Speed Comparison: BitLinear and nn.Linear

[ZiqingChang]

Hello,

I measured the time of your BitLinear and BitLinearBitBLAS against nn.Linear, and it seems that the time for smaller input_features and out_features is slower than nn.Linear. Is there a solution for this?

I used the quant_utils from your BitNet integration: https://github.com/microsoft/BitBLAS/blob/main/integration/BitNet/utils_quant.py

My GPU is NVIDIA GeForce RTX 3090

import time
import torch
import torch.nn as nn
from torch.autograd import Variable

# from bitlinear import BitLinear, BitLinearBitBLAS
from utils_quant import BitLinear, BitLinearBitBLAS

# Function to measure computation time
def measure_time(layer, input_tensor, num_runs=100):
    with torch.no_grad():
        # Warm up
        for _ in range(100): ## 100
            _ = layer(input_tensor)
        
        start_time = time.time()
        for _ in range(num_runs):
            _ = layer(input_tensor)
        torch.cuda.synchronize()  #### new
        end_time = time.time()
        
        avg_time = (end_time - start_time) / num_runs
    return avg_time

# # # Test parameters
input_features = 512
output_features = 256
batch_size = 8

# input_features = 1024
# output_features = 512
# batch_size = 32

# input_features = 10240
# output_features = 5120
# batch_size = 32

# input_features = 20480
# output_features = 10240
# batch_size = 32


# Create random input tensor
input_tensor = torch.randn(batch_size, input_features).cuda()

# Initialize layers
nn_linear_layer = nn.Linear(input_features, output_features).cuda()
bit_linear_layer = BitLinear(input_features, output_features).cuda()

bitblas_linear_layer = BitLinearBitBLAS.from_bit_linear(bit_linear_layer)

# Measure computation time
num_runs = 100
nn_linear_time = measure_time(nn_linear_layer, input_tensor, num_runs)
bit_linear_time = measure_time(bit_linear_layer, input_tensor, num_runs)
bitblas_linear_time = measure_time(bitblas_linear_layer, input_tensor, num_runs)

print('input_features, output_features, batch_size: ', input_features, output_features, batch_size)
print(f"Average computation time for nn.Linear: {nn_linear_time * 1000:.4f} ms")
print(f"Average computation time for fp32 simulated BitLinear: {bit_linear_time * 1000:.4f} ms")
print(f"Average computation time Bitblas BitLinear: {bitblas_linear_time * 1000:.4f} ms")

Here are the testing results:

input_features, output_features, batch_size:  512 256 8
Average computation time for nn.Linear: 0.0230 ms
Average computation time for fp32 simulated BitLinear: 0.3450 ms
Average computation time Bitblas BitLinear: 0.3091 ms

input_features, output_features, batch_size:  1024 512 32
Average computation time for nn.Linear: 0.0265 ms
Average computation time for fp32 simulated BitLinear: 0.3427 ms
Average computation time Bitblas BitLinear: 0.3137 ms

input_features, output_features, batch_size:  10240 5120 32
Average computation time for nn.Linear: 0.5421 ms
Average computation time for fp32 simulated BitLinear: 6.3314 ms
Average computation time Bitblas BitLinear: 0.3170 ms

input_features, output_features, batch_size:  20480 10240 32
Average computation time for nn.Linear: 2.1726 ms
Average computation time for fp32 simulated BitLinear: 25.2509 ms
Average computation time Bitblas BitLinear: 0.5633 ms

Thanks for your reply in advance.

[LeiWang1999]

Hi, @ZiqingChang , I guess the overhead comes from input quant and output rescaling, we can improve the performance through torch.compile, this update is not applied in this version, to test the efficient bitnet layer you may want to check out the branch at https://github.com/LeiWang1999/vllm-bitblas/tree/bitblas-intg , we provide a more efficient bitlinear.

[ZiqingChang]

Hi, @LeiWang1999 ,thanks for your quick reply, I was just wondering, will BitLinearBitBLAS speed up models with the majority of linear layers with input < 1024, because based on the computational time of the overhead for small inputs, I'm not certain.

I was wondering about this because I tested the BitLinearBitBLAS on my model with most of the linear layers having inputs and outputs < 1024, and the inference time is 2x slower than the same model with nn.Linear.

[LeiWang1999]

Hi @ZiqingChang , absolutely it should be.

[ZiqingChang]

Hi @LeiWang1999 , can you provide an example of how to test the speed of your more efficient BitLinear? I tried torch.compile, but it does not seem to speed up BitBlas BitLinear:

opt_nn_linear_layer = torch.compile(nn_linear_layer, mode="reduce-overhead")
opt_bit_linear_layer = torch.compile(bit_linear_layer, mode="reduce-overhead")
opt_bitblas_linear_layer = torch.compile(bitblas_linear_layer, mode="reduce-overhead")

torch.set_float32_matmul_precision('high')

# Measure computation time
num_runs = 100
opt_nn_linear_time = measure_time(opt_nn_linear_layer, input_tensor, num_runs)
opt_bit_linear_time = measure_time(opt_bit_linear_layer, input_tensor, num_runs)
opt_bitblas_linear_time = measure_time(opt_bitblas_linear_layer, input_tensor, num_runs)

print('*' * 50)
print(f"Average computation time for compiled nn.Linear: {opt_nn_linear_time * 1000:.4f} ms")
print(f"Average computation time for compiled fp32 simulated BitLinear: {opt_bit_linear_time * 1000:.4f} ms")
print(f"Average computation time for compiled Bitblas BitLinear: {opt_bitblas_linear_time * 1000:.4f} ms")

The results are :

input_features, output_features, batch_size:  512 256 8
**************************************************
Average computation time for nn.Linear: 0.0221 ms
Average computation time for fp32 simulated BitLinear: 0.3338 ms
Average computation time for Bitblas BitLinear: 0.3025 ms
**************************************************
Average computation time for compiled nn.Linear: 0.1108 ms
Average computation time for compiled fp32 simulated BitLinear: 0.1114 ms
Average computation time for compiled Bitblas BitLinear: 0.3155 ms

[LeiWang1999]

@ZiqingChang , sry I missed this message! it's strange, would you mind provide an intact script that we can reproduce it :)

[ZiqingChang]

Hello @LeiWang1999 , yes, here is the full script.

import time
import torch
import torch.nn as nn
from torch.autograd import Variable

# from bitlinear import BitLinear, BitLinearBitBLAS
from utils_quant import BitLinear, BitLinearBitBLAS

# Function to measure computation time
def measure_time(layer, input_tensor, num_runs=100):
    with torch.no_grad():
        # Warm up
        for _ in range(100): ## 100
            _ = layer(input_tensor)
        
        start_time = time.time()
        for _ in range(num_runs):
            _ = layer(input_tensor)
        torch.cuda.synchronize()  #### new
        end_time = time.time()
        
        avg_time = (end_time - start_time) / num_runs
    return avg_time

# # # Test parameters
input_features = 512
output_features = 256
batch_size = 8

# Create random input tensor
input_tensor = torch.randn(batch_size, input_features).cuda()

# Initialize layers
nn_linear_layer = nn.Linear(input_features, output_features).cuda()
bit_linear_layer = BitLinear(input_features, output_features).cuda()

bitblas_linear_layer = BitLinearBitBLAS.from_bit_linear(bit_linear_layer)

# Measure computation time
num_runs = 100
nn_linear_time = measure_time(nn_linear_layer, input_tensor, num_runs)
bit_linear_time = measure_time(bit_linear_layer, input_tensor, num_runs)
bitblas_linear_time = measure_time(bitblas_linear_layer, input_tensor, num_runs)

print('input_features, output_features, batch_size: ', input_features, output_features, batch_size)
print(f"Average computation time for nn.Linear: {nn_linear_time * 1000:.4f} ms")
print(f"Average computation time for fp32 simulated BitLinear: {bit_linear_time * 1000:.4f} ms")
print(f"Average computation time Bitblas BitLinear: {bitblas_linear_time * 1000:.4f} ms")


opt_nn_linear_layer = torch.compile(nn_linear_layer, mode="reduce-overhead")
opt_bit_linear_layer = torch.compile(bit_linear_layer, mode="reduce-overhead")
opt_bitblas_linear_layer = torch.compile(bitblas_linear_layer, mode="reduce-overhead")

torch.set_float32_matmul_precision('high')

# Measure computation time
num_runs = 100
opt_nn_linear_time = measure_time(opt_nn_linear_layer, input_tensor, num_runs)
opt_bit_linear_time = measure_time(opt_bit_linear_layer, input_tensor, num_runs)
opt_bitblas_linear_time = measure_time(opt_bitblas_linear_layer, input_tensor, num_runs)

print('*' * 50)
print(f"Average computation time for compiled nn.Linear: {opt_nn_linear_time * 1000:.4f} ms")
print(f"Average computation time for compiled fp32 simulated BitLinear: {opt_bit_linear_time * 1000:.4f} ms")
print(f"Average computation time for compiled Bitblas BitLinear: {opt_bitblas_linear_time * 1000:.4f} ms")

[LeiWang1999]

Take some info from profiling:

nn.Linear

 Time (%)  Total Time (ns)  Instances  Avg (ns)  Med (ns)  Min (ns)  Max (ns)  StdDev (ns)                                                  Name                                                
 --------  ---------------  ---------  --------  --------  --------  --------  -----------  ----------------------------------------------------------------------------------------------------
     60.1           456110        105    4343.9    4320.0      4319      5120         88.9  void cutlass::Kernel<cutlass_80_tensorop_s1688gemm_64x64_16x6_tn_align4>(T1::Params)                
     32.9           250178        105    2382.6    2368.0      2367      2496         19.4  void splitKreduce_kernel<(int)32, (int)16, int, float, float, float, float, (bool)1, (bool)0, (bool…
      2.0            14816          2    7408.0    7408.0      7360      7456         67.9  void at::native::reduce_kernel<(int)512, (int)1, at::native::ReduceOp<float, at::native::MeanOps<fl…
      0.9             6688          3    2229.3    2304.0      1984      2400        217.8  void at::native::vectorized_elementwise_kernel<(int)4, at::native::<unnamed>::launch_clamp_scalar(a…
      0.8             5984          2    2992.0    2992.0      2304      3680        973.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::AbsFunctor<float>, at::detail::A…
      0.5             4161          2    2080.5    2080.5      2080      2081          0.7  void at::native::vectorized_elementwise_kernel<(int)4, at::native::reciprocal_kernel_cuda(at::Tenso…
      0.5             3969          2    1984.5    1984.5      1984      1985          0.7  void at::native::vectorized_elementwise_kernel<(int)4, at::native::AUnaryFunctor<float, float, floa…
      0.5             3936          1    3936.0    3936.0      3936      3936          0.0  void at::native::unrolled_elementwise_kernel<at::native::direct_copy_kernel_cuda(at::TensorIterator…
      0.5             3488          1    3488.0    3488.0      3488      3488          0.0  void at::native::elementwise_kernel<(int)128, (int)2, void at::native::gpu_kernel_impl_nocast<at::n…
      0.4             2911          1    2911.0    2911.0      2911      2911          0.0  void at::native::elementwise_kernel<(int)128, (int)2, void at::native::gpu_kernel_impl_nocast<at::n…
      0.3             2401          1    2401.0    2401.0      2401      2401          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::round_kernel_cuda(at::TensorIter…
      0.3             2400          1    2400.0    2400.0      2400      2400          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::<unnamed>::launch_clamp_scalar(a…
      0.3             2241          1    2241.0    2241.0      2241      2241          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::CUDAFunctorOnSelf_add<signed cha…

BitBLASLinear

 Time (%)  Total Time (ns)  Instances  Avg (ns)  Med (ns)  Min (ns)  Max (ns)  StdDev (ns)                                                  Name                                                
 --------  ---------------  ---------  --------  --------  --------  --------  -----------  ----------------------------------------------------------------------------------------------------
     58.4           446394        210    2125.7    2128.0      1951      2463        159.0  triton_                                                                                             
     34.6           264545        105    2519.5    2528.0      2495      2624         19.5  matmul_n256k512_i8xi2_simt_opt_m_16                                                                 
      1.9            14784          2    7392.0    7392.0      7360      7424         45.3  void at::native::reduce_kernel<(int)512, (int)1, at::native::ReduceOp<float, at::native::MeanOps<fl…
      0.9             6689          3    2229.7    2304.0      1985      2400        217.3  void at::native::vectorized_elementwise_kernel<(int)4, at::native::<unnamed>::launch_clamp_scalar(a…
      0.8             6304          2    3152.0    3152.0      2304      4000       1199.3  void at::native::vectorized_elementwise_kernel<(int)4, at::native::AbsFunctor<float>, at::detail::A…
      0.6             4225          2    2112.5    2112.5      2112      2113          0.7  void at::native::vectorized_elementwise_kernel<(int)4, at::native::reciprocal_kernel_cuda(at::Tenso…
      0.5             3967          2    1983.5    1983.5      1983      1984          0.7  void at::native::vectorized_elementwise_kernel<(int)4, at::native::AUnaryFunctor<float, float, floa…
      0.5             3936          1    3936.0    3936.0      3936      3936          0.0  void at::native::unrolled_elementwise_kernel<at::native::direct_copy_kernel_cuda(at::TensorIterator…
      0.5             3456          1    3456.0    3456.0      3456      3456          0.0  void at::native::elementwise_kernel<(int)128, (int)2, void at::native::gpu_kernel_impl_nocast<at::n…
      0.4             2912          1    2912.0    2912.0      2912      2912          0.0  void at::native::elementwise_kernel<(int)128, (int)2, void at::native::gpu_kernel_impl_nocast<at::n…
      0.3             2400          1    2400.0    2400.0      2400      2400          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::<unnamed>::launch_clamp_scalar(a…
      0.3             2400          1    2400.0    2400.0      2400      2400          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::round_kernel_cuda(at::TensorIter…
      0.3             2208          1    2208.0    2208.0      2208      2208          0.0  void at::native::vectorized_elementwise_kernel<(int)4, at::native::CUDAFunctorOnSelf_add<signed cha…

Looks like the shape that you're benchmarking is too small (especially N and K, and while M is smaller, the gain is bigger), make the overhead of activation dequantize and quantize be non-neligible.

[LeiWang1999]

The kernel time is similar, must be something slow of the cpu runtime, we should dig further.

[LeiWang1999]

I tested on a big shape, the performance is normal, though looks like there still exist some cpu runtime overhead.

BitBLAS Tuning done, appended operator to global_operator_cache.
input_features, output_features, batch_size:  16384 16384 1
Average computation time for nn.Linear: 0.6810 ms
Average computation time for fp32 simulated BitLinear: 12.7460 ms
Average computation time Bitblas BitLinear: 0.1746 ms

[ZiqingChang]

For my model, the input and output size of Linear modules are all <= 1024, so due to the small shape, can BitBlas actually speed up the inference time?

[LeiWang1999]

For my model, the input and output size of Linear modules are all <= 1024, so due to the small shape, can BitBlas actually speed up the inference time?

@ZiqingChang , from the kernel side, absolutely it should be, but we need to understand why the torch bitnet linear forward is introducing some extra overhead.

[alexfanqi]

Friendly ping. Any update on this? We are planning to use this in spiking neural network since often the forward step is bool x float while backward is some STE float x float and connection weight matrix is almost always the bottleneck.

I had similar results on nvidia 4090.

BitBLAS Tuning done, appended operator to global_operator_cache.                     
input_features, output_features, batch_size:  512 256 8
Average computation time for nn.Linear: 0.0215 ms                                    
Average computation time for fp32 simulated BitLinear: 0.2595 ms                     
Average computation time Bitblas BitLinear: 0.1508 ms

input_features, output_features, batch_size:  10240 5120 32
Average computation time for nn.Linear: 0.4902 ms
Average computation time for fp32 simulated BitLinear: 5.7793 ms
Average computation time Bitblas BitLinear: 0.1744 ms

with some compilation error messages

2025-12-28 11:38:18 [BitBLAS:ERROR]: An exception occurred for hint {block_M=16,block
_N=16,warp_M=16,warp_N=16,block_K=640,threads=32,num_stages=0,enable_rasterization=Fa
lse,split_k_factor=1}: Traceback (most recent call last):                            
  76: tvm::runtime::PackedFuncObj::Extractor<tvm::runtime::PackedFuncSubObj<tvm::runt
ime::TypedPackedFunc<tvm::IRModule (tvm::transform::Pass, tvm::IRModule)>::AssignType
dLambda<tvm::transform::__mk_TVM9::{lambda(tvm::transform::Pass, tvm::IRModule)#1}>(t
vm::transform::__mk_TVM9::{lambda(tvm::transform::Pass, tvm::IRModule)#1}, std::__cxx
11::basic_string<char, std::char_traits<char>, std::allocator<char> >)::{lambda(tvm::
runtime::TVMArgs const&, tvm::runtime::T        ...     opy::Lower(tvm::tl::LowerArgs
 const&, tvm::arith::Analyzer*) const                                                
  2: tvm::tl::PartitionLoop(tvm::tir::For, tvm::tir::Var, tvm::arith::Analyzer*, tvm:
:tl::Fragment)                                                                       
  1: tvm::tl::FragmentNode::Inverse() const                                          
  0: tvm::tl::LayoutNode::Inverse() const                                            
  File "/home/t-leiwang/BitBLAS/3rdparty/tilelang/src/layout/layout.cc", line 204    
InternalError: Check failed: (res->errors.empty()) is false: ["The iterations do not 
traverse full iter space", "Index mapping does not form a bijective transform."]     
2025-12-28 11:38:18 [BitBLAS:ERROR]: An exception occurred for hint {block_M=16,block
_N=16,warp_M=16,warp_N=16,block_K=640,threads=32,num_stages=0,enable_rasterization=Fa
lse,split_k_factor=2}: Traceback (most recent call last):                            
  76: tvm::runtime::PackedFuncObj::Extractor<tvm::runtime::PackedFuncSubObj<tvm::runt
ime::TypedPackedFunc<tvm::IRModule (tvm::transform::Pass, tvm::IRModule)>::AssignType
dLambda<tvm::transform::__mk_TVM9::{lambda(tvm::transform::Pass, tvm::IRModule)#1}>(t
vm::transform::__mk_TVM9::{lambda(tvm::transform::Pass, tvm::IRModule)#1}, std::__cxx
11::basic_string<char, std::char_traits<char>, std::allocator<char> >)::{lambda(tvm::
runtime::TVMArgs const&, tvm::runtime::T        ...     opy::Lower(tvm::tl::LowerArgs
 const&, tvm::arith::Analyzer*) const                                                
  2: tvm::tl::PartitionLoop(tvm::tir::For, tvm::tir::Var, tvm::arith::Analyzer*, tvm:
:tl::Fragment)                                                                       
  1: tvm::tl::FragmentNode::Inverse() const
  0: tvm::tl::LayoutNode::Inverse() const
  File "/home/t-leiwang/BitBLAS/3rdparty/tilelang/src/layout/layout.cc", line 204
InternalError: Check failed: (res->errors.empty()) is false: ["The iterations do not 
traverse full iter space", "Index mapping does not form a bijective transform."]
2025-12-28 11:38:18 [BitBLAS:ERROR]: An exception occurred for hint {block_M=16,block
_N=160,warp_M=16,warp_N=32,block_K=128,threads=160,num_stages=0,enable_rasterization=
False,split_k_factor=4}: Traceback (most recent call last):