16
software simulation mode is available on CPUs with Intel® AVX-512 instruction set that do not support the
native avx512_bf16
instruction. This mode is used for development purposes and it does not guarantee good performance.
To enable the simulation, the ov::hint::inference_precision
has to be explicitly set to ov::element::bf16
.
Note
If ov::hint::inference_precision
is set to ov::element::bf16
on a CPU without native bfloat16 support or bfloat16 simulation mode, an exception is thrown.
Note
Due to the reduced mantissa size of half-precision data types (bfloat16
or float16
), the resulting half-precision inference accuracy may differ from the f32
inference,
especially for models that were not trained using half-precision data types. If half-precision inference accuracy is not acceptable,
it is recommended to switch to the f32
precision. Also, the performance/accuracy balance can be managed using the ov::hint::execution_mode
hint,
see the Execution Mode Hint.
Execution Mode Hint#
In case ov::hint::inference_precision
is not explicitly set, one can use ov::hint::execution_mode
hint to direct the run-time optimizations toward either better accuracy or better performance.
If ov::hint::execution_mode
is set to ov::hint::ExecutionMode::PERFORMANCE
(default behavior) and the platform natively supports half-precision
calculations (bfloat16
or float16
) then bf16
or f16
type is automatically used instead of f32
to achieve better performance.
If the accuracy in this mode is not good enough, then set ov::hint::execution_mode
to ov::hint::ExecutionMode::ACCURACY
to enforce the plugin to
use the f32
precision in floating point calculations.
For more details and code examples, see the Precision Control.
Supported Features#
Automatic Device Selection#
If a system includes OpenVINO-supported devices other than the CPU (e.g. an integrated GPU), then any supported model can be executed on all the devices simultaneously.
This can be achieved by specifying AUTO:CPU,GPU.0
as a target device, and adding the CUMULATIVE_THROUGHPUT
parameter.
core = ov.Core()
compiled_model = core.compile_model(model, "AUTO:CPU,GPU.0", {hints.performance_mode: hints.PerformanceMode.CUMULATIVE_THROUGHPUT})
ov::Core core;
auto model = core.read_model("model.xml");
auto compiled_model = core.compile_model(model, "AUTO:CPU,GPU.0", ov::hint::performance_mode(ov::hint::PerformanceMode::CUMULATIVE_THROUGHPUT));
For more details, see the Automatic Device Selection.
Multi-stream Execution#
If either ov::num_streams(n_streams)
with n_streams > 1
or ov::hint::performance_mode(ov::hint::PerformanceMode::THROUGHPUT)
property is set for CPU plugin, then multiple streams are created for the model. In case of CPU plugin, each stream has its own
host thread, which means that incoming infer requests can be processed simultaneously. Each stream is pinned to its own group of
physical cores with respect to NUMA nodes physical memory usage to minimize overhead on data transfer between NUMA nodes.
For more details, see the optimization guide and thread scheduling introduction.
Note
When it comes to latency, be aware that running only one stream on multi-socket platform may introduce additional overheads
on data transfer between NUMA nodes. In that case it is better to use the ov::hint::PerformanceMode::LATENCY
performance hint.
For more details see the performance hints overview.
Dynamic Shapes#
CPU provides full functional support for models with dynamic shapes in terms of the opset coverage.
Note
The CPU plugin does not support tensors with dynamically changing rank. In case of an attempt to infer a model with such tensors, an exception will be thrown.
Some runtime optimizations work better if the model shapes are known in advance. Therefore, if the input data shape is not changed between inference calls, it is recommended to use a model with static shapes or reshape the existing model with the static input shape to get the best performance.
import openvino as ov
core = ov.Core()
model.reshape([10, 20, 30, 40])
ov::Core core;
auto model = core.read_model("model.xml");
ov::Shape static_shape = {10, 20, 30, 40};
model->reshape(static_shape);
For more details, see the dynamic shapes guide.
Preprocessing Acceleration#
CPU plugin supports a full set of the preprocessing operations, providing high performance implementations for them. For more details, see preprocessing API guide.
The CPU plugin support for handling tensor precision conversion is limited to the following ov::element types:
bf16
f16
f32
f64
i8
i16
i32
i64
u8
u16
u32
u64
boolean
Model Caching#
CPU supports Import/Export network capability. If model caching is enabled via the common OpenVINO™ ov::cache_dir
property,
the plugin automatically creates a cached blob inside the specified directory during model compilation. This cached blob contains
partial representation of the network, having performed common runtime optimizations and low precision transformations.
The next time the model is compiled, the cached representation will be loaded to the plugin instead of the initial OpenVINO IR,
so the aforementioned transformation steps will be skipped. These transformations take a significant amount of time during
model compilation, so caching this representation reduces time spent for subsequent compilations of the model, thereby reducing
first inference latency (FIL).
For more details, see the model caching overview.
Extensibility#
CPU plugin supports fallback on ov::Op
reference implementation if the plugin does not have its own implementation for such operation.
That means that OpenVINO™ Extensibility Mechanism can be used for the plugin extension as well.
Enabling fallback on a custom operation implementation is possible by overriding the ov::Op::evaluate
method in the derived operation
class (see custom OpenVINO™ operations for details).
Stateful Models#
The CPU plugin supports stateful models without any limitations.
For details, see stateful models guide.
Supported Properties#
The plugin supports the following properties:
Read-write Properties#
All parameters must be set before calling ov::Core::compile_model()
in order to take effect or passed as additional argument to ov::Core::compile_model()
ov::enable_profiling
ov::hint::inference_precision
ov::hint::performance_mode
ov::hint::execution_mode
ov::hint::num_request
ov::hint::scheduling_core_type
ov::hint::enable_hyper_threading
ov::hint::enable_cpu_pinning
ov::num_streams
ov::affinity
ov::inference_num_threads
ov::cache_dir
ov::intel_cpu::denormals_optimization
ov::intel_cpu::sparse_weights_decompression_rate
Read-only properties#
ov::supported_properties
ov::available_devices
ov::range_for_async_infer_requests
ov::range_for_streams
ov::device::full_name
ov::device::capabilities
Note
ov::affinity
is replaced by ov::hint::enable_cpu_pinning
. As such, it is deprecated in the 2024.0 release and will be removed in the 2025 release.
External Dependencies#
For some performance-critical DL operations, the CPU plugin uses third-party libraries:
oneDNN (Intel® x86-64, Arm®)
Compute Library (Arm®)
Optimization guide#
Multi-Threading Optimization#
CPU inference will infer an input or multiple inputs in parallel on multiple logical processors.
For more details, see the thread scheduling introduction.
Denormals Optimization#
Denormal numbers (denormals) are non-zero, finite float numbers that are very close to zero, i.e. the numbers in (0, 1.17549e-38) and (0, -1.17549e-38). In such cases, normalized-number encoding format does not have a capability to encode the number and underflow will happen. The computation involving such numbers is extremely slow on much hardware.
As a denormal number is extremely close to zero, treating a denormal directly as zero is a straightforward
and simple method to optimize computation of denormals. This optimization does not comply with IEEE 754 standard.
If it causes unacceptable accuracy degradation, the denormals_optimization
property is introduced to control this behavior.
If there are denormal numbers in use cases, and no or acceptable accuracy drop is seen, set the property to True
to improve performance, otherwise set it to False
. If it is not set explicitly by the property and the application
does not perform any denormals optimization as well, the optimization is disabled by default. After enabling
the denormals_optimization
property, OpenVINO will provide a cross operation system/ compiler and safe optimization
on all platform when applicable.
There are cases when the application in which OpenVINO is used also performs this low-level denormals optimization.
If it is optimized by setting the FTZ(Flush-To-Zero) and DAZ(Denormals-As-Zero) flags in MXCSR register at the beginning
of the thread where OpenVINO is called, OpenVINO will inherit this setting in the same thread and sub-thread,
so there is no need to set the denormals_optimization
property. In such cases, you are responsible for the
effectiveness and safety of the settings.
Note
The denormals_optimization
property must be set before calling compile_model()
.
To enable denormals optimization in the application, the denormals_optimization
property must be set to True
:
import openvino.properties.intel_cpu as intel_cpu
core = ov.Core()
core.set_property("CPU", intel_cpu.denormals_optimization(True))
compiled_model = core.compile_model(model=model, device_name=device_name)
ov::Core core; // Step 1: create ov::Core object
core.set_property(ov::intel_cpu::denormals_optimization(true)); // Step 1b: Enable denormals optimization
auto model = core.read_model(modelPath); // Step 2: Read Model
//... // Step 3: Prepare inputs/outputs
//... // Step 4: Set device configuration
auto compiled = core.compile_model(model, device, config); // Step 5: LoadNetwork
Sparse weights decompression (Intel® x86-64)#
Sparse weights
are weights where most of the elements are zero. The ratio of the number of zero elements
to the number of all elements is called sparse rate
. Thus, we assume that sparse weights
are weights
with a high sparse rate. In case of sparse weights
, we can store only non-zero values in memory using
special storage structures, which allows us to use memory more efficiently. In turn, this can give us better
performance in the high memory bound workloads (e.g., throughput scenario).
Sparse weights decompression feature
allows to pack weights for Matrix Multiplication operations directly
in the CPU plugin at the model compilation stage and store non-zero values in a special packed format. Then,
during the execution of the model, the weights are unpacked and used in the computational kernel. Since the
weights are loaded from DDR/L3 cache in the packed format this significantly decreases memory consumption
and as a consequence improve inference performance.
To use this feature, the user is provided with property sparse_weights_decompression_rate
, which can take
values from the interval [0, 1]. sparse_weights_decompression_rate
defines sparse rate threshold: only operations
with higher sparse rate will be executed using sparse weights decompression feature
. The default value is 1
,
which means the option is disabled.
Note
Sparse weights decompression feature
is disabled by default since overall speed-up highly depends on
particular workload and for some cases the feature may introduce performance degradations.
Code examples of how to use sparse_weights_decompression_rate
:
import openvino.properties.intel_cpu as intel_cpu
core = ov.Core()
core.set_property("CPU", intel_cpu.sparse_weights_decompression_rate(0.8))
compiled_model = core.compile_model(model=model, device_name=device_name)
ov::Core core; // Step 1: create ov::Core object
core.set_property(ov::intel_cpu::sparse_weights_decompression_rate(0.8f)); // Step 1b: Enable sparse weights decompression feature
auto model = core.read_model(modelPath); // Step 2: Read Model
//... // Step 3: Prepare inputs/outputs
//... // Step 4: Set device configuration
auto compiled = core.compile_model(model, device, config); // Step 5: LoadNetwork
Note
The sparse_weights_decompression_rate
property must be set before calling compile_model()
.
Information about the layers in which the sparse weights decompression feature
was applied can be obtained
from perf counters log. The “exec type” field will contain the implementation type with the “sparse” particle
(“brgemm_avx512_amx_sparse_I8” in the example below):
MatMul_1800 EXECUTED layerType: FullyConnected execType: brgemm_avx512_amx_sparse_I8 realTime (ms): 0.050000 cpuTime (ms): 0.050000
Limitations#
Currently, the sparse weights decompression feature
is supported with the following limitations:
Model should be quantized to int8 precision.
Feature is only supported for Matrix Multiplication operations.
HW target must have Intel AMX extension support (e.g., Intel® 4th Generation Xeon® processors (code name Sapphire Rapids)).
The number of input and output channels of the weights must be a multiple of 64.