How does parallel sort() scale on your PC

[Bonita Montero]

#include <iostream>
#include <algorithm>
#include <execution>
#include <vector>
#include <random>
#include <chrono>
#include <charconv>
#include <functional>

#if defined(_MSC_VER)
#pragma warning(disable: 4996)
#endif

using namespace std;
using namespace chrono;

int main( int argc, char **argv )
{
if( argc < 2 )
return EXIT_FAILURE;
size_t upper;
bool b16 = strnicmp( argv[1], "0x", 2 ) == 0;
argv[1] += b16 ? 2 : 0;
if( from_chars_result fcr = from_chars( argv[1], argv[1] + strlen(
argv[1] ), upper, b16 ? 16 : 10 ); (bool)fcr.ec || *fcr.ptr )
return EXIT_FAILURE;
vector<size_t> vs;
vs.reserve( upper );
size_t size = 1;
mt19937_64 mt;
uniform_int_distribution<size_t> uid( 0, -1 );
do
{
size = size * 2 <= upper ? size * 2 : upper;
vs.resize( size );
auto perf = [&]( function<void ()> const &fn )
{
for( size_t &s : vs )
s = uid( mt );
auto start = high_resolution_clock::now();
fn();
return duration_cast<nanoseconds>( high_resolution_clock::now() -
start ).count() / 1.0e9;
};
double
tParallel = perf( [&]() { sort( execution::parallel_policy(),
vs.begin(), vs.end() ); } ),
tSerial = perf( [&]() { sort( vs.begin(), vs.end() ); } );
auto s2 = []( double d ) -> double { return trunc( 100.0 * d + 0.5 ) /
100.0; };
cout << "0x" << hex << size << ": " << tSerial / tParallel << " (" <<
s2( tSerial ) << " : " << s2( tParallel ) << ")" << endl;
} while( size < upper );
}

I get only a scaling up to 8x on a 3990X (Windows) and a 7950X (Linux).