1 // Mini-benchmark for tsan VTS worst case performance
2 // Idea:
3 // 1) Spawn M + N threads (M >> N)
4 //    We'll call the 'M' threads as 'garbage threads'.
5 // 2) Make sure all threads have created thus no TIDs were reused
6 // 3) Join the garbage threads
7 // 4) Do many sync operations on the remaining N threads
8 //
9 // It turns out that due to O(M+N) VTS complexity the (4) is much slower with
10 // when N is large.
11 //
12 // Some numbers:
13 // a) clang++ native O1 with n_iterations=200kk takes
14 //      5s regardless of M
15 //    clang++ tsanv2 O1 with n_iterations=20kk takes
16 //      23.5s with M=200
17 //      11.5s with M=1
18 //    i.e. tsanv2 is ~23x to ~47x slower than native, depends on M.
19 // b) g++ native O1 with n_iterations=200kk takes
20 //      5.5s regardless of M
21 //    g++ tsanv1 O1 with n_iterations=2kk takes
22 //      39.5s with M=200
23 //      20.5s with M=1
24 //    i.e. tsanv1 is ~370x to ~720x slower than native, depends on M.
25 
26 #include <assert.h>
27 #include <pthread.h>
28 #include <stdio.h>
29 #include <stdlib.h>
30 
31 class __attribute__((aligned(64))) Mutex {
32  public:
Mutex()33   Mutex()  { pthread_mutex_init(&m_, NULL); }
~Mutex()34   ~Mutex() { pthread_mutex_destroy(&m_); }
Lock()35   void Lock() { pthread_mutex_lock(&m_); }
Unlock()36   void Unlock() { pthread_mutex_unlock(&m_); }
37 
38  private:
39   pthread_mutex_t m_;
40 };
41 
42 const int kNumMutexes = 1024;
43 Mutex mutexes[kNumMutexes];
44 
45 int n_threads, n_iterations;
46 
47 pthread_barrier_t all_threads_ready, main_threads_ready;
48 
GarbageThread(void * unused)49 void* GarbageThread(void *unused) {
50   pthread_barrier_wait(&all_threads_ready);
51   return 0;
52 }
53 
Thread(void * arg)54 void *Thread(void *arg) {
55   long idx = (long)arg;
56   pthread_barrier_wait(&all_threads_ready);
57 
58   // Wait for the main thread to join the garbage threads.
59   pthread_barrier_wait(&main_threads_ready);
60 
61   printf("Thread %ld go!\n", idx);
62   int offset = idx * kNumMutexes / n_threads;
63   for (int i = 0; i < n_iterations; i++) {
64     mutexes[(offset + i) % kNumMutexes].Lock();
65     mutexes[(offset + i) % kNumMutexes].Unlock();
66   }
67   printf("Thread %ld done\n", idx);
68   return 0;
69 }
70 
main(int argc,char ** argv)71 int main(int argc, char **argv) {
72   int n_garbage_threads;
73   if (argc == 1) {
74     n_threads = 2;
75     n_garbage_threads = 200;
76     n_iterations = 20000000;
77   } else if (argc == 4) {
78     n_threads = atoi(argv[1]);
79     assert(n_threads > 0 && n_threads <= 32);
80     n_garbage_threads = atoi(argv[2]);
81     assert(n_garbage_threads > 0 && n_garbage_threads <= 16000);
82     n_iterations = atoi(argv[3]);
83   } else {
84     printf("Usage: %s n_threads n_garbage_threads n_iterations\n", argv[0]);
85     return 1;
86   }
87   printf("%s: n_threads=%d n_garbage_threads=%d n_iterations=%d\n",
88          __FILE__, n_threads, n_garbage_threads, n_iterations);
89 
90   pthread_barrier_init(&all_threads_ready, NULL, n_garbage_threads + n_threads + 1);
91   pthread_barrier_init(&main_threads_ready, NULL, n_threads + 1);
92 
93   pthread_t *t = new pthread_t[n_threads];
94   {
95     pthread_t *g_t = new pthread_t[n_garbage_threads];
96     for (int i = 0; i < n_garbage_threads; i++) {
97       int status = pthread_create(&g_t[i], 0, GarbageThread, NULL);
98       assert(status == 0);
99     }
100     for (int i = 0; i < n_threads; i++) {
101       int status = pthread_create(&t[i], 0, Thread, (void*)i);
102       assert(status == 0);
103     }
104     pthread_barrier_wait(&all_threads_ready);
105     printf("All threads started! Killing the garbage threads.\n");
106     for (int i = 0; i < n_garbage_threads; i++) {
107       pthread_join(g_t[i], 0);
108     }
109     delete [] g_t;
110   }
111   printf("Resuming the main threads.\n");
112   pthread_barrier_wait(&main_threads_ready);
113 
114 
115   for (int i = 0; i < n_threads; i++) {
116     pthread_join(t[i], 0);
117   }
118   delete [] t;
119   return 0;
120 }
121