前言
最近项目上有大量的字符串数据需要存储到内存，并且需要储存至一定时间，于是自然而然的想到了使用字符串压缩算法对“源串”进行压缩存储。由此触发了对一些优秀压缩算法的调研。
字符串压缩，我们通常的需求有几个，一是高压缩率，二是压缩速率高，三是解压速率高。不过高压缩率与高压缩速率是鱼和熊掌的关系，不可皆得，优秀的算法一般也是采用压缩率与性能折中的方案。从压缩率、压缩速率、解压速率考虑，zstd与lz4有较好的压缩与解压性能，最终选取zstd与lz4进行调研。
zstd是facebook开源的提供高压缩比的快速压缩算法（参考https://github.com/facebook/zstd），很想了解一下它在压缩与解压方面的实际表现。
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 // compress13 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 // decompress35 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 << "comparess 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 content33 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 space40 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 file48 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 parameter55 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。
下一篇介绍调研的LZ4压缩算法。码字不易，还请各位技术爱好者登录点个赞呀！
