using BenchmarkTools is the recommended way to benchmark Julia functions. Unless you are timing something that takes quite a while, use either @benchmark or the less verbose @btime macros exported from it. Because the machinery behind these macros evaluates the target function many times, @time is useful for benchmarking things that run slowly (e.g. where disk access or very time-consuming calculations are involved).
It is important to use @btime or @benchmark correctly, this avoids misleading results. Usually, you are benchmarking a function that takes one or more arguments. When benchmarking, all arguments should be external variables: (without the benchmark macro)
x = 1
f(x)
# do not use f(1)
The function will be evaluated many times. To prevent the function arguments from being re-evaluated whenever the function is evaluated, we must mark each argument by prefixing a $ to the name of each variable that is used as an argument. The benchmarking macros use this to indicate that the variable should be evaluated (resolved) once, at the start of the benchmarking process and then the result is to be reused directly as is:
julia> using BenchmarkTools
julia> a = 1/2;
julia> b = 1/4;
julia> c = 1/8;
julia> a, b, c
(0.5, 0.25, 0.125)
julia> function sum_cosines(x, y, z)
return cos(x) + cos(y) + cos(z)
end;
julia> @btime sum_cosines($a, $b, $c); # the `;` suppresses printing the returned value
11.899 ns (0 allocations: 0 bytes) # calling the function takes ~12 ns (nanoseconds)
# the function does not allocate any memory
# if we omit the '$', what we see is misleading
julia> @btime sum_cosines(a, b, c); # the function appears more than twice slower
28.441 ns (1 allocation: 16 bytes) # the function appears to be allocating memory
# @benchmark can be used the same way that @btime is used
julia> @benchmark sum_cosines($a,$b,$c) # do not use a ';' here
BenchmarkTools.Trial:
memory estimate: 0 bytes
allocs estimate: 0
--------------
minimum time: 12.111 ns (0.00% GC)
median time: 12.213 ns (0.00% GC)
mean time: 12.500 ns (0.00% GC)
maximum time: 39.741 ns (0.00% GC)
--------------
samples: 1500
evals/sample: 999
While there are parameters than can be adjusted, the default values usually work well. For additional information about BenchmarkTools for experienced ursers, see the manual.
