redis 性能问题查找

使用redis出现性能问题，可以使用redis的slowlog命令来查看哪些命令阻塞了redis。

使用redis作为数据库时，系统出现少量超时，通过日志信息发现，超时发生在bgsave时。bgsave命令会fork一个子进程，子进程会将redis数据库信息dump到rdb文件中。因此不能确定使用bgsave命令时，是fork一个子进程引起超时，还是dump文件时与主进程的sync同步同时写磁盘引起的超时。

这时就可以使用redis自带的slowlog命令了，但是需要在redis 配置文件中加入slowlog使用的参数。

################################## SLOW LOG ###################################

# The Redis Slow Log is a system to log queries that exceeded a specified
# execution time. The execution time does not include the I/O operations
# like talking with the client, sending the reply and so forth,
# but just the time needed to actually execute the command (this is the only
# stage of command execution where the thread is blocked and can not serve
# other requests in the meantime).
# 
# You can configure the slow log with two parameters: one tells Redis
# what is the execution time, in microseconds, to exceed in order for the
# command to get logged, and the other parameter is the length of the
# slow log. When a new command is logged the oldest one is removed from the
# queue of logged commands.

# The following time is expressed in microseconds, so 1000000 is equivalent
# to one second. Note that a negative number disables the slow log, while
# a value of zero forces the logging of every command.
#如果一个命令执行操作10ms，需要记录下这个命令和执行时间
slowlog-log-slower-than 10000

# There is no limit to this length. Just be aware that it will consume memory.
# You can reclaim memory used by the slow log with SLOWLOG RESET.
#保留最近的1024个超时的命令
slowlog-max-len 1024

下面开始使用slowlog查看超时的命令有哪些，先使用redis-cli连接服务器，在使用slowlog命令查看超时的命令有哪些。

redis 127.0.0.1:7771> SLOWLOG GET 3
1) 1) (integer) 174            //编号
   2) (integer) 1413914462     //发生日期，UnixTime类型，需要使用工具转换成日期
   3) (integer) 724446         //命令次持续时间，单位是微妙
   4) 1) "bgSave"              //执行的命令和参数
2) 1) (integer) 173
   2) (integer) 1413828062
   3) (integer) 740908
   4) 1) "bgSave"
3) 1) (integer) 172
   2) (integer) 1413741662
   3) (integer) 737333
   4) 1) "bgSave"

问题发现了，bgsave命令执行时间用了700多ms，原来是fork时使用了太长时间，从而阻塞了主进程。

-------------------------------------------------showlog命令实现细节------------------------------------------------------

执行时间的记录。所有redis命令都会调用call函数来执行，而call函数就在执行前后记录下时间，并放在slowlog时间队列中，队列的最大长度就是配置文件定义的长度。

/* Call() is the core of Redis execution of a command */
void call(redisClient *c, int flags) {
    long long dirty, start, duration;
    int client_old_flags = c->flags;

   。。。
    /* Call the command. */
    c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL);
    redisOpArrayInit(&server.also_propagate);
    dirty = server.dirty;
    start = ustime();   //记录命令执行开始时间
    c->cmd->proc(c);    //执行命令
    duration = ustime()-start;   //记录命令执行时间
    dirty = server.dirty-dirty;

    /* When EVAL is called loading the AOF we don't want commands called
     * from Lua to go into the slowlog or to populate statistics. */
    if (server.loading && c->flags & REDIS_LUA_CLIENT)
        flags &= ~(REDIS_CALL_SLOWLOG | REDIS_CALL_STATS);

    /* If the caller is Lua, we want to force the EVAL caller to propagate
     * the script if the command flag or client flag are forcing the
     * propagation. */
    if (c->flags & REDIS_LUA_CLIENT && server.lua_caller) {
        if (c->flags & REDIS_FORCE_REPL)
            server.lua_caller->flags |= REDIS_FORCE_REPL;
        if (c->flags & REDIS_FORCE_AOF)
            server.lua_caller->flags |= REDIS_FORCE_AOF;
    }

    /* Log the command into the Slow log if needed, and populate the
     * per-command statistics that we show in INFO commandstats. */
    if (flags & REDIS_CALL_SLOWLOG && c->cmd->proc != execCommand)
        slowlogPushEntryIfNeeded(c->argv,c->argc,duration);   //保存命令执行时间
     。。。
}

执行时间的查看。slowlogCommand 函数对应slowlog命令。

/* The SLOWLOG command. Implements all the subcommands needed to handle the
 * Redis slow log. */
void slowlogCommand(redisClient *c) {
    if (c->argc == 2 && !strcasecmp(c->argv[1]->ptr,"reset")) {
        slowlogReset();
        addReply(c,shared.ok);
    } else if (c->argc == 2 && !strcasecmp(c->argv[1]->ptr,"len")) {
        addReplyLongLong(c,listLength(server.slowlog));
    } else if ((c->argc == 2 || c->argc == 3) &&
               !strcasecmp(c->argv[1]->ptr,"get"))
    {
        long count = 10, sent = 0;
        listIter li;
        void *totentries;
        listNode *ln;
        slowlogEntry *se;

        if (c->argc == 3 &&
            getLongFromObjectOrReply(c,c->argv[2],&count,NULL) != REDIS_OK)
            return;

        listRewind(server.slowlog,&li);
        totentries = addDeferredMultiBulkLength(c);
        //打印count数量的超时命令
        while(count-- && (ln = listNext(&li))) {
            int j;

            se = ln->value;
            addReplyMultiBulkLen(c,4);
            addReplyLongLong(c,se->id);
            addReplyLongLong(c,se->time);
            addReplyLongLong(c,se->duration);
            addReplyMultiBulkLen(c,se->argc);
            for (j = 0; j < se->argc; j++)
                addReplyBulk(c,se->argv[j]);
            sent++;
        }
        setDeferredMultiBulkLength(c,totentries,sent);
    } else {
        addReplyError(c,
            "Unknown SLOWLOG subcommand or wrong # of args. Try GET, RESET, LEN.");
    }
}

redis 性能问题查找

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