C語言字符串壓縮之ZSTD算法詳解

前言

最近項目上有大量的字符串數據需要存儲到內存,並且需要儲存至一定時間,於是自然而然的想到瞭使用字符串壓縮算法對“源串”進行壓縮存儲。由此觸發瞭對一些優秀壓縮算法的調研。

字符串壓縮,我們通常的需求有幾個,一是高壓縮率,二是壓縮速率高,三是解壓速率高。不過高壓縮率與高壓縮速率是魚和熊掌的關系,不可皆得,優秀的算法一般也是采用壓縮率與性能折中的方案。從壓縮率、壓縮速率、解壓速率考慮,zstd與lz4有較好的壓縮與解壓性能,最終選取zstd與lz4進行調研。

zstd是facebook開源的提供高壓縮比的快速壓縮算法(參考https://github.com/facebook/zstd),很想瞭解一下它在壓縮與解壓方面的實際表現。

一、zstd壓縮與解壓

ZSTD_compress屬於ZSTD的Simple API范疇,隻有壓縮級別可以設置。

ZSTD_compress函數原型如下:

size_t ZSTD_compress(void* dst, size_t dstCapacity, const void* src, size_t srcSize, int compressionLevel)

ZSTD_decompress函數原型如下:

size_t ZSTD_decompress( void* dst, size_t dstCapacity, const void* src, size_t compressedSize);  我們先來看看zstd的壓縮與解壓縮示例。

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>

using namespace std;

int main()
{
    // compress
    size_t com_space_size;
    size_t peppa_pig_text_size;

    char *com_ptr = NULL;
    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.";

    peppa_pig_text_size = strlen(peppa_pig_buf);
    com_space_size= ZSTD_compressBound(peppa_pig_text_size);
    com_ptr = (char *)malloc(com_space_size);
    if(NULL == com_ptr) {
        cout << "compress malloc failed" << endl;
        return -1;
    }

    size_t com_size;
    com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);
    cout << "peppa pig text size:" << peppa_pig_text_size << endl;
    cout << "compress text size:" << com_size << endl;
    cout << "compress ratio:" << (float)peppa_pig_text_size / (float)com_size << endl << endl;


    // decompress
    char* decom_ptr = NULL;
    unsigned long long decom_buf_size;
    decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);

    decom_ptr = (char *)malloc((size_t)decom_buf_size);
    if(NULL == decom_ptr) {
        cout << "decompress malloc failed" << endl;
        return -1;
    }

    size_t decom_size;
    decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
    cout << "decompress text size:" << decom_size << endl;

    if(strncmp(peppa_pig_buf, decom_ptr, peppa_pig_text_size)) {
        cout << "decompress text is not equal peppa pig text" << endl;
    }

    free(com_ptr);
    free(decom_ptr);
    return 0;
}

執行結果:

從結果可以發現,壓縮之前的peppa pig文本長度為1827,壓縮後的文本長度為759,壓縮率為2.4,解壓後的長度與壓縮前相等。

另外,上文提到可以調整ZSTD_compress函數的壓縮級別,zstd的默認級別為ZSTD_CLEVEL_DEFAULT = 3,最小值為0,最大值為ZSTD_MAX_CLEVEL = 22。另外也提供一些策略設置,例如 ZSTD_fast, ZSTD_greedy, ZSTD_lazy, ZSTD_lazy2, ZSTD_btlazy2。壓縮級別越高,壓縮率越高,但是壓縮速率越低。

二、ZSTD壓縮與解壓性能探索

上面探索瞭zstd的基礎壓縮與解壓方法,接下來再摸索一下zstd的壓縮與解壓縮性能。

測試方法是,使用ZSTD_compress連續壓縮同一段文本並持續10秒,最後得到每一秒的平均壓縮速率。測試壓縮性能的代碼示例如下:

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>

using namespace std;

int main()
{
    int cnt = 0;

    size_t com_size;
    size_t com_space_size;
    size_t peppa_pig_text_size;

    char *com_ptr = NULL;
    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.";

    timeval st, et;

    peppa_pig_text_size = strlen(peppa_pig_buf);
    com_space_size= ZSTD_compressBound(peppa_pig_text_size);

    gettimeofday(&st, NULL);
    while(1) {

        com_ptr = (char *)malloc(com_space_size);
        com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);

        free(com_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << "compress per second:" << cnt/10 << " times" << endl;
    return 0;
}

執行結果:

結果顯示ZSTD的壓縮性能大概在每秒6-7萬次左右,這個結果其實並不是太理想。需要說明的是壓縮性能與待壓縮文本的長度、字符內容也是有關系的。

我們再來探索一下ZSTD的解壓縮性能。與上面的測試方法類似,先對本文進行壓縮,然後連續解壓同一段被壓縮過的數據並持續10秒,最後得到每一秒的平均解壓速率。測試解壓性能的代碼示例如下:

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>

using namespace std;

int main()
{
    int cnt = 0;

    size_t com_size;
    size_t com_space_size;
    size_t peppa_pig_text_size;

    timeval st, et;

    char *com_ptr = NULL;
    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.";

    size_t decom_size;
    char* decom_ptr = NULL;
    unsigned long long decom_buf_size;

    peppa_pig_text_size = strlen(peppa_pig_buf);
    com_space_size= ZSTD_compressBound(peppa_pig_text_size);
    com_ptr = (char *)malloc(com_space_size);

    com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, 1);

    gettimeofday(&st, NULL);
    decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);

    while(1) {

        decom_ptr = (char *)malloc((size_t)decom_buf_size);

        decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
        if(decom_size != peppa_pig_text_size) {

            cout << "decompress error" << endl;
            break;
        }

        free(decom_ptr);

        cnt++;
        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << "decompress per second:" << cnt/10 << " times" << endl;

    free(com_ptr);
    return 0;
}

執行結果:

結果顯示ZSTD的解壓縮性能大概在每秒12萬次左右,解壓性能比壓縮性能高。

三、zstd的高級用法

zstd提供瞭一個名為PZSTD的壓縮和解壓工具。PZSTD(parallel zstd),並行壓縮的zstd,是一個使用多線程對待壓縮文本進行切片分段,且進行並行壓縮的命令行工具。

其實高版本(v1.4.0及以上)的zstd也提供瞭指定多線程對文本進行並行壓縮的相關API接口,也就是本小節要介紹的zstd高級API用法。下面我們再來探索一下zstd的多線程壓縮使用方法。

多線程並行壓縮的兩個關鍵API,一個是參數設置API,另一個是壓縮API。

參數設置API的原型是:

size_t ZSTD_CCtx_setParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int value)

壓縮API的原型是:

size_t ZSTD_compress2(ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize)

下面給出zstd並行壓縮的示例demo,通過ZSTD_CCtx_setParameter設置線程數為3,即指定宏ZSTD_c_nbWorkers為3,通過ZSTD_compress2壓縮相關文本。另外,為瞭展示zstd確實使用瞭多線程,需要先讀取一個非常大的文件,作為zstd的壓縮文本源,盡量使zstd運行較長時間。

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>

using namespace std;

int main()
{
    size_t com_size;
    size_t com_space_size;

    FILE *fp = NULL;
    unsigned int file_len;

    char *com_ptr = NULL;
    char *file_text_ptr = NULL;

    fp = fopen("xxxxxx", "r");
    if(NULL == fp){
         cout << "file open failed" << endl;
         return -1;
    }

    fseek(fp, 0, SEEK_END);
    file_len = ftell(fp);
    fseek(fp, 0, SEEK_SET);
    cout << "file length:" << file_len << endl;

    // malloc space for file content
    file_text_ptr = (char *)malloc(file_len);
    if(NULL == file_text_ptr) {
        cout << "malloc failed" << endl;
        return -1;
    }

    // malloc space for compress space
    com_space_size = ZSTD_compressBound(file_len);
    com_ptr = (char *)malloc(com_space_size);
    if(NULL == com_ptr) {
        cout << "malloc failed" << endl;
        return -1;
    }

    // read text from source file
    fread(file_text_ptr, 1, file_len, fp);
    fclose(fp);

    ZSTD_CCtx* cctx;
    cctx = ZSTD_createCCtx();

    // set multi-thread parameter
    ZSTD_CCtx_setParameter(cctx, ZSTD_c_nbWorkers, 3);
    ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, ZSTD_btlazy2);

    com_size = ZSTD_compress2(cctx, com_ptr, com_space_size, file_text_ptr, file_len);

    free(com_ptr);
    free(file_text_ptr);
    return 0;
}

運行上述demo,可見zstd確實啟動瞭3個線程對文本進行瞭並行壓縮。且設置的線程數越多,壓縮時間越短,這裡就不詳細展示瞭,讀者可以自行實驗。

需要說明的是,zstd當前默認編譯單線程的庫文件,要實現多線程的API調用,需要在make的時候指定編譯參數ZSTD_MULTITHREAD。

另外,zstd還支持線程池的方式,線程池的函數原型:

POOL_ctx* ZSTD_createThreadPool(size_t numThreads)

線程池可以避免在多次、連續壓縮場景時頻繁的去創建線程、撤銷線程產生的非必要開銷,使得算力主要開銷在文本壓縮方面。

四、總結

本篇分享瞭zstd壓縮與解壓縮使用的基本方法,對壓縮與解壓的性能進行瞭摸底,最後探索瞭zstd多線程壓縮的使用方法。

從壓縮測試來看,zstd的壓縮比其實已經比較好瞭,比原文所占用空間縮小瞭一半以上,當然壓縮比也跟待壓縮文本的內容有關。

從性能執行結果來看,zstd的壓縮與解壓性能表現比較勉強,我認為zstd在魚(性能)和熊掌(壓縮比)之間更偏向熊掌一些,不過對一些性能要求不太高的,但是要高壓縮比的場景是比較符合的。

多線程並行壓縮,在有大文本需要連續多次壓縮的場景下,結合線程池可以很好的提升壓縮速率。

以上就是C語言字符串壓縮之ZSTD算法詳解的詳細內容,更多關於C語言字符串壓縮的資料請關註WalkonNet其它相關文章!

推薦閱讀: