pthread vs openMP之我见
前两天看了些并行计算的文章，了解了一些并行计算的方法和原理。然后发现多线程实现里面还有个openMP，这个以前从来没见过（火星了），之前只是知道pthread线程库和微软也实现了一套线程。又看了看openMP的一些教程才知道它是怎么回事。
pthread全称应该是POSIX THREAD，顾名思义这个肯定是按照POSIX对线程的标准而设计的。目前我所知道的有两个版本：Linux Thread（较早）和NPTL（主流？）。pthread库是一套关于线程的API，提供“遵循”（各平台实现各异）POSIX标准的线程相关的功能。
openMP不同于pthread的地方是，它是根植于编译器的（也要包含头文件omp.h），而不是在各系统平台是做文章。它貌似更偏向于将原来串行化的程序，通过加入一些适当的编译器指令（compiler directive）变成并行执行，从而提高代码运行的速率。如：
1
#include
<
omp.h
>
2
#include
<
stdio.h
>
3
4
#define
ARRAY_SIZE 1000
5
#define
CHUNK_SIZE 100
6
7
int
main()
8
{
9
int
array[ARRAY_SIZE];
10
int
thread_num
=
ARRAY_SIZE
/
CHUNK_SIZE
+
1
;
11
omp_set_num_threads(thread_num);
12
13
//
init array
14
int
i;
15
for
(i
=
0
;i
<
ARRAY_SIZE;i
++
)
16
{
17
array[i]
=
i;
18
}
19
20
#pragma
omp parallel for schedule(guided,CHUNK_SIZE) private(i)
21
for
(i
=
0
;i
<
ARRAY_SIZE;i
++
)
22
{
23
int
n
=
array[i];
24
int
num_of_one
=
0
;
25
if
(n
!=
0
)
26
{
27
num_of_one
++
;
28
while
((n
=
n
&
(n
-
1
))
!=
0
)
29
{
30
num_of_one
++
;
31
}
32
}
33
array[i]
=
num_of_one;
34
35
}
36
for
(i
=
0
;i
<
ARRAY_SIZE;i
++
)
37
{
38
printf(
"
%d
"
,array[i]);
39
}
40
printf(
"
\n
"
);
41
return
0
;
42
43
}
44
45
上面一段代码是通过加了一条函数调用（11行）和一条编译器指令（20行），从而将原来的循环分给多个线程来做。（本程序是计算0~ArraySize-1的每个数中二进制包含1个数）。
而对于一开始就打算用并行方法来实现的程序，用pthread应该是更方便和更清晰。
下面是分别用pthread和openMP实现的worker_and_consumer:
pthread版：
代码
1
#include
<
unistd.h
>
2
#include
<
pthread.h
>
3
#include
<
stdio.h
>
4
#include
<
stdlib.h
>
5
6
#define
SIZE 100
7
#define
THREAD_NUM_WORKER 15
8
#define
THREAD_NUM_CONSUMER 10
9
#define
SLEEP_WORKERS 2
10
#define
SLEEP_CONSUMERS 1
11
12
int
warehouse[SIZE];
13
int
at
=-
1
;
14
int
is_end
=
0
;
15
pthread_mutex_t space
=
PTHREAD_MUTEX_INITIALIZER;
16
pthread_mutex_t end
=
PTHREAD_MUTEX_INITIALIZER;
17
18
void
*
consumer_func(
void
*
);
19
void
*
worker_func(
void
*
);
20
21
int
main()
22
{
23
pthread_t workers[THREAD_NUM_WORKER];
24
pthread_t consumers[THREAD_NUM_CONSUMER];
25
int
i,j;
26
int
n;
27
for
(i
=
0
;i
<
THREAD_NUM_WORKER;i
++
)
28
pthread_create(
&
workers[i],NULL,worker_func,NULL);
29
for
(j
=
0
;j
<
THREAD_NUM_CONSUMER;j
++
)
30
pthread_create(
&
consumers[j],NULL,consumer_func,NULL);
31
while
(is_end
==
0
)
32
{
33
scanf(
"
%d
"
,
&
n);
34
if
(n
==
0
)
35
{
36
pthread_mutex_lock(
&
end);
37
is_end
=
1
;
38
pthread_mutex_unlock(
&
end);
39
}
40
}
41
for
(i
=
0
;i
<
THREAD_NUM_WORKER;i
++
)
42
pthread_join(workers[i],NULL);
43
for
(j
=
0
;j
<
THREAD_NUM_CONSUMER;j
++
)
44
pthread_join(consumers[j],NULL);
45
return
0
;
46
}
47
48
void
*
worker_func(
void
*
var)
49
{
50
while
(
1
)
51
{
52
if
(is_end)
53
break
;
54
//
保护at变量
55
pthread_mutex_lock(
&
space);
56
if
(SIZE
-
at
-
1
>
0
)
57
{
58
printf(
"
Make %d by worker %lld
"
,warehouse[
++
at]
=
rand(),pthread_self());
59
printf(
"
and at is %d\n
"
,at);
60
}
61
pthread_mutex_unlock(
&
space);
62
sleep(SLEEP_WORKERS);
63
}
64
return
NULL;
65
}
66
67
68
void
*
consumer_func(
void
*
var)
69
{
70
while
(
1
)
71
{
72
if
(is_end)
73
break
;
74
pthread_mutex_lock(
&
space);
75
if
(at
>=
0
)
76
{
77
printf(
"
Got %d by consumer %lld\n
"
,warehouse[at
--
],pthread_self());
78
printf(
"
and at is %d\n
"
,at);
79
}
80
pthread_mutex_unlock(
&
space);
81
sleep(SLEEP_CONSUMERS);
82
}
83
return
NULL;
84
}
85
86
87
88
openMP版：
代码
1
#include
<
unistd.h
>
2
#include
<
stdio.h
>
3
#include
<
stdlib.h
>
4
#include
<
omp.h
>
5
6
7
#define
SIZE 100
8
#define
THREAD_NUM_WORKER 15
9
#define
THREAD_NUM_CONSUMER 10
10
#define
SLEEP_WORKERS 2
11
#define
SLEEP_CONSUMERS 1
12
13
int
warehouse[SIZE];
14
int
at
=-
1
;
15
int
is_end
=
0
;
16
17
void
start_workers()
18
{
19
omp_set_num_threads(THREAD_NUM_WORKER);
20
#pragma
omp parallel default(shared)
21
{
22
if
(omp_get_thread_num()
==
0
)
23
printf(
"
worker num is %d\n
"
,omp_get_num_threads());
24
while
(
1
)
25
{
26
if
(is_end)
27
break
;
28
//
保护at变量
29
#pragma
omp critical(space)
30
{
31
if
(SIZE
-
at
-
1
>
0
)
32
{
33
printf(
"
Make %d by worker %d
"
,warehouse[
++
at]
=
rand(),omp_get_thread_num());
34
printf(
"
and at is %d\n
"
,at);
35
}
36
}
37
sleep(SLEEP_WORKERS);
38
}
39
}
40
}
41
42
43
void
start_consumers(
void
)
44
{
45
omp_set_num_threads(THREAD_NUM_CONSUMER);
46
#pragma
omp parallel default(shared)
47
{
48
if
(omp_get_thread_num()
==
0
)
49
printf(
"
consumer num is %d\n
"
,omp_get_num_threads());
50
while
(
1
)
51
{
52
if
(is_end)
53
break
;
54
#pragma
omp critical(space)
55
{
56
if
(at
>=
0
)
57
{
58
printf(
"
Got %d by consumer %d\n
"
,warehouse[at
--
],omp_get_thread_num());
59
printf(
"
and at is %d\n
"
,at);
60
}
61
}
62
sleep(SLEEP_CONSUMERS);
63
}
64
}
65
}
66
67
int
main()
68
{
69
omp_set_dynamic(
0
);
70
omp_set_nested(
1
);
//
这个不设置的话，就不能嵌套fork子线程咯
71
//
先设置3个线程，每个线程完成一个section
72
omp_set_num_threads(
3
);
73
#pragma
omp parallel sections
74
{
75
#pragma
omp section
76
{
77
start_workers();
78
}
79
#pragma
omp section
80
{
81
start_consumers();
82
}
83
#pragma
omp section
84
{
85
int
in
;
86
scanf(
"
%d
"
,
&
in
);
87
if
(
!
in
)
88
{
89
//
保护is_end
90
#pragma
omg critical(end)
91
is_end
=
1
;
92
}
93
}
94
}
95
return
0
;
96
}
97
98
99
最后说一下，用openMP，编译时要加上选项-fopenmp，编译pthread时加上链接-lpthread。另外openMP有个缺点是若是代码中编译指令出错时，找错还是挺麻烦的，就像昨晚我把#pragma omp parallel写成了#pragma omg parallel，结果编译链接通过后却始终只有一个线程（主线程），找了好久...囧！