libcruft-util/test/thread/ticketlock.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018 Danny Robson <danny@nerdcruft.net>
 */

#include "thread/flag.hpp"
#include "thread/ticketlock.hpp"

#include "tap.hpp"

#include <mutex>
#include <thread>


///////////////////////////////////////////////////////////////////////////////
using ffs_t = std::chrono::high_resolution_clock;


//-----------------------------------------------------------------------------
void
fight (cruft::thread::flag &start, cruft::thread::ticketlock &l, int total, ffs_t::time_point &finish) {
    start.wait ();

    for (int count = 0; count < total; ++count)
        std::lock_guard g (l);

    finish = std::chrono::high_resolution_clock::now ();
};



//-----------------------------------------------------------------------------
int
main ()
{
    cruft::TAP::logger tap;

    cruft::thread::ticketlock l;
    l.lock ();
    tap.expect (true, "locked without contention");

    l.unlock ();
    tap.expect (true, "unlocked without contention");

    if (true || std::thread::hardware_concurrency () < 2) {
        tap.skip ("reasonably fair under contention");
    } else {
        // measure fairness under contention is below some threshold
        //
        // * create two threads that will fight over a lock
        // * measure the start time
        // * fire an event to start the threads
        // * record the time the threads finish
        // * compare the difference in runtimes as a percentage
        //
        // we have to be reasonably generous with our test at the end because
        // there's liable to be a lot of noise in the measurement with a test
        // that runs in a short enough time period.
        constexpr int iterations = 1 << 16;
        cruft::thread::flag start_flag;

        ffs_t::time_point a_finish, b_finish;

        std::thread a (
            fight,
            std::ref (start_flag),
            std::ref (l),
            iterations,
            std::ref (a_finish)
        );

        std::thread b (
            fight,
            std::ref (start_flag),
            std::ref (l),
            iterations,
            std::ref (b_finish)
        );

        auto start = std::chrono::high_resolution_clock::now ();
        start_flag.notify_all ();

        a.join ();
        b.join ();

        auto a_total = std::chrono::duration_cast<std::chrono::nanoseconds> (a_finish - start).count ();
        auto b_total = std::chrono::duration_cast<std::chrono::nanoseconds> (b_finish - start).count ();

        auto diff = cruft::max (a_total, b_total) / float (cruft::min (a_total, b_total));
        auto rel  = cruft::abs (1 - diff);
        tap.expect_lt (rel, 0.1f, "reasonably fair under contention");
    }

    return tap.status ();
}