[英]Unable to set the OpenMP threads affinity in a forked process
我試圖使用openMP在不同的CPU上運行兩個進程。 在這種情況下,每個CPU都有6個帶有超線程的內核(因此有12個硬件線程)。 他們需要做一些同步,如果他們知道彼此的PID就會更容易。 所以我使用fork()
和execve()
從sigS
啟動進程sigC
,並為GOMP_CPU_AFFINITY
環境變量調用不同的值。 在fork()/execve()
調用之后, sigS
仍具有正確的親和力,但sigC
打印
libgomp: no cpus left for affinity setting
並且所有線程都在同一個核心上。
sigS
的代碼:
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <omp.h>
#include <sched.h>
int main( void )
{
omp_set_num_threads(12); //12 hardware threads per CPU
//this loop runs as expected
#pragma omp parallel for
for( int i = 0; i<12; i++ ) {
#pragma omp critical
{
printf("TEST PRE-FORK: I am thread %2d running on core %d\n",
omp_get_thread_num(), sched_getcpu());
}
}
pid_t childpid = fork();
if( childpid < 0 ) {
perror("Fork failed");
} else {
if( childpid == 0 ) { //<------ attempt to set affinity for child
//change the affinity for the other process so it runs
//on the other cpu
char ompEnv[] = "GOMP_CPU_AFFINITY=6-11 18-23";
char * const args[] = { "./sigC", (char*)0 };
char * const envArgs[] = { ompEnv, (char*)0 };
execve(args[0], args, envArgs);
perror("Returned from execve");
exit(1);
} else {
omp_set_num_threads(12);
printf("PARENT: my pid = %d\n", getpid());
printf("PARENT: child pid = %d\n", childpid);
sleep(5); //sleep for a bit so child process prints first
//This loop gives the same thread core/pairings as above
//this is expected
#pragma omp parallel for
for( int i = 0; i < 12; i++ ) {
#pragma omp critical
{
printf("PARENT: I'm thread %2d, on core %d.\n",
omp_get_thread_num(), sched_getcpu());
}
}
}
}
return 0;
}
sigC
的代碼只有一個omp並行for循環,但為了完整性:
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <omp.h>
#include <sched.h>
int main( void )
{
omp_set_num_threads(12);
printf("CHILD: my pid = %d\n", getpid());
printf("CHILD: parent pid = %d\n", getppid());
//I expect this loop to have the core pairings as I specified in execve
//i.e thread 0 -> core 6, 1 -> 7, ... 6 -> 18, 7 -> 19 ... 11 -> 23
#pragma omp parallel for
for( int i = 0; i < 12; i++ ) {
#pragma omp critical
{
printf("CHILD: I'm thread %2d, on core %d.\n",
omp_get_thread_num(), sched_getcpu());
}
}
return 0;
}
輸出:
$ env GOMP_CPU_AFFINITY="0-5 12-17" ./sigS
這部分是預期的
TEST PRE-FORK: I'm thread 0, on core 0.
TEST PRE-FORK: I'm thread 11, on core 17.
TEST PRE-FORK: I'm thread 5, on core 5.
TEST PRE-FORK: I'm thread 6, on core 12.
TEST PRE-FORK: I'm thread 3, on core 3.
TEST PRE-FORK: I'm thread 1, on core 1.
TEST PRE-FORK: I'm thread 8, on core 14.
TEST PRE-FORK: I'm thread 10, on core 16.
TEST PRE-FORK: I'm thread 7, on core 13.
TEST PRE-FORK: I'm thread 2, on core 2.
TEST PRE-FORK: I'm thread 4, on core 4.
TEST PRE-FORK: I'm thread 9, on core 15.
PARENT: my pid = 11009
PARENT: child pid = 11021
這就是問題 - 孩子中的所有線程都在核心0上運行
libgomp: no CPUs left for affinity setting
CHILD: my pid = 11021
CHILD: parent pid = 11009
CHILD: I'm thread 1, on core 0.
CHILD: I'm thread 0, on core 0.
CHILD: I'm thread 4, on core 0.
CHILD: I'm thread 5, on core 0.
CHILD: I'm thread 6, on core 0.
CHILD: I'm thread 7, on core 0.
CHILD: I'm thread 8, on core 0.
CHILD: I'm thread 9, on core 0.
CHILD: I'm thread 10, on core 0.
CHILD: I'm thread 11, on core 0.
CHILD: I'm thread 3, on core 0.
(我省略了父線程打印,因為它與pre-fork相同)
關於我如何解決這個問題或是否是正確方法的任何想法?
fork()
ed子進程繼承其父關聯掩碼。 libgomp
將此親和力掩碼與GOMP_CPU_AFFINITY
的集合GOMP_CPU_AFFINITY
,最后得到一個空集,因為兩個集都是互補的。 這種行為沒有記錄,但是看一下libgomp
的源代碼libgomp
證實確實如此。
解決方案是在進行execve()
調用之前重置子進程的affinity mask:
if (childpid == 0) { //<------ attempt to set affinity for child
cpu_set_t *mask;
size_t size;
int nrcpus = 256; // 256 CPUs should be more than enough
// Reset the CPU affinity mask
mask = CPU_ALLOC(nrcpus);
size = CPU_ALLOC_SIZE(nrcpus);
for (int i = 0; i < nrcpus; i++)
CPU_SET_S(i, size, mask);
if (sched_setaffinity(0, size, mask) == -1) { handle error }
CPU_FREE(mask);
//change the affinity for the other process so it runs
//on the other cpu
char ompEnv[] ="GOMP_CPU_AFFINITY=6-11 18-23";
char * const args[] = {"./sigC", (char*)0};
char * const envArgs[] = {ompEnv, (char*)0};
execve(args[0], args, envArgs);
perror("Returned from execve");
exit(1);
} else {
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