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libmemcached的MEMCACHED_MAX_BUFFER问题

时间:2021-07-01 10:21:17 帮助过:38人阅读

最近给服务增加了一个cache_put_latency指标,加了之后,吓了一跳。发现往memcached put一个10KB左右的数据,latency居然有7ms左右,难于理解,于是花了一些精力找原因。我分别写了一个shell和C++的测试程序。 1、shell脚本使用nc发送set命令。 #/bin/env ba

最近给服务增加了一个cache_put_latency指标,加了之后,吓了一跳。发现往memcached put一个10KB左右的数据,latency居然有7ms左右,难于理解,于是花了一些精力找原因。我分别写了一个shell和C++的测试程序。

1、shell脚本使用nc发送set命令。

#/bin/env bash
let s=1
let i=0
let len=8*1024
while true
do
	if (( i >= $len ))
	then
		break
	fi
	str=${str}1
	let i++
done
let i=0
begin_time=`date +%s`
while true
do
	if (( i >= 1000 ))
	then
		break
	fi
	printf "set $i 0 0 $len\r\n${str}\r\n" | nc 10.234.4.24 11211
	if [[ $? -eq 0 ]]
	then
		echo "echo key: $i"
	fi
	let i++
done
end_time=`date +%s`
let use_time=end_time-begin_time
echo "set time consumed: $use_time"
let i=0
begin_time=`date +%s`
while true
do
	if (( i >= 1000 ))
	then
		break
	fi
	printf "get $i\r\n" | nc 10.234.4.22 11211 > /dev/null 2>&1
	let i++
done
end_time=`date +%s`
let use_time=end_time-begin_time
echo "get time consumed: $use_time"

2、C++程序则通过libmemcached set。

#include 
#include 
#include 
#include 
#include 
#include 
#include "libmemcached/memcached.h"
using namespace std;
uint32_t item_size = 0;
uint32_t loop_num = 0;
bool single_server = false;
std::string local_ip;
std::map servers;
int64_t getCurrentTime()
{
    struct timeval tval;
    gettimeofday(&tval, NULL);
    return (tval.tv_sec * 1000000LL + tval.tv_usec);
}
memcached_st* mc_init()
{
	memcached_st * mc = memcached_create(NULL);
	if (mc == NULL)
	{
		cout << "create mc error" << endl;
		return NULL;
	}
    std::map::iterator iter;
    for (iter = servers.begin(); iter != servers.end(); ++iter)
    {
        if (single_server && iter->first != local_ip)
        {
            continue;
        }
	    memcached_return rc = memcached_server_add(mc, iter->first.c_str(), iter->second);
        if(rc != MEMCACHED_SUCCESS)
        {
            cout << "add server " << iter->first << " error" << endl;
            return NULL;
        }else
        {
            cout << "add server " << iter->first << " success" << endl;
        }
    }
	return mc;
}
void test_put(memcached_st * mc)
{
	char* ptr = new char[item_size];
	memset(ptr, 'a', item_size);
	char buf[32];
    memset(buf, 0, 32);
	struct iovec curkey, curval;
	curval.iov_base = ptr;
	curval.iov_len = item_size;
	curkey.iov_base = buf;
	curkey.iov_len = 32;
	uint64_t begin_time = getCurrentTime();
	for (uint32_t i=0; i < loop_num; ++i)
	{
		sprintf(buf, "%d", i);
		memcached_return rc = memcached_set(mc,(const char *)curkey.iov_base, curkey.iov_len,
 		    	        (const char *)curval.iov_base, curval.iov_len, 600, (uint32_t)0);
		if (rc != MEMCACHED_SUCCESS)
		{
			cout << "set key error: " << buf << endl;
            continue;
		}else
        {
            cout << "set key: " << buf << endl;
        }
	}
	uint64_t end_time = getCurrentTime();
	cout << "put time comsumed: " <<  end_time - begin_time << endl;
}
void test_get(memcached_st * mc)
{
}
int main(int argc, const char* argv[])
{
    //
    if (strcmp(argv[1], "s") == 0)
    {
        single_server = true;
    }else
    {
        single_server = false;
    }
    item_size = atoi(argv[2])*1024;
    loop_num = atoi(argv[3]);
    //
    servers["10.232.42.91"] = 11211;
    /*servers["10.234.4.22"] = 11211;
    servers["10.234.4.23"] = 11211;
    servers["10.234.4.24"] = 11211;*/
    //
    local_ip = "10.232.42.91";
    //
	memcached_st* mc = mc_init();
    if (! mc)
    {
        cout << "mc_init error" << endl;
        return -1;
    }
	test_put(mc);
	test_get(mc);
}

测试发现二者的结果是相背的。shell脚本set 1000次8KB的item,只要3s左右,平均需要3ms。而C++版本则需要39s左右,平均耗时39ms。照理说shell脚本需要不断连接服务器和启动nc进程,应该更慢才对。我用ltrace跟踪了一下,发现8KB的数据需要发送两次,两次write都是非常快的,但是等memcached返回时用了很多时间,主要的时间就耗费在这个地方。

23:32:37.069922 [0x401609]
memcached_set(0x19076200, 0x7fffdad68560, 32, 0x1907a570, 8192 
23:32:37.070034 [0x3f280c5f80]
SYS_write(3, "set 29 0 600
8192\r\naaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 8196) = 8196 <0.000022>
23:32:37.071657 [0x3f280c5f80]
SYS_write(3, "aaaaaaaaaaaaaaa\r\n", 17) = 17 <0.000012>
23:32:37.071741 [0x3f280c5f00]
SYS_read(3, "STORED\r\n", 8196) = 8 <0.039765>(39ms)

和剑豪讨论下之后,剑豪马上去grep了一把代码,发现原来libmemcached居然有MEMCACHED_MAX_BUFFER这样一个常量,其值为8196。并且它还没有对应的memcached_behavior_set函数。在memcached_constants.h中将其直接改成81960,然后就欣喜地发现cache_put_latency从7ms降低到1ms左右。

问题完美虽然地解决了,但是意犹未尽,于是想搞明白为什么会出现这种奇怪的现象。瓶颈貌似在服务器端,于是对memcached做了一些修改。在状态切换的时候加上一个精确到微秒的时间。

static int64_t getCurrentTime()
{
    struct timeval tval;
    gettimeofday(&tval, NULL);
    return (tval.tv_sec * 1000000LL + tval.tv_usec);
}
static void conn_set_state(conn *c, enum conn_states state) {
    assert(c != NULL);
    assert(state >= conn_listening && state < conn_max_state);
    if (state != c->state) {
        if (settings.verbose > 2) { 
            fprintf(stderr, "%d: going from %s to %s, time: %lu\n",
                    c->sfd, state_text(c->state),
                    state_text(state), getCurrentTime());
        }    
        c->state = state;
        if (state == conn_write || state == conn_mwrite) {
            MEMCACHED_PROCESS_COMMAND_END(c->sfd, c->wbuf, c->wbytes);
        }    
    }    
}

从打印的时间戳可以看出来,时间主要花在conn_nread状态处理代码中。最后定位到第二次read花费的时间非常多。

15: going from conn_waiting to conn_read, time: 1348466584440118
15: going from conn_read to conn_parse_cmd, time: 1348466584440155
<15 set 98 0 600 8192
15: going from conn_parse_cmd to conn_nread, time: 1348466584440177
conn_nread: 17
> NOT FOUND 98
>15 STORED
15: going from conn_nread to conn_write, time: 1348466584480099(36ms)
15: going from conn_write to conn_new_cmd, time: 1348466584480145
15: going from conn_new_cmd to conn_waiting, time: 1348466584480152

value的数据可能在conn_read中读完了,这个时候只需要memmove一下就好了。如果没有在conn_read状态中读完,那么就需要conn_nread自己来一次read了(因为套接字被设置成了异步,所以还可能需要多次read),关键就是这个read太慢了。

        case conn_nread:
            if (c->rlbytes == 0) { 
                complete_nread(c);
                break;
            }    
            /* first check if we have leftovers in the conn_read buffer */
            if (c->rbytes > 0) { 
                int tocopy = c->rbytes > c->rlbytes ? c->rlbytes : c->rbytes;
                if (c->ritem != c->rcurr) {
                    memmove(c->ritem, c->rcurr, tocopy);
                }    
                c->ritem += tocopy;
                c->rlbytes -= tocopy;
                c->rcurr += tocopy;
                c->rbytes -= tocopy;
                if (c->rlbytes == 0) { 
                    break;
                }    
            }    
            /*  now try reading from the socket */
            res = read(c->sfd, c->ritem, c->rlbytes);
            if (res > 0) { 
                pthread_mutex_lock(&c->thread->stats.mutex);
                c->thread->stats.bytes_read += res; 
                pthread_mutex_unlock(&c->thread->stats.mutex);
                if (c->rcurr == c->ritem) {
                    c->rcurr += res; 
                }    
                c->ritem += res; 
                c->rlbytes -= res; 
                break;
            }

折腾了好久,在libmemcached的io_flush函数前后也打了不少时间戳,发现libmemcached发送数据是非常快的。突然灵感闪现,我想起来了TCP_NODELAY这个参数,于是在libmemcached memcached_connect.c文件中的set_socket_options函数中增加了这个参数(事实上set_socket_options函数里面可以设置TCP_NODELAY,没有仔细看)。

    int flag = 1;
    int error = setsockopt(ptr->fd, IPPROTO_TCP, TCP_NODELAY, (char *)&flag, sizeof(flag) );
    if (error == -1) {
          printf("Couldn't setsockopt(TCP_NODELAY)\n");
            exit(-1);
    }else
    {   
          printf("set setsockopt(TCP_NODELAY)\n");
    }

在不改MEMCACHED_MAX_BUFFER的情况下,现在set 100KB的item也是一瞬间的事情了。不过新的困惑又出现了,Nagle算法什么情况会起作用呢?为什么第一个包没被缓存,第二个包一定会被缓存呢?

libmemcached发送一个set命令是分成三部分的,首先是header(set 0 0 600 8192\r\n,共18个字节),然后是value(8192个字节),最后是’\r\n’(两个字节),一共是8212个字节。memcached在conn_read状态一共能读取2048+2048+4096+8196=16KB的数据,因此对于8KB的数据是完全可以在conn_read状态读完的。通过在conn_read状态处理的代码中增加下面的打印语句可以发现有些情况下,conn_read最后一次只读取了4个字节(正常情况应该是2048+2048+4096+20),剩下的16个字节放到conn_nread中读了。

        res = read(c->sfd, c->rbuf + c->rbytes, avail);
        if (res > 0) {
            char buf[10240] = {0};
            sprintf(buf, "%.*s", res, c->rbuf + c->rbytes);
            printf("avail=%d, read=%d, str=%s\n", avail, res, buf);

未设置TCP_NODELAY选项时,使用netstat可以看到客户端socket的Send-Q一直会维持在8214和8215之间。

tcp        0   8215 10.232.42.91:59836          10.232.42.91:11211          ESTABLISHED 25800/t

设置TCP_NODELAY选项时,客户端socket的Send-Q就一直为0了。

tcp        0      0 10.232.42.91:59890          10.232.42.91:11211          ESTABLISHED 26554/t.quick

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