怎麼突然研究memory ordering?
因為我幻想在M1 macbook pro可以用QEMU多核心模擬x86並且在上面操作,結論是可以用但是只能單核心慢到爆炸,開機可能要十分鐘
UTM模拟器(QEMU for iOS)的二三事上面聊到utm issue說無法在主機為arm(weaker memory order)模擬x86(weaker memory order)時利用multi-threaded Tiny Code Generator(MTTCG)所導致無法利用多核心.
所以我們先來探討memory ordering
MEMORY ORDERING
參考INTEL手冊 Volume 3 8.2.3.4
機械碼指令的順序可能為了效能被編譯器或是cpu(Dynamic Pipeline Scheduling)重新排序執行,在單線程的程式沒有執行問題,但是在多核心多線程的程式可能會發生問題這個就是memory reorder造成的錯誤
參考浅谈Memory Reordering,Weak vs. Strong Memory Models
- program ordering(strong ordering))
Intel386 processor enforces program ordering - processor ordering
gantt
section Section
Completed :done, des1, 2014-01-06,2014-01-08
Active :active, des2, 2014-01-07, 3d
Parallel 1 : des3, after des1, 1d
Parallel 2 : des4, after des1, 1d
Parallel 3 : des5, after des3, 1d
Parallel 4 : des6, after des4, 1d
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| #include <pthread.h>
#include <semaphore.h>
#include <cstdio>
#include <cstdlib>
#define USE_CPU_FENCE 0
#define USE_SINGLE_PROCESSOR 0
#if USE_SINGLE_PROCESSOR
#include <sched.h>
#endif
// Mersenne Twister Parameters
#define MT_N 624
#define MT_M 397
/* A thread-safe random number generator
*/
class MersenneTwister {
public:
explicit MersenneTwister(int seed);
unsigned int Integer();
private:
unsigned int buffer_[MT_N];
int index_;
};
MersenneTwister::MersenneTwister(int seed) {
buffer_[0] = seed;
for (index_ = 1; index_ < MT_N; ++index_) {
buffer_[index_] = (1812433253UL * (buffer_[index_-1]
^ (buffer_[index_-1] >> 30)) + index_);
}
}
unsigned int MersenneTwister::Integer() {
if (index_ >= MT_N) {
unsigned int i;
unsigned int x;
for (i = 0; i < MT_N - MT_M; ++i) {
x = (buffer_[i] & 0x80000000UL) | (buffer_[i+1] & 0x7fffffffUL);
buffer_[i] = buffer_[i+MT_M] ^ (x >> 1) ^ ((x & 1) * 0x9908b0dfUL);
}
for (; i < MT_N - 1; ++i) {
x = (buffer_[i] & 0x80000000UL) | (buffer_[i+1] & 0x7fffffffUL);
buffer_[i] = buffer_[i+MT_M-MT_N] ^ (x >> 1) ^ ((x & 1) * 0x9908b0dfUL);
}
x = (buffer_[MT_N-1] & 0x80000000UL) | (buffer_[0] & 0x7fffffffUL);
buffer_[MT_N-1] = buffer_[MT_M-1] ^ (x >> 1) ^ ((x & 1) * 0x9908b0dfUL);
index_ = 0;
}
unsigned int y = buffer_[index_++];
y ^= (y >> 11);
y ^= (y << 7) & 0x9d2c5680UL;
y ^= (y << 15) & 0xefc60000UL;
y ^= (y >> 18);
return y;
}
sem_t begin_sem1;
sem_t begin_sem2;
sem_t end_sem;
int X, Y;
int r1, r2;
void *ThreadFunc1(void *param) {
MersenneTwister random(1);
for (;;) {
sem_wait(&begin_sem1);
// random delay
while (random.Integer() % 8 != 0) {
}
X = 1;
#if USE_CPU_FENCE
asm volatile("mfence" ::: "memory"); // prevent CPU ordering
#else
asm volatile("" ::: "memory"); // prevent compiler ordering
#endif
r1 = Y;
sem_post(&end_sem);
}
return NULL;
}
void *ThreadFunc2(void *param) {
MersenneTwister random(2);
for (;;) {
sem_wait(&begin_sem2);
// random delay
while (random.Integer() % 8 != 0) {
}
Y = 1;
#if USE_CPU_FENCE
asm volatile("mfence" ::: "memory"); // prevent CPU ordering
#else
asm volatile("" ::: "memory"); // prevent compiler ordering
#endif
r2 = X;
sem_post(&end_sem);
}
return NULL;
}
int main(int argc, char *argv[]) {
sem_init(&begin_sem1, 0, 0);
sem_init(&begin_sem2, 0, 0);
sem_init(&end_sem, 0, 0);
pthread_t thread[2];
pthread_create(&thread[0], NULL, ThreadFunc1, NULL);
pthread_create(&thread[1], NULL, ThreadFunc2, NULL);
#if USE_SINGLE_PROCESSOR
cpu_set_t cpus;
CPU_ZERO(&cpus);
CPU_SET(0, &cpus);
pthread_setaffinity_np(thread[0], sizeof(cpu_set_t), &cpus);
pthread_setaffinity_np(thread[1], sizeof(cpu_set_t), &cpus);
#endif
int detected = 0;
for (int i = 1; ; ++i) {
X = 0;
Y = 0;
sem_post(&begin_sem1);
sem_post(&begin_sem2);
sem_wait(&end_sem);
sem_wait(&end_sem);
if (r1 == 0 && r2 == 0) {
detected++;
printf("%d reorders detected after %d iterations\n", detected, i);
}
}
return 0;
}
|
參考資源
