比如，当 Redis 命令通过 call 函数（在 server.c 文件中）执行时，call 函数就会调用 latencyAddSampleIfNeeded 函数进行采样，如下所示：

if (flags & CMD_CALL_SLOWLOG) {
    char *latency_event = (real_cmd->flags & CMD_FAST) ?
            "fast-command" : "command";
    // 记录常规命令和fast-command命令的延迟
    latencyAddSampleIfNeeded(latency_event,duration/1000);
}

如果 AOF 文件刷盘的配置项是 AOF_FSYNC_ALWAYS，会记录aof-fsync-always事件的延迟：

/* Perform the fsync if needed. */
    if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
        /* redis_fsync is defined as fdatasync() for Linux in order to avoid
         * flushing metadata. */
        latencyStartMonitor(latency);
        /* Let's try to get this data on the disk. To guarantee data safe when
         * the AOF fsync policy is 'always', we should exit if failed to fsync
         * AOF (see comment next to the exit(1) after write error above). */
        if (redis_fsync(server.aof_fd) == -1) {
            serverLog(LL_WARNING,"Can't persist AOF for fsync error when the "
              "AOF fsync policy is 'always': %s. Exiting...", strerror(errno));
            exit(1);
        }
        latencyEndMonitor(latency);
        latencyAddSampleIfNeeded("aof-fsync-always",latency);
        server.aof_fsync_offset = server.aof_current_size;
        server.aof_last_fsync = server.unixtime;
    }

延迟分析和提供应对措施建议

Redis 是提供了 latency doctor 命令，来给出延迟分析结果和应对方法建议的。当我们执行这条命令的时候，Redis 就会使用 latencyCommand 函数来处理。而在处理这个命令时，latencyCommand 函数会调用 createLatencyReport 函数，来生成延迟分析报告和应对方法建议。

createLatencyReport 函数会针对 latency_events 哈希表中记录的每一类事件，先调用 analyzeLatencyForEvent 函数，计算获得采样的延迟事件执行时长的均值、最大 / 最小值等统计结果。在 createLatencyReport 函数中，它定义了多个 int 变量，当这些变量的值为 1 时，就表示建议 Redis 使用者采用一种应对高延迟的措施，代码如下：

/* Create a human readable report of latency events for this Redis instance. */
sds createLatencyReport(void) {
    sds report = sdsempty();
    int advise_better_vm = 0;       /* Better virtual machines. */
    int advise_slowlog_enabled = 0; /* Enable slowlog. */
    int advise_slowlog_tuning = 0;  /* Reconfigure slowlog. */
    int advise_slowlog_inspect = 0; /* Check your slowlog. */
    int advise_disk_contention = 0; /* Try to lower disk contention. */
    int advise_scheduler = 0;       /* Intrinsic latency. */
    int advise_data_writeback = 0;  /* data=writeback. */
    int advise_no_appendfsync = 0;  /* don't fsync during rewrites. */
    int advise_local_disk = 0;      /* Avoid remote disks. */
    int advise_ssd = 0;             /* Use an SSD drive. */
    int advise_write_load_info = 0; /* Print info about AOF and write load. */
    int advise_hz = 0;              /* Use higher HZ. */
    int advise_large_objects = 0;   /* Deletion of large objects. */
    int advise_mass_eviction = 0;   /* Avoid mass eviction of keys. */
    int advise_relax_fsync_policy = 0; /* appendfsync always is slow. */
    int advise_disable_thp = 0;     /* AnonHugePages detected. */
    int advices = 0;
    ...
    }

慢命令日志的实现

Redis 是使用了一个较为简单的方法来记录慢命令日志，也就是用一个列表，把执行时间超出慢命令日志执行时间阈值的命令记录下来，记录在redisServer中的slowlog变量中：

struct redisServer {
    …
    list *slowlog;
    …}

// slowlog中的元素数据结构如下：
/* This structure defines an entry inside the slow log list */
typedef struct slowlogEntry {
    robj **argv;     //日志项对应的命令及参数
    int argc;        //日志项对应的命令及参数个数
    long long id;       //日志项的唯一ID
    long long duration;  //日志项对应命令的执行时长（以微秒为单位）
    time_t time;        //日志项对应命令的执行时间戳
    sds cname;          //日志项对应命令的发送客户端名称
    sds peerid;         //日志项对应命令的发送客户端网络地址
} slowlogEntry;

实现慢命令日志记录功能的代码是在slowlog.c文件中。这里的主要函数是 slowlogPushEntryIfNeeded，函数的参数包含了当前执行命令及其参数 argv，以及当前命令的执行时长 duration

/* Push a new entry into the slow log.
* This function will make sure to trim the slow log accordingly to the
* configured max length. */
void slowlogPushEntryIfNeeded(client *c, robj **argv, int argc, long long duration) {
    // 慢日志没有开启直接返回
    if (server.slowlog_log_slower_than < 0) return; /* Slowlog disabled */
    // 如果超过阈值,则插入一条慢日志
    if (duration >= server.slowlog_log_slower_than)
        listAddNodeHead(server.slowlog,
                        slowlogCreateEntry(c,argv,argc,duration));

    // 超过长度则删除老的
    /* Remove old entries if needed. */
    while (listLength(server.slowlog) > server.slowlog_max_len)
        listDelNode(server.slowlog,listLast(server.slowlog));
}

如果我们发现日志中记录的命令参数非常多，那么它就可能是一条操作 bigkey 的命令.

slowlogCreateEntry 函数在创建慢命令日志项时，也会判断命令参数个数。如果命令参数个数，超出了阈值 SLOWLOG_ENTRY_MAX_ARGC 这个宏定义的大小（默认 32）时，它就不会记录超出阈值的参数了，而是记录下剩余的参数个数。这样一来，慢命令日志项中就既记录了部分命令参数，有助于排查问题，也避免了记录过多参数，占用过多内存。

/* Create a new slowlog entry.
 * Incrementing the ref count of all the objects retained is up to
 * this function. */
slowlogEntry *slowlogCreateEntry(client *c, robj **argv, int argc, long long duration) {
    slowlogEntry *se = zmalloc(sizeof(*se));
    int j, slargc = argc;

    if (slargc > SLOWLOG_ENTRY_MAX_ARGC) slargc = SLOWLOG_ENTRY_MAX_ARGC;
    se->argc = slargc;
    se->argv = zmalloc(sizeof(robj*)*slargc);
    for (j = 0; j < slargc; j++) {
        /* Logging too many arguments is a useless memory waste, so we stop
         * at SLOWLOG_ENTRY_MAX_ARGC, but use the last argument to specify
         * how many remaining arguments there were in the original command. */
        if (slargc != argc && j == slargc-1) {
            se->argv[j] = createObject(OBJ_STRING,
                sdscatprintf(sdsempty(),"... (%d more arguments)",
                argc-slargc+1));
        } else {
            /* Trim too long strings as well... */
            if (argv[j]->type == OBJ_STRING &&
                sdsEncodedObject(argv[j]) &&
                sdslen(argv[j]->ptr) > SLOWLOG_ENTRY_MAX_STRING)
            {
                sds s = sdsnewlen(argv[j]->ptr, SLOWLOG_ENTRY_MAX_STRING);

                s = sdscatprintf(s,"... (%lu more bytes)",
                    (unsigned long)
                    sdslen(argv[j]->ptr) - SLOWLOG_ENTRY_MAX_STRING);
                se->argv[j] = createObject(OBJ_STRING,s);
            } else if (argv[j]->refcount == OBJ_SHARED_REFCOUNT) {
                se->argv[j] = argv[j];
            } else {
                /* Here we need to duplicate the string objects composing the
                 * argument vector of the command, because those may otherwise
                 * end shared with string objects stored into keys. Having
                 * shared objects between any part of Redis, and the data
                 * structure holding the data, is a problem: FLUSHALL ASYNC
                 * may release the shared string object and create a race. */
                se->argv[j] = dupStringObject(argv[j]);
            }
        }
    }
    se->time = time(NULL);
    se->duration = duration;
    se->id = server.slowlog_entry_id++;
    se->peerid = sdsnew(getClientPeerId(c));
    se->cname = c->name ? sdsnew(c->name->ptr) : sdsempty();
    return se;
}

记录函数slowlogPushEntryIfNeeded会被slowlogPushCurrentCommand调用

/* Log the last command a client executed into the slowlog. */
void slowlogPushCurrentCommand(client *c, struct redisCommand *cmd, ustime_t duration) {
    /* Some commands may contain sensitive data that should not be available in the slowlog. */
    if (cmd->flags & CMD_SKIP_SLOWLOG)
        return;

    /* If command argument vector was rewritten, use the original
* arguments. */
    robj **argv = c->original_argv ? c->original_argv : c->argv;
    int argc = c->original_argv ? c->original_argc : c->argc;
    slowlogPushEntryIfNeeded(c,argv,argc,duration);
}

void call(client *c, int flags) {
    ...
        elapsedStart(&call_timer);
    c->cmd->proc(c);  // 执行命令
    const long duration = elapsedUs(call_timer); // 得到命令的执行时间
    c->duration = duration; 
    ...
        /* Log the command into the Slow log if needed.
* If the client is blocked we will handle slowlog when it is unblocked. */
        if ((flags & CMD_CALL_SLOWLOG) && !(c->flags & CLIENT_BLOCKED))
        slowlogPushCurrentCommand(c, real_cmd, duration);
    ...
    }

即call函数 -> slowlogPushCurrentCommand -> slowlogPushEntryIfNeeded进行记录慢命令日志

总结

本文介绍了事件延迟框架的实现和慢命令日志的实现，针对延迟框架，会记录像fork、AOF重写、过期key事件等事件的延迟事件，当然要延迟超过阈值才会记录，每类事件会记录到一个数组中。针对慢命令日志，针对超过阈值的慢命令，使用一个列表把它们记录下来，这里面包括了命令及参数，以及发送命令的客户端信息，这样可以方便运维人员查看分析。Redis 的 INFO 命令也提供了 Redis 运行时的监控信息，不过你要知道，INFO 命令的实现，主要是在全局变量 server 的成员变量中，用来记录 Redis 实例的实时运行状态或是资源使用情况的。