On Thursday, 18 June 2015 at 10:27:58 UTC, Russel Winder wrote:
On a given machine, the code:
double sequential_loop(const int n, const double delta) {
auto sum = 0.0;
foreach (immutable i; 1 .. n + 1) {
immutable x = (i - 0.5) * delta;
sum += 1.0 / (1.0 + x * x);
}
return 4.0 * delta * sum;
}
runs in about 6.70s. However the code:
double sequential_reduce(const int n, const double delta) {
return 4.0 * delta * reduce!((double t, int i){immutable x =
(i -
0.5) * delta; return t + 1.0 / (1.0 + x * x);})(0.0, iota(1, n
+ 1));
}
runs in about 17.03s, whilst:
double sequential_reduce_alt(const int n, const double delta) {
return 4.0 * delta * reduce!"a + b"(
map!((int i){immutable x = (i - 0.5) * delta; return
1.0 /
(1.0 + x * x);})(iota(1, n + 1)));
}
takes about 28.02s. Unless I am missing something (very
possible), this
is not going to create a good advert for D as an imperative
language
with declarative (internal iteration) expression.
import std.stdio, std.datetime, std.algorithm, std.range,
std.conv;
double sequential_loop(const int n, const double delta) {
auto sum = 0.0;
foreach (immutable i; 1 .. n + 1) {
immutable x = (i - 0.5) * delta;
sum += 1.0 / (1.0 + x * x);
}
return 4.0 * delta * sum;
}
//runs in about 6.70s. However the code:
double sequential_reduce(const int n, const double delta) {
return 4.0 * delta * reduce!((double t, int i){immutable x = (i
-
0.5) * delta; return t + 1.0 / (1.0 + x * x);})(0.0, iota(1, n +
1));
}
//runs in about 17.03s, whilst:
double sequential_reduce_alt(const int n, const double delta) {
return 4.0 * delta * reduce!"a + b"(
map!((int i){immutable x = (i - 0.5) * delta; return 1.0
/
(1.0 + x * x);})(iota(1, n + 1)));
}
void main() {
auto res = benchmark!(
{sequential_loop(1000, 10);},
{sequential_reduce(1000, 10);},
{sequential_reduce_alt(1000, 10);},
)(1000);
writeln(res[].map!(to!Duration));
}
$ dmd -run test.d
[9 ms, 305 μs, and 9 hnsecs, 16 ms, 625 μs, and 3 hnsecs, 27 ms,
417 μs, and 3 hnsecs]
$ dmd -O -inline -release -run test.d
[4 ms, 567 μs, and 6 hnsecs, 4 ms, 853 μs, and 8 hnsecs, 5 ms, 52
μs, and 5 hnsecs]
OS X, DMD64 D Compiler v2.067.1
Looks like you forgot optimisation troika "-O -inline -release"
Regards,
Ilya
