Redis 源码阅读（一）

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Redis 作为常用的内存数据库，代码精简易于阅读，所以成为我 Linux C 开发学习之首选。
Redis源码版本：4.0.2

本章主要看内存相关操作。

zmalloc.c & zmalloc.h

定义各种内存申请操作。

提供了三种库，一种是google的tcmalloc，一种是jemalloc，还有一种就是malloc

zmalloc

最简单的申请内存。加了一个size_t的空间，记录当前申请的空间的大小。

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void *zmalloc(size_t size) {
    void *ptr = malloc(size+PREFIX_SIZE);

    //oom : out of memory
    //zmalloc_oom 是一个错误处理函数，输出错误信息
    if (!ptr) zmalloc_oom_handler(size);
#ifdef HAVE_MALLOC_SIZE
    update_zmalloc_stat_alloc(zmalloc_size(ptr));
    return ptr;
#else
    //如果未定义malloc_size，那么在第一个位置写上size，更新used_memory后返回真正内容的地址
    *((size_t*)ptr) = size;
    update_zmalloc_stat_alloc(size+PREFIX_SIZE);
    return (char*)ptr+PREFIX_SIZE;
#endif
}

如果未提供zmalloc_size函数，默认方法是向前取PREFIX_SIZE拿到size，相加得到真正的size（在刚才的情况中并不会被调用到）

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#ifndef HAVE_MALLOC_SIZE
size_t zmalloc_size(void *ptr) {
    void *realptr = (char*)ptr-PREFIX_SIZE;
    size_t size = *((size_t*)realptr);
    /* Assume at least that all the allocations are padded at sizeof(long) by
     * the underlying allocator. */
    if (size&(sizeof(long)-1)) size += sizeof(long)-(size&(sizeof(long)-1));
    return size+PREFIX_SIZE;
}
#endif

其中update_zmalloc_stat_alloc作用是更新used_memory。首先将n用long对齐，然后原子加used_memory

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#define update_zmalloc_stat_alloc(__n) do { \
    size_t _n = (__n); \
    if (_n&(sizeof(long)-1)) _n += sizeof(long)-(_n&(sizeof(long)-1)); \
    atomicIncr(used_memory,__n); \
} while(0)

atomicIncr定义如下，__atomic_add_fetch是内建函数，原型为type __atomic_add_fetch (type *ptr, type val, int memorder)，详见文档。

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#define atomicIncr(var,count) __atomic_add_fetch(&var,(count),__ATOMIC_RELAXED)

zfree

释放内存

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void zfree(void *ptr) {
#ifndef HAVE_MALLOC_SIZE
    void *realptr;
    size_t oldsize;
#endif

    if (ptr == NULL) return;
#ifdef HAVE_MALLOC_SIZE
    update_zmalloc_stat_free(zmalloc_size(ptr));
    free(ptr);
#else
    realptr = (char*)ptr-PREFIX_SIZE;
    oldsize = *((size_t*)realptr);
    update_zmalloc_stat_free(oldsize+PREFIX_SIZE);
    free(realptr);
#endif
}

zmalloc_get_rss

获取RSS（resident set size，常驻内存大小）

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#if defined(HAVE_PROC_STAT)
//通过proc/<pid>/stat获取RSS
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

size_t zmalloc_get_rss(void) {
    int page = sysconf(_SC_PAGESIZE);
    size_t rss;
    char buf[4096];
    char filename[256];
    int fd, count;
    char *p, *x;

    snprintf(filename,256,"/proc/%d/stat",getpid());
    if ((fd = open(filename,O_RDONLY)) == -1) return 0;
    if (read(fd,buf,4096) <= 0) {
        close(fd);
        return 0;
    }
    close(fd);

    p = buf;
    count = 23; /* RSS is the 24th field in /proc/<pid>/stat */
    while(p && count--) {
        p = strchr(p,' ');
        if (p) p++;
    }
    if (!p) return 0;
    x = strchr(p,' ');
    if (!x) return 0;
    *x = '\0';

    rss = strtoll(p,NULL,10);
    rss *= page;
    return rss;
}
#elif defined(HAVE_TASKINFO)
//MACH内核可以通过task_info获取大小
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/sysctl.h>
#include <mach/task.h>
#include <mach/mach_init.h>

size_t zmalloc_get_rss(void) {
    task_t task = MACH_PORT_NULL;
    struct task_basic_info t_info;
    mach_msg_type_number_t t_info_count = TASK_BASIC_INFO_COUNT;

    if (task_for_pid(current_task(), getpid(), &task) != KERN_SUCCESS)
        return 0;
    task_info(task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count);

    return t_info.resident_size;
}
#else
//使用used_memory值，并不是准确的RSS，会使得碎片率固定为1
size_t zmalloc_get_rss(void) {
    /* If we can't get the RSS in an OS-specific way for this system just
     * return the memory usage we estimated in zmalloc()..
     *
     * Fragmentation will appear to be always 1 (no fragmentation)
     * of course... */
    return zmalloc_used_memory();
}
#endif

zmalloc_get_fragmentation_ratio

碎片率

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/* Fragmentation = RSS / allocated-bytes */
float zmalloc_get_fragmentation_ratio(size_t rss) {
    return (float)rss/zmalloc_used_memory();
}

zmalloc_get_smap_bytes_by_field

按字段名计算总字节数，字段名需要以”:”结尾，pid为-1表示当前进程。若不支持直接返回0。（利用"proc/<pid>/smaps"或"proc/self/smaps"信息）

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/* Get the sum of the specified field (converted form kb to bytes) in
 * /proc/self/smaps. The field must be specified with trailing ":" as it
 * apperas in the smaps output.
 *
 * If a pid is specified, the information is extracted for such a pid,
 * otherwise if pid is -1 the information is reported is about the
 * current process.
 *
 * Example: zmalloc_get_smap_bytes_by_field("Rss:",-1);
 */
#if defined(HAVE_PROC_SMAPS)
size_t zmalloc_get_smap_bytes_by_field(char *field, long pid) {
    char line[1024];
    size_t bytes = 0;
    int flen = strlen(field);
    FILE *fp;

    if (pid == -1) {
        fp = fopen("/proc/self/smaps","r");
    } else {
        char filename[128];
        snprintf(filename,sizeof(filename),"/proc/%ld/smaps",pid);
        fp = fopen(filename,"r");
    }

    if (!fp) return 0;
    while(fgets(line,sizeof(line),fp) != NULL) {
        if (strncmp(line,field,flen) == 0) {
            char *p = strchr(line,'k');
            if (p) {
                *p = '\0';
                bytes += strtol(line+flen,NULL,10) * 1024;
            }
        }
    }
    fclose(fp);
    return bytes;
}
#else
size_t zmalloc_get_smap_bytes_by_field(char *field, long pid) {
    ((void) field);
    ((void) pid);
    return 0;
}
#endif

zmalloc_get_memory_size

返回物理内存字节数。（声称虽然看起来丑但是已经是最干净的方式）

通过sysctl或sysconf查询物理内存使用

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size_t zmalloc_get_memory_size(void) {
#if defined(__unix__) || defined(__unix) || defined(unix) || \
    (defined(__APPLE__) && defined(__MACH__))
#if defined(CTL_HW) && (defined(HW_MEMSIZE) || defined(HW_PHYSMEM64))
    int mib[2];
    mib[0] = CTL_HW;
#if defined(HW_MEMSIZE)
    mib[1] = HW_MEMSIZE;            /* OSX. --------------------- */
#elif defined(HW_PHYSMEM64)
    mib[1] = HW_PHYSMEM64;          /* NetBSD, OpenBSD. --------- */
#endif
    int64_t size = 0;               /* 64-bit */
    size_t len = sizeof(size);
    if (sysctl( mib, 2, &size, &len, NULL, 0) == 0)
        return (size_t)size;
    return 0L;          /* Failed? */

#elif defined(_SC_PHYS_PAGES) && defined(_SC_PAGESIZE)
    /* FreeBSD, Linux, OpenBSD, and Solaris. -------------------- */
    return (size_t)sysconf(_SC_PHYS_PAGES) * (size_t)sysconf(_SC_PAGESIZE);

#elif defined(CTL_HW) && (defined(HW_PHYSMEM) || defined(HW_REALMEM))
    /* DragonFly BSD, FreeBSD, NetBSD, OpenBSD, and OSX. -------- */
    int mib[2];
    mib[0] = CTL_HW;
#if defined(HW_REALMEM)
    mib[1] = HW_REALMEM;        /* FreeBSD. ----------------- */
#elif defined(HW_PYSMEM)
    mib[1] = HW_PHYSMEM;        /* Others. ------------------ */
#endif
    unsigned int size = 0;      /* 32-bit */
    size_t len = sizeof(size);
    if (sysctl(mib, 2, &size, &len, NULL, 0) == 0)
        return (size_t)size;
    return 0L;          /* Failed? */
#else
    return 0L;          /* Unknown method to get the data. */
#endif
#else
    return 0L;          /* Unknown OS. */
#endif
}