job/ticketlock: add a trivial ticketlock

CMakeLists.txt

@@ -270,6 +270,8 @@ list (
     job/flag.hpp
     job/queue.cpp
     job/queue.hpp
+    job/ticketlock.cpp
+    job/ticketlock.hpp
     job/spinlock.cpp
     job/spinlock.hpp
     json/fwd.hpp
@@ -483,6 +485,7 @@ if (TESTS)
         job/flag
         job/queue
         job/spinlock
+        job/ticketlock
         json_types
         json2/event
         maths

job/ticketlock.cpp

@@ -0,0 +1,45 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "ticketlock.hpp"
+
+using util::job::ticketlock;
+
+
+///////////////////////////////////////////////////////////////////////////////
+ticketlock::ticketlock ():
+    current (0),
+    next (0)
+{ }
+
+
+///////////////////////////////////////////////////////////////////////////////
+void
+ticketlock::lock (void)
+{
+    auto self = next++;
+
+    while (current != self)
+        __asm__ volatile ("pause");
+}
+
+
+//-----------------------------------------------------------------------------
+void
+ticketlock::unlock (void)
+{
+    ++current;
+}

job/ticketlock.hpp

@@ -0,0 +1,36 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#ifndef CRUFT_UTIL_JOB_TICKETLOCK_HPP
+#define CRUFT_UTIL_JOB_TICKETLOCK_HPP
+
+#include <atomic>
+
+namespace util::job {
+    class ticketlock {
+    public:
+        ticketlock ();
+
+        void lock (void);
+        void unlock (void);
+
+    private:
+        std::atomic<unsigned> current;
+        std::atomic<unsigned> next;
+    };
+};
+
+#endif

test/job/ticketlock.cpp

@@ -0,0 +1,93 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Copyright 2018 Danny Robson <danny@nerdcruft.net>
+ */
+
+#include "job/flag.hpp"
+#include "job/ticketlock.hpp"
+#include "tap.hpp"
+
+#include <iostream>
+
+#include <thread>
+#include <mutex>
+
+
+///////////////////////////////////////////////////////////////////////////////
+using ffs_t = std::chrono::high_resolution_clock;
+
+
+//-----------------------------------------------------------------------------
+void
+fight (util::job::flag &start, util::job::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 ()
+{
+    util::TAP::logger tap;
+
+    util::job::ticketlock l;
+    l.lock ();
+    tap.expect (true, "locked without contention");
+
+    l.unlock ();
+    tap.expect (true, "unlocked without contention");
+
+    if (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;
+        util::job::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 ();
+
+        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 = util::max (a_total, b_total) / float (util::min (a_total, b_total));
+        auto rel  = util::abs (1 - diff);
+        std::cout << rel << '\n';
+        tap.expect_lt (rel, 0.1f, "reasonably fair under contention");
+    }
+}