学习总结
在redis中使用到缓冲区的功能主要有：
- 客户端缓冲区
- 复制积压缓冲区
- AOF缓冲区
其中客户端缓冲区指客户端通过TCP连接到redis的输入缓冲区和输出缓存区。
参数client-query-buffer-limit用来控制客户端传递给redis的数据大小，默认为1G，在使用Redis时应尽量避免超大Key。
参数client-output-buffer-limit用来控制输出缓冲区大小，可以按照不同的使用场景来进行控制：
# The client output buffer limits can be used to force disconnection of clients
# that are not reading data from the server fast enough for some reason (a
# common reason is that a Pub/Sub client can't consume messages as fast as the
# publisher can produce them).
#
# The limit can be set differently for the three different classes of clients:
#
# normal -> normal clients including MONITOR clients
# replica -> replica clients
# pubsub -> clients subscribed to at least one pubsub channel or pattern
#
# The syntax of every client-output-buffer-limit directive is the following:
#
# client-output-buffer-limit <class> <hard limit> <soft limit> <soft seconds>
#
# A client is immediately disconnected once the hard limit is reached, or if
# the soft limit is reached and remains reached for the specified number of
# seconds (continuously).
# So for instance if the hard limit is 32 megabytes and the soft limit is
# 16 megabytes / 10 seconds, the client will get disconnected immediately
# if the size of the output buffers reach 32 megabytes, but will also get
# disconnected if the client reaches 16 megabytes and continuously overcomes
# the limit for 10 seconds.
#
# By default normal clients are not limited because they don't receive data
# without asking (in a push way), but just after a request, so only
# asynchronous clients may create a scenario where data is requested faster
# than it can read.
#
# Instead there is a default limit for pubsub and replica clients, since
# subscribers and replicas receive data in a push fashion.
#
# Both the hard or the soft limit can be disabled by setting them to zero.
client-output-buffer-limit normal 0 0 0
client-output-buffer-limit replica 256mb 64mb 60
client-output-buffer-limit pubsub 32mb 8mb 60
# Client query buffers accumulate new commands. They are limited to a fixed
# amount by default in order to avoid that a protocol desynchronization (for
# instance due to a bug in the client) will lead to unbound memory usage in
# the query buffer. However you can configure it here if you have very special
# needs, such us huge multi/exec requests or alike.
#
# client-query-buffer-limit 1gb
默认配置中，不会对normal的客户端进行限制，在使用monitor监听redis的命令请求时，如果redis的QPS较高或者请求数据较多时，客户端无法快速接收这些数据，则会在客户端缓冲区中形成积压，严重消耗redis内存。
在请求频繁的生产环境，应尽量避免使用monitor命令并严格控制monitor命令执行时间。
通过命令client listhe info clients来获取客户端列表和客户端输入输出缓冲区的资源消耗。
源码学习
首先在redisServer结构中会保存所有的从节点列表和监控客户端：
struct redisServer {
list *slaves, *monitors; /* List of slaves and MONITORs */
}
当客户端使用monitor请求服务时，会将该客户端加入到monitor列表中：
void monitorCommand(client *c) {
/* ignore MONITOR if already slave or in monitor mode */
if (c->flags & CLIENT_SLAVE) return;
c->flags |= (CLIENT_SLAVE|CLIENT_MONITOR);
listAddNodeTail(server.monitors,c);
addReply(c,shared.ok);
}
在命令执行前，会将要执行的命令发送给所有的Monitor：
/* Call() is the core of Redis execution of a command.
*/
void call(client *c, int flags) {
long long dirty;
ustime_t start, duration;
int client_old_flags = c->flags;
struct redisCommand *real_cmd = c->cmd;
server.fixed_time_expire++;
/* Sent the command to clients in MONITOR mode, only if the commands are
* not generated from reading an AOF. */
if (listLength(server.monitors) &&
!server.loading &&
!(c->cmd->flags & (CMD_SKIP_MONITOR|CMD_ADMIN)))
{
replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc);
}
}
replicationFeedMonitor函数会将命令进行封装，使用addReply函数将封装结果发送到所有monitor的output buffer中。
void replicationFeedMonitors(client *c, list *monitors, int dictid, robj **argv, int argc) {
listNode *ln;
listIter li;
int j;
sds cmdrepr = sdsnew("+");
robj *cmdobj;
struct timeval tv;
gettimeofday(&tv,NULL);
cmdrepr = sdscatprintf(cmdrepr,"%ld.%06ld ",(long)tv.tv_sec,(long)tv.tv_usec);
if (c->flags & CLIENT_LUA) {
cmdrepr = sdscatprintf(cmdrepr,"[%d lua] ",dictid);
} else if (c->flags & CLIENT_UNIX_SOCKET) {
cmdrepr = sdscatprintf(cmdrepr,"[%d unix:%s] ",dictid,server.unixsocket);
} else {
cmdrepr = sdscatprintf(cmdrepr,"[%d %s] ",dictid,getClientPeerId(c));
}
for (j = 0; j < argc; j++) {
if (argv[j]->encoding == OBJ_ENCODING_INT) {
cmdrepr = sdscatprintf(cmdrepr, ""%ld"", (long)argv[j]->ptr);
} else {
cmdrepr = sdscatrepr(cmdrepr,(char*)argv[j]->ptr,
sdslen(argv[j]->ptr));
}
if (j != argc-1)
cmdrepr = sdscatlen(cmdrepr," ",1);
}
cmdrepr = sdscatlen(cmdrepr,"
",2);
cmdobj = createObject(OBJ_STRING,cmdrepr);
listRewind(monitors,&li);
while((ln = listNext(&li))) {
client *monitor = ln->value;
addReply(monitor,cmdobj);
}
decrRefCount(cmdobj);
}
在将监控日志放入客户端output buffer前会检查当前buffer是否有足够空间来存放数据(_addReplyToBuffer)，如果空间不足则跳过。当监控日志存入outpu buffer后，会异步检查output buffer的水位线(asyncCloseClientOnOutputBufferLimitReached)
/* Add the object 'obj' string representation to the client output buffer. */
void addReply(client *c, robj *obj) {
if (prepareClientToWrite(c) != C_OK) return;
if (sdsEncodedObject(obj)) {
if (_addReplyToBuffer(c,obj->ptr,sdslen(obj->ptr)) != C_OK)
_addReplyStringToList(c,obj->ptr,sdslen(obj->ptr));
} else if (obj->encoding == OBJ_ENCODING_INT) {
/* For integer encoded strings we just convert it into a string
* using our optimized function, and attach the resulting string
* to the output buffer. */
char buf[32];
size_t len = ll2string(buf,sizeof(buf),(long)obj->ptr);
if (_addReplyToBuffer(c,buf,len) != C_OK)
_addReplyStringToList(c,buf,len);
} else {
serverPanic("Wrong obj->encoding in addReply()");
}
}
/* -----------------------------------------------------------------------------
* Low level functions to add more data to output buffers.
* -------------------------------------------------------------------------- */
int _addReplyToBuffer(client *c, const char *s, size_t len) {
size_t available = sizeof(c->buf)-c->bufpos;
if (c->flags & CLIENT_CLOSE_AFTER_REPLY) return C_OK;
/* If there already are entries in the reply list, we cannot
* add anything more to the static buffer. */
if (listLength(c->reply) > 0) return C_ERR;
/* Check that the buffer has enough space available for this string. */
if (len > available) return C_ERR;
memcpy(c->buf+c->bufpos,s,len);
c->bufpos+=len;
return C_OK;
}
void _addReplyStringToList(client *c, const char *s, size_t len) {
if (c->flags & CLIENT_CLOSE_AFTER_REPLY) return;
listNode *ln = listLast(c->reply);
clientReplyBlock *tail = ln? listNodeValue(ln): NULL;
/* Note that 'tail' may be NULL even if we have a tail node, becuase when
* addDeferredMultiBulkLength() is used, it sets a dummy node to NULL just
* fo fill it later, when the size of the bulk length is set. */
/* Append to tail string when possible. */
if (tail) {
/* Copy the part we can fit into the tail, and leave the rest for a
* new node */
size_t avail = tail->size - tail->used;
size_t copy = avail >= len? len: avail;
memcpy(tail->buf + tail->used, s, copy);
tail->used += copy;
s += copy;
len -= copy;
}
if (len) {
/* Create a new node, make sure it is allocated to at
* least PROTO_REPLY_CHUNK_BYTES */
size_t size = len < PROTO_REPLY_CHUNK_BYTES? PROTO_REPLY_CHUNK_BYTES: len;
tail = zmalloc(size + sizeof(clientReplyBlock));
/* take over the allocation's internal fragmentation */
tail->size = zmalloc_usable(tail) - sizeof(clientReplyBlock);
tail->used = len;
memcpy(tail->buf, s, len);
listAddNodeTail(c->reply, tail);
c->reply_bytes += tail->size;
}
asyncCloseClientOnOutputBufferLimitReached(c);
}
当水位线超过阈值后，会异步关闭客户端，防止output buffer内存消耗过多引发实例异常。
/* Asynchronously close a client if soft or hard limit is reached on the
* output buffer size. The caller can check if the client will be closed
* checking if the client CLIENT_CLOSE_ASAP flag is set.
*
* Note: we need to close the client asynchronously because this function is
* called from contexts where the client can't be freed safely, i.e. from the
* lower level functions pushing data inside the client output buffers. */
void asyncCloseClientOnOutputBufferLimitReached(client *c) {
if (c->fd == -1) return; /* It is unsafe to free fake clients. */
serverAssert(c->reply_bytes < SIZE_MAX-(1024*64));
if (c->reply_bytes == 0 || c->flags & CLIENT_CLOSE_ASAP) return;
if (checkClientOutputBufferLimits(c)) {
sds client = catClientInfoString(sdsempty(),c);
freeClientAsync(c);
serverLog(LL_WARNING,"Client %s scheduled to be closed ASAP for overcoming of output buffer limits.", client);
sdsfree(client);
}
}