一、zstd压缩与解压
ZSTD_compress属于ZSTD的Simple API范畴，只有压缩级别可以设置。
ZSTD_compress函数原型如下：
size_t ZSTD_compress(void* dst, size_t dstCapacity, const void* src, size_t srcSize, int compressionLevel)
ZSTD_decompress函数原型如下：
size_t ZSTD_decompress( void* dst, size_t dstCapacity, const void* src, size_t compressedSize);
我们先来看看zstd的压缩与解压缩示例。
1 #include <stdio.h>
2 #include <string.h>
3 #include <sys/time.h>
4 #include <malloc.h>
5 #include <zstd.h>
6 #include <iostream>
7
8 using namespace std;
9
10 int main()
11 {
12 // compress
13 size_t com_space_size;
14 size_t peppa_pig_text_size;
15
16 char *com_ptr = NULL;
17 char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
18
19 peppa_pig_text_size = strlen(peppa_pig_buf);
20 com_space_size= ZSTD_compressBound(peppa_pig_text_size);
21 com_ptr = (char *)malloc(com_space_size);
22 if(NULL == com_ptr) {
23 cout << "compress malloc failed" << endl;
24 return -1;
25 }
26
27 size_t com_size;
28 com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);
29 cout << "peppa pig text size:" << peppa_pig_text_size << endl;
30 cout << "compress text size:" << com_size << endl;
31 cout << "compress ratio:" << (float)peppa_pig_text_size / (float)com_size << endl << endl;
32
33
34 // decompress
35 char* decom_ptr = NULL;
36 unsigned long long decom_buf_size;
37 decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);
38
39 decom_ptr = (char *)malloc((size_t)decom_buf_size);
40 if(NULL == decom_ptr) {
41 cout << "decompress malloc failed" << endl;
42 return -1;
43 }
44
45 size_t decom_size;
46 decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
47 cout << "decompress text size:" << decom_size << endl;
48
49 if(strncmp(peppa_pig_buf, decom_ptr, peppa_pig_text_size)) {
50 cout << "decompress text is not equal peppa pig text" << endl;
51 }
52
53 free(com_ptr);
54 free(decom_ptr);
55 return 0;
56 }
执行结果：
从结果可以发现，压缩之前的peppa pig文本长度为1827，压缩后的文本长度为759，压缩率为2.4，解压后的长度与压缩前相等。
另外，上文提到可以调整ZSTD_compress函数的压缩级别，zstd的默认级别为ZSTD_CLEVEL_DEFAULT = 3，最小值为0，最大值为ZSTD_MAX_CLEVEL = 22。另外也提供一些策略设置，例如 ZSTD_fast, ZSTD_greedy, ZSTD_lazy, ZSTD_lazy2, ZSTD_btlazy2。压缩级别越高，压缩率越高，但是压缩速率越低。
二、ZSTD压缩与解压性能探索
上面探索了zstd的基础压缩与解压方法，接下来再摸索一下zstd的压缩与解压缩性能。
测试方法是，使用ZSTD_compress连续压缩同一段文本并持续10秒，最后得到每一秒的平均压缩速率。测试压缩性能的代码示例如下：
1 #include <stdio.h>
2 #include <string.h>
3 #include <sys/time.h>
4 #include <malloc.h>
5 #include <zstd.h>
6 #include <iostream>
7
8 using namespace std;
9
10 int main()
11 {
12 int cnt = 0;
13
14 size_t com_size;
15 size_t com_space_size;
16 size_t peppa_pig_text_size;
17
18 char *com_ptr = NULL;
19 char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
20
21 timeval st, et;
22
23 peppa_pig_text_size = strlen(peppa_pig_buf);
24 com_space_size= ZSTD_compressBound(peppa_pig_text_size);
25
26 gettimeofday(&st, NULL);
27 while(1) {
28
29 com_ptr = (char *)malloc(com_space_size);
30 com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);
31
32 free(com_ptr);
33 cnt++;
34
35 gettimeofday(&et, NULL);
36 if(et.tv_sec - st.tv_sec >= 10) {
37 break;
38 }
39 }
40
41 cout << "compress per second:" << cnt/10 << " times" << endl;
42 return 0;
43 }
执行结果：
结果显示ZSTD的压缩性能大概在每秒6-7万次左右，这个结果其实并不是太理想。需要说明的是压缩性能与待压缩文本的长度、字符内容也是有关系的。
我们再来探索一下ZSTD的解压缩性能。与上面的测试方法类似，先对本文进行压缩，然后连续解压同一段被压缩过的数据并持续10秒，最后得到每一秒的平均解压速率。测试解压性能的代码示例如下：
1 #include <stdio.h>
2 #include <string.h>
3 #include <sys/time.h>
4 #include <malloc.h>
5 #include <zstd.h>
6 #include <iostream>
7
8 using namespace std;
9
10 int main()
11 {
12 int cnt = 0;
13
14 size_t com_size;
15 size_t com_space_size;
16 size_t peppa_pig_text_size;
17
18 timeval st, et;
19
20 char *com_ptr = NULL;
21 char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
22
23 size_t decom_size;
24 char* decom_ptr = NULL;
25 unsigned long long decom_buf_size;
26
27 peppa_pig_text_size = strlen(peppa_pig_buf);
28 com_space_size= ZSTD_compressBound(peppa_pig_text_size);
29 com_ptr = (char *)malloc(com_space_size);
30
31 com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, 1);
32
33 gettimeofday(&st, NULL);
34 decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);
35
36 while(1) {
37
38 decom_ptr = (char *)malloc((size_t)decom_buf_size);
39
40 decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
41 if(decom_size != peppa_pig_text_size) {
42
43 cout << "decompress error" << endl;
44 break;
45 }
46
47 free(decom_ptr);
48
49 cnt++;
50 gettimeofday(&et, NULL);
51 if(et.tv_sec - st.tv_sec >= 10) {
52 break;
53 }
54 }
55
56 cout << "decompress per second:" << cnt/10 << " times" << endl;
57
58 free(com_ptr);
59 return 0;
60 }
执行结果：
结果显示ZSTD的解压缩性能大概在每秒12万次左右，解压性能比压缩性能高。
三、zstd的高级用法
zstd提供了一个名为PZSTD的压缩和解压工具。PZSTD（parallel zstd），并行压缩的zstd，是一个使用多线程对待压缩文本进行切片分段，且进行并行压缩的命令行工具。
其实高版本（v1.4.0及以上）的zstd也提供了指定多线程对文本进行并行压缩的相关API接口，也就是本小节要介绍的zstd高级API用法。下面我们再来探索一下zstd的多线程压缩使用方法。
多线程并行压缩的两个关键API，一个是参数设置API，另一个是压缩API。
参数设置API的原型是：
size_t ZSTD_CCtx_setParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int value)
压缩API的原型是：
size_t ZSTD_compress2(ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize)
下面给出zstd并行压缩的示例demo，通过ZSTD_CCtx_setParameter设置线程数为3，即指定宏ZSTD_c_nbWorkers为3，通过ZSTD_compress2压缩相关文本。另外，为了展示zstd确实使用了多线程，需要先读取一个非常大的文件，作为zstd的压缩文本源，尽量使zstd运行较长时间。
1 #include <stdio.h>
2 #include <string.h>
3 #include <sys/time.h>
4 #include <malloc.h>
5 #include <zstd.h>
6 #include <iostream>
7
8 using namespace std;
9
10 int main()
11 {
12 size_t com_size;
13 size_t com_space_size;
14
15 FILE *fp = NULL;
16 unsigned int file_len;
17
18 char *com_ptr = NULL;
19 char *file_text_ptr = NULL;
20
21 fp = fopen("xxxxxx", "r");
22 if(NULL == fp){
23 cout << "file open failed" << endl;
24 return -1;
25 }
26
27 fseek(fp, 0, SEEK_END);
28 file_len = ftell(fp);
29 fseek(fp, 0, SEEK_SET);
30 cout << "file length:" << file_len << endl;
31
32 // malloc space for file content
33 file_text_ptr = (char *)malloc(file_len);
34 if(NULL == file_text_ptr) {
35 cout << "malloc failed" << endl;
36 return -1;
37 }
38
39 // malloc space for compress space
40 com_space_size = ZSTD_compressBound(file_len);
41 com_ptr = (char *)malloc(com_space_size);
42 if(NULL == com_ptr) {
43 cout << "malloc failed" << endl;
44 return -1;
45 }
46
47 // read text from source file
48 fread(file_text_ptr, 1, file_len, fp);
49 fclose(fp);
50
51 ZSTD_CCtx* cctx;
52 cctx = ZSTD_createCCtx();
53
54 // set multi-thread parameter
55 ZSTD_CCtx_setParameter(cctx, ZSTD_c_nbWorkers, 3);
56 ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, ZSTD_btlazy2);
57
58 com_size = ZSTD_compress2(cctx, com_ptr, com_space_size, file_text_ptr, file_len);
59
60 free(com_ptr);
61 free(file_text_ptr);
62 return 0;
63 }
运行上述demo，可见zstd确实启动了3个线程对文本进行了并行压缩。且设置的线程数越多，压缩时间越短，这里就不详细展示了，读者可以自行实验。
需要说明的是，zstd当前默认编译单线程的库文件，要实现多线程的API调用，需要在make的时候指定编译参数ZSTD_MULTITHREAD。
另外，zstd还支持线程池的方式，线程池的函数原型：
POOL_ctx* ZSTD_createThreadPool(size_t numThreads)
线程池可以避免在多次、连续压缩场景时频繁的去创建线程、撤销线程产生的非必要开销，使得算力主要开销在文本压缩方面。
四、总结
本篇分享了zstd压缩与解压缩使用的基本方法，对压缩与解压的性能进行了摸底，最后探索了zstd多线程压缩的使用方法。
从压缩测试来看，zstd的压缩比其实已经比较好了，比原文所占用空间缩小了一半以上，当然压缩比也跟待压缩文本的内容有关。
从性能执行结果来看，zstd的压缩与解压性能表现比较勉强，我认为zstd在鱼（性能）和熊掌（压缩比）之间更偏向熊掌一些，不过对一些性能要求不太高的，但是要高压缩比的场景是比较符合的。
多线程并行压缩，在有大文本需要连续多次压缩的场景下，结合线程池可以很好的提升压缩速率。