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江湖传言，在redis中，一个command（"get or set key value"）中value的长度超过1k的时候，其性能急剧下降。
稍微看了下redis的代码，发现它的网络模块关闭了nagel算法，如果修改redis的源码启动这个算法会如何呢？
为了验证这个算法启用与否，是否影响了redis的性能。先不改变redis的代码，利用redis-benchmark进行一番测试，执行下面的命令：
./redis-benchmark -p 30000 -n 50000 -k 1 -d 2048 -q
上面命令相关参数意义为：连接服务器断开30000、测试50000次、打开keepalive选项、value长度2k、只输出结果不输出测试过程。
测试结果是：
SET: 51599.59 requests per second GET: 50403.23 requests per second INCR: 55493.89 requests per second LPUSH: 54112.55 requests per second LPOP: 48875.86 requests per second SADD: 54644.81 requests per second SPOP: 54112.55 requests per second LPUSH (needed to benchmark LRANGE): 49067.71 requests per second LRANGE_600 (first 600 elements): 400.68 requests per second MSET (10 keys): 33134.53 requests per second
下面是/redis-2.8.19/networking.c的createClient函数：
53 redisClient *createClient(int fd) {
54 redisClient *c = zmalloc(sizeof(redisClient));
55
56 /* passing -1 as fd it is possible to create a non connected client.
57 * This is useful since all the Redis commands needs to be executed
58 * in the context of a client. When commands are executed in other
59 * contexts (for instance a Lua script) we need a non connected client. */
60 if (fd != -1) {
61 anetNonBlock(NULL,fd);
62 // anetEnableTcpNoDelay(NULL,fd);
63 if (server.tcpkeepalive)
64 anetKeepAlive(NULL,fd,server.tcpkeepalive);
65 if (aeCreateFileEvent(server.el,fd,AE_READABLE,
66 readQueryFromClient, c) == AE_ERR)
67 {
68 close(fd);
69 zfree(c);
70 return NULL;
71 }
72 }
请注意，我已经把line 62注释掉，采用系统默认设置。
执行同样的命令，其测试结果为：
SET: 50968.40 requests per second GET: 50607.29 requests per second INCR: 55432.37 requests per second LPUSH: 47438.33 requests per second LPOP: 49950.05 requests per second SADD: 54945.05 requests per second SPOP: 55187.64 requests per second LPUSH (needed to benchmark LRANGE): 47709.93 requests per second LRANGE_600 (first 600 elements): 398.41 requests per second MSET (10 keys): 26766.60 requests per second
同样地，可以修改代码以明确启用Nagel算法，同样的代码块修改后为：
53 redisClient *createClient(int fd) {
54 redisClient *c = zmalloc(sizeof(redisClient));
55
56 /* passing -1 as fd it is possible to create a non connected client.
57 * This is useful since all the Redis commands needs to be executed
58 * in the context of a client. When commands are executed in other
59 * contexts (for instance a Lua script) we need a non connected client. */
60 if (fd != -1) {
61 anetNonBlock(NULL,fd);
62 // anetEnableTcpNoDelay(NULL,fd);
63 anetDisableTcpNoDelay(NULL,fd);
64 if (server.tcpkeepalive)
65 anetKeepAlive(NULL,fd,server.tcpkeepalive);
66 if (aeCreateFileEvent(server.el,fd,AE_READABLE,
67 readQueryFromClient, c) == AE_ERR)
68 {
69 close(fd);
70 zfree(c);
71 return NULL;
72 }
73 }
明确地启用Nagel算法后，其测试结果为：
SET: 49164.21 requests per second GET: 48496.61 requests per second INCR: 52910.05 requests per second LPUSH: 50556.12 requests per second LPOP: 49164.21 requests per second SADD: 53937.43 requests per second SPOP: 53248.14 requests per second LPUSH (needed to benchmark LRANGE): 48169.56 requests per second LRANGE_600 (first 600 elements): 406.73 requests per second MSET (10 keys): 34106.41 requests per second
从以上对比结果来看，大部分情况下启用nagel算法后，redis的性能约有百分之二的提升。
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原文地址：http://my.oschina.net/u/875730/blog/387715
