Carefully-tuned primitives for running tensor and image-processing code on CPU, GPUs and accelerators.
The library is in heavy development. For now the CPU backend is being optimised.
cpuinfo
for runtime CPU feature detection for x86, x86-64 and ARMimport laser/simd
Laser includes a wrapper for x86 and x86-64 to operate on 128-bit (SSE) and 256-bit (AVX) vectors of floats and integers. SIMD are added on a as-needed basis for Laser optimisation needs.
import laser/openmp
Laser includes several OpenMP templates to easu data-parallel programming in Nim:
omp parallel
, omp critical
, omp master
, omp barrier
and omp flush
for fine-grained control over parallelismattachGC
and detachGC
if you need to use Nim GC-ed types in a non-master thread.Examples:
cpuinfo
for runtime CPU feature detection for x86, x86-64 and ARMimport laser/cpuinfo
Laser includes a wrapper for cpuinfo
by Facebook's PyTorch team.
This allows to query runtime information about CPU SIMD capabilities and various L1, L2, L3, L4 CPU cache sizes
to optimize your compute-bound algorithms.
Example: ex01_cpuinfo.nim
import laser/photon_jit
Laser offers its own JIT assembler with features being added on a as needed basis. It is very lightweight and easy to extend. Currently it only supports x86-64 with the following opcodes.
Examples:
import laser/strided_iteration/foreach
import laser/strided_iteration/foreach_staged
Usage - forEach:
forEach x in a, y in b, z in c:
x += y * z
Laser includes optimised macros to iterate on contiguous and strided tensors. The iterators work with normal Nim syntax, are parallelized via OpenMP when it makes sense.
Any tensor type works as long as it exposes the following interface:
[]
indexingptr UncheckedArray[T]
or
any type with []
indexing implemented, including mutable indexing.A advanced iterator forEach_staged
provides a lot of flexibility to deal with advanced need, for example for parallel reduction:
proc reduction_localsum_critical[T](x, y: Tensor[T]): T =
forEachStaged xi in x, yi in y:
openmp_config:
use_openmp: true
use_simd: false
nowait: true
omp_grain_size: OMP_MEMORY_BOUND_GRAIN_SIZE
iteration_kind:
{contiguous, strided} # Default, "contiguous", "strided" are also possible
before_loop:
var local_sum = 0.T
in_loop:
local_sum += xi + yi
after_loop:
omp_critical:
result += local_sum
Examples:
Benchmarks:
import laser/tensor/[datatypes, allocator, initialization] # WIP
Laser includes a low-level tensor type with only the low-level allocation and initialization needed:
RawImmutablePtr
and mutable objects return a RawMutablePtr
to prevent you from accidentally modifying an immutable object when accessing raw memory.An example of how to use that to build higher-level newTensor
or randomTensor
, transpose
and []
is give in the iter_bench
in the previous section.
import laser/primitives/reductions
Floating-point reductions are not optimised by compilers by default because they can't assume that
result = (a+b) + c
is equivalent to result = a + (b + c)
due to how floating-point rounding work.
This forces serial evaluation of reductions unless -ffast-math
flag is passed to the compiler.
The primitives work around that by keeping several accumulators in parallel to avoid waiting for a previous serial evaluation. This allows those kernels to maximise memory-bandwith of your computer.
Benchmarks:
In heavy development.
Unfortunately the default logarithm and exponential functions included in C and C++ standard <math.h> library are extremely slow.
Benchmarks shows that a 10x speed improvement is possible while keeping excellent accuracy.
Benchmarks:
import laser/primitives/swapaxes
While logical transpose (just swapping the shape
and strides
metadata of the tensor/matrix) is often enough, we sometimes might need to transpose data physically in-memory.
Laser provides Optimised routines for physical transpose, batched transpose (N matrices) and also transposition of images from and to NCHW and NHWC i.e. [Image id, Color, Height, Width] and [Image id, Height, Width, Color].
90% of ML libraries including Nvidia's CuDNN prefer to work in NCHW while often images are decoded in HWC.
Benchmarks:
import laser/primitives/matrix_multiplication/gemm
Matrix multiplication is the at the base of Machine Learning and numerical computing.
The Dense/Linear/Affine layer of neural network is just a matrix-multiplication and often convolutions are reframed into matrix multiplication to use the 20 years of optimisation research gone into BLAS libraries.
Laser implements its own multithreaded BLAS with the following details:
myTensor[0::2, :]
.
This is very useful when doing cross-validation as you don't need an extra copy before matrix-multiplication.int32
and int64
using SSE2 or AVX2 instructionstemplate int32x8_muladd_unfused_avx2(a, b, c: m256i): m256i =
mm256_add_epi32(mm256_mullo_epi32(a, b), c)
ukernel_generator(
x86_AVX2,
typ = int32,
vectype = m256i,
nb_scalars = 8,
simd_setZero = mm256_setzero_si256,
simd_broadcast_value = mm256_set1_epi32,
simd_load_aligned = mm256_load_si256,
simd_load_unaligned = mm256_loadu_si256,
simd_store_unaligned = mm256_storeu_si256,
simd_mul = mm256_mullo_epi32,
simd_add = mm256_add_epi32,
simd_fma = int32x8_muladd_unfused_avx2
)
The BLAS will allow easily fusing unary operations (like max/relu
, tanh
or sigmoid
) and binary operations (like adding a bias) at the end of the matrix multiplication kernels.
As those operations are memory-bound and not compute-bound, and for matrix multiplication we already have all the data in memory (in the unary case) or half the data (in the binary case), we basically save lots by not looping once again on the matrix to apply them.
Similarly, you will be able to fuse operations before the matrix multiplication kernel, during the prepacking when data is being re-ordered for high performance processing. This will be useful
for backward propagation when before each matrix multiplication we must apply the derivatives of relu
, tanh
and sigmoid
.
Also pre-packing matrices and working on pre-packed matrices is being added. This is useful for matrices that are being used repeatedly, for example for batched matrix multiplication.
im2col
prepacker that fuses the convolution->matrix multiplication
(im2col) step with the matrix multiplication packing is also planned to get very efficient convolutions.
We often have to bached matrix multiplication for examples N tensors A multiplied by a tensor B, or N tensors A multiplied by N tensors B, this is planned.
In many cases we don't deal with 1000x1000 matrices. For example the traditional image size is 224x224 and the overhead to re-pack matrices in an efficient format is not justified.
When reframing convolutions in terms of matrix multiplication this is even worse as the main convolution kernels are 1x1, 3x3, 5x5.
Optimised small matrix-multiplication is planned.
In heavy development.
Benchmarks:
In heavy development
Benchmarks of multinomial sampling for Natural Language Processing and Reinforcement Learning: -bench_multinomial_samplers
The library is split in relatively independant modules that can be used without the others.
For example to just use the SIMD and cpu-detection portion, just do:
import laser/simd
import laser/cpuinfo
To just use OpenMP
import laser/openmp
The library is unstable and will be published on nimble when more mature. Basically it will be published when it's ready to be the CPU backend of Arraymancer, it will automatically profit from the dozens of tests and edge cases handled in Arraymancer test suite.