libcruft-util/test/thread/ticketlock.cpp

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/*
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* 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");
}
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return tap.status ();
}