Bfloat16#
Note
In this section we abbreviate data types names for readability. For example,
dnnl::memory::data_type::f32 is abbreviated to f32.
Bfloat16 (bf16) is a 16-bit floating point data type based on the IEEE 32-bit
single-precision floating point data type (f32).
Both bf16 and f32 have an 8-bit exponent. However, while f32 has a 23-bit
mantissa, bf16 has only a 7-bit one, keeping only the most significant bits.
As a result, while these data types support a very close numerical range of
values, bf16 has a significantly reduced precision. Therefore, bf16 occupies
a spot between f32 and the IEEE 16-bit half-precision floating point data
type, f16. Compared directly to f16, which has a 5-bit exponent and a
10-bit mantissa, bf16 trades increased range for reduced precision.
More details of the bfloat16 data type can be found here.
One of the advantages of using bf16 versus f32 is reduced memory footprint
and, hence, increased memory access throughput.
Workflow#
The main difference between implementing training with the f32 data type and
with the bf16 data type is the way the weights updates are treated. With the
f32 data type, the weights gradients have the same data type as the weights
themselves. This is not necessarily the case with the bf16 data type as oneDNN
allows some flexibility here. For example, one could maintain a master copy of
all the weights, computing weights gradients in f32 and converting the result
to bf16 afterwards.
Support#
Most of the primitives can support the bf16 data type for source and weights
tensors. Destination tensors can be specified to have either the bf16 or f32
data type. The latter is intended for cases in which the output is to be fed to
operations that do not support bfloat16 or require higher precision.