Given a matrix A, say:
A = torch.randn(5,5)
What is the difference between A.T and A.t()?
1 Like
From the docs:
tensor.t:
Expects
inputto be <= 2-D tensor and transposes dimensions 0 and 1.
0-D and 1-D tensors are returned as is. When input is a 2-D tensor this is equivalent totranspose(input, 0, 1).
Returns a view of this tensor with its dimensions reversed.
Ifnis the number of dimensions inx,x.Tis equivalent tox.permute(n-1, n-2, ..., 0).
In your use case both will yield the same result.
5 Likes
I think also it is valuable ti add that t() won’t work for dim >= 2, but transpose will. so so then it is necessary to use transpose(input, 0, 1). as ptrblck kinda wrote above
1 Like
By testing on my GPU machine, for 2-D Tensors x.T is consistently, slightly faster than x.t().
x.T is also syntactically shorter and more intuitive so it should be preferred.
Could you show your profiling code to measure this overhead?
Yes, here:
import time
import pandas as pd
import torch
def benchmark_transpose(dims_list, num_iterations=100000):
results = []
for dims in dims_list:
x = torch.randn(*dims, device="cuda")
# Warmup
for _ in range(100):
_ = x.T
_ = x.t()
# Benchmark x.T
torch.cuda.synchronize()
start_time = time.perf_counter()
for _ in range(num_iterations):
_ = x.T
torch.cuda.synchronize()
end_time = time.perf_counter()
t_property_time = (end_time - start_time) / num_iterations
# Benchmark x.t()
torch.cuda.synchronize()
start_time = time.perf_counter()
for _ in range(num_iterations):
_ = x.t()
torch.cuda.synchronize()
end_time = time.perf_counter()
t_method_time = (end_time - start_time) / num_iterations
results.append(
{
"dimensions": dims,
"tensor.T (ms)": t_property_time * 1000,
"tensor.t() (ms)": t_method_time * 1000,
"size": x.nelement() * x.element_size(),
}
)
return pd.DataFrame(results)
if __name__ == "__main__":
assert torch.cuda.is_available()
dims_to_test = [
(5, 5),
(55, 55),
(555, 555),
(5555, 5555),
(25555, 25555),
]
results_df = benchmark_transpose(dims_to_test)
# Format and display results
pd.set_option("display.float_format", lambda x: "{:.4f}".format(x))
results_df["size_mb"] = results_df["size"] / (1024 * 1024)
results_df["T/t() ratio"] = (
results_df["tensor.T (ms)"] / results_df["tensor.t() (ms)"]
)
print("\nTranspose Operations Benchmark Results:")
print("======================================")
print(
results_df[
["dimensions", "tensor.T (ms)", "tensor.t() (ms)", "size_mb", "T/t() ratio"]
]
)
Local results:
Transpose Operations Benchmark Results:
======================================
dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
0 (5, 5) 0.0080 0.0083 0.0001 0.9664
1 (55, 55) 0.0081 0.0083 0.0115 0.9656
2 (555, 555) 0.0080 0.0083 1.1750 0.9675
3 (5555, 5555) 0.0079 0.0082 117.7140 0.9619
4 (25555, 25555) 0.0080 0.0083 2491.2187 0.9648
I don’t think your benchmark shows a real difference and would claim the noise is to large when you are measuring us intervals with host timers.
E.g. if I move the syncs outside of the for loop right before stopping the host timers I get the opposite results:
# Transpose Operations Benchmark Results:
# ======================================
# dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
# 0 (5, 5) 0.0007 0.0007 0.0001 1.0178
# 1 (55, 55) 0.0007 0.0007 0.0115 1.0174
# 2 (555, 555) 0.0007 0.0007 1.1750 1.0191
# 3 (5555, 5555) 0.0007 0.0007 117.7140 1.0181
# 4 (25555, 25555) 0.0007 0.0007 2491.2187 1.0254
Interesting when I do the same on different architectures. On my Tesla T4, I still get similar results:
Transpose Operations Benchmark Results:
======================================
dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
0 (5, 5) 0.0013 0.0014 0.0001 0.9411
1 (55, 55) 0.0013 0.0014 0.0115 0.9168
2 (555, 555) 0.0013 0.0014 1.1750 0.9211
3 (5555, 5555) 0.0013 0.0014 117.7140 0.9319
4 (25555, 25555) 0.0013 0.0014 2491.2187 0.9386
But on an A100, results are similar to yours:
Transpose Operations Benchmark Results:
======================================
dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
0 (5, 5) 0.0008 0.0007 0.0001 1.0309
1 (55, 55) 0.0007 0.0007 0.0115 1.0317
2 (555, 555) 0.0008 0.0007 1.1750 1.0375
3 (5555, 5555) 0.0007 0.0007 117.7140 1.0349
4 (25555, 25555) 0.0007 0.0007 2491.2187 1.0163
Anyway, then I tried the recommended torch.cuda.Event for timing, and finally got results that drew the same conclusion on both architectures: x.T is indeed (slightly) faster than x.t(); the difference is yet less significant on newer architectures.
# T4:
Transpose Operations Benchmark Results:
======================================
dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
0 (5, 5) 1.2723 1.3940 0.0001 0.9127
1 (55, 55) 1.2873 1.3875 0.0115 0.9278
2 (555, 555) 1.2917 1.3887 1.1750 0.9302
3 (5555, 5555) 1.2803 1.3813 117.7140 0.9269
4 (25555, 25555) 1.1278 1.3752 2491.2187 0.8201
# A100:
Transpose Operations Benchmark Results:
======================================
dimensions tensor.T (ms) tensor.t() (ms) size_mb T/t() ratio
0 (5, 5) 0.6749 0.6923 0.0001 0.9748
1 (55, 55) 0.6860 0.6945 0.0115 0.9877
2 (555, 555) 0.6919 0.6969 1.1750 0.9929
3 (5555, 5555) 0.6956 0.6996 117.7140 0.9943
4 (25555, 25555) 0.6814 0.6970 2491.2187 0.9776
Full code:
import pandas as pd
import torch
def benchmark_transpose(dims_list, num_iterations=100000):
results = []
for dims in dims_list:
x = torch.randn(*dims, device="cuda")
# Warmup
for _ in range(100):
_ = x.T
_ = x.t()
start_event = torch.cuda.Event(enable_timing=True)
end_event = torch.cuda.Event(enable_timing=True)
# Benchmark x.T
start_event.record()
for _ in range(num_iterations):
_ = x.T
end_event.record()
torch.cuda.synchronize()
t_property_time = start_event.elapsed_time(end_event) / num_iterations
# Benchmark x.t()
start_event.record()
for _ in range(num_iterations):
_ = x.t()
end_event.record()
torch.cuda.synchronize()
t_method_time = start_event.elapsed_time(end_event) / num_iterations
results.append(
{
"dimensions": dims,
"tensor.T (ms)": t_property_time * 1000,
"tensor.t() (ms)": t_method_time * 1000,
"size": x.nelement() * x.element_size(),
}
)
return pd.DataFrame(results)
if __name__ == "__main__":
assert torch.cuda.is_available()
dims_to_test = [
(5, 5),
(55, 55),
(555, 555),
(5555, 5555),
(25555, 25555),
]
results_df = benchmark_transpose(dims_to_test)
# Format and display results
pd.set_option("display.float_format", lambda x: "{:.4f}".format(x))
results_df["size_mb"] = results_df["size"] / (1024 * 1024)
results_df["T/t() ratio"] = (
results_df["tensor.T (ms)"] / results_df["tensor.t() (ms)"]
)
print("\nTranspose Operations Benchmark Results:")
print("======================================")
print(
results_df[
["dimensions", "tensor.T (ms)", "tensor.t() (ms)", "size_mb", "T/t() ratio"]
]
)
Edit: typo slower → faster @ptrblck