parameter/test/efficiency.cpp

// Copyright David Abrahams, Matthias Troyer, Michael Gauckler 2005.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#include <boost/parameter/name.hpp>
#include <boost/config/workaround.hpp>
#include <boost/timer.hpp>
#include <iostream>

namespace test {

    //
    // This test measures the abstraction overhead of using the named
    // parameter interface.  Some actual test results have been recorded
    // in timings.txt in this source file's directory, or
    // http://www.boost.org/libs/parameter/test/timings.txt.
    //
    // Caveats:
    //
    //   1. This test penalizes the named parameter library slightly, by
    //      passing two arguments through the named interface, while
    //      only passing one through the plain C++ interface.
    //
    //   2. This test does not measure the case where an ArgumentPack is
    //      so large that it doesn't fit in the L1 cache.
    //
    //   3. Although we've tried to make this test as general as possible,
    //      we are targeting it at a specific application.  Where that
    //      affects design decisions, we've noted it below in ***...***.
    //
    //   4. The first time you run this program, the time may not be
    //      representative because of disk and memory cache effects, so
    //      always run it multiple times and ignore the first
    //      measurement.  This approach will also allow you to estimate
    //      the statistical error of your test by observing the
    //      variation in the valid times.
    //
    //   5. Try to run this program on a machine that's otherwise idle,
    //      or other processes and even device hardware interrupts may
    //      interfere by causing caches to be flushed.

    // Accumulator function object with plain C++ interface 
    template <typename T>
    struct plain_weight_running_total
    {
        plain_weight_running_total()
#if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
          : sum(T())
#else
          : sum()
#endif
        {
        }

        void operator()(T w)
        {
            this->sum += w;
        }

        T sum;
    };

    BOOST_PARAMETER_NAME(weight)
    BOOST_PARAMETER_NAME(value)

    // Accumulator function object with named parameter interface
    template <typename T>
    struct named_param_weight_running_total
    {
        named_param_weight_running_total()
#if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
          : sum(T())
#else
          : sum()
#endif
        {
        }

        template <typename ArgumentPack>
        void operator()(ArgumentPack const& variates)
        {
            this->sum += variates[test::_weight];
        }

        T sum;
    };

    // This value is required to ensure that a smart compiler's dead code
    // elimination doesn't optimize away anything we're testing.  We'll use it
    // to compute the return code of the executable to make sure it's needed.
    double live_code;

    // Call objects of the given Accumulator type repeatedly
    // with x an argument.
    template <typename Accumulator, typename Arg>
    void hammer(Arg const& x, long const repeats)
    {
        // Strategy: because the sum in an accumulator after each call
        // depends on the previous value of the sum, the CPU's pipeline
        // might be stalled while waiting for the previous addition to
        // complete.  Therefore, we allocate an array of accumulators,
        // and update them in sequence, so that there's no dependency
        // between adjacent addition operations.
        //
        // Additionally, if there were only one accumulator, the compiler or
        // CPU might decide to update the value in a register rather than
        // writing it back to memory.  We want each operation to at least
        // update the L1 cache.  *** Note: This concern is specific to the
        // particular application at which we're targeting the test. ***

        // This has to be at least as large as the number of simultaneous
        // accumulations that can be executing in the compiler pipeline.  A
        // safe number here is larger than the machine's maximum pipeline
        // depth.  If you want to test the L2 or L3 cache, or main memory,
        // you can increase the size of this array.  1024 is an upper limit
        // on the pipeline depth of current vector machines.
        std::size_t const number_of_accumulators = 1024;

        Accumulator a[number_of_accumulators];

        for (long iteration = 0; iteration < repeats; ++iteration)
        {
            for (Accumulator* ap = a; ap < a + number_of_accumulators; ++ap)
            {
                (*ap)(x);
            }
        }

        // Accumulate all the partial sums to avoid dead code elimination.
        for (Accumulator* ap = a; ap < a + number_of_accumulators; ++ap)
        {
            test::live_code += ap->sum;
        }
    }

    // Measure the time required to hammer accumulators of the given
    // type with the argument x.
    template <typename Accumulator, typename T>
    double measure(T const& x, long const repeats)
    {
        // Hammer accumulators a couple of times to ensure the instruction
        // cache is full of our test code, and that we don't measure the cost
        // of a page fault for accessing the data page containing the memory
        // where the accumulators will be allocated.
        test::hammer<Accumulator>(x, repeats);
        test::hammer<Accumulator>(x, repeats);

        // Now start a timer.
        boost::timer time;
        test::hammer<Accumulator>(x, repeats);  // This time, we'll measure.
        return time.elapsed();
    }
}

int main()
{
    // First decide how many repetitions to measure.
    long repeats = 100;
    double measured = 0;

    while (measured < 1.0 && repeats <= 10000000)
    {
        repeats *= 10;

        boost::timer time;

        test::hammer<test::plain_weight_running_total<double> >(.1, repeats);
        test::hammer<test::named_param_weight_running_total<double> >(
            (test::_weight = .1, test::_value = .2), repeats
        );

        measured = time.elapsed();
    }
    
    std::cout
        << "plain time:           "
        << test::measure<test::plain_weight_running_total<double> >(
            .1, repeats
        )
        << std::endl;

    std::cout
        << "named parameter time: "
        << test::measure<test::named_param_weight_running_total<double> >(
            (test::_weight = .1, test::_value = .2), repeats
        )
        << std::endl;

    // This is ultimately responsible for preventing all the test code
    // from being optimized away.  Change this to return 0 and you
    // unplug the whole test's life support system.
    return test::live_code < 0.;
}