一、actor对等调度

actor的调度由线程池来调度。actor是被调度对象，skynet把所有活跃的actor通过链表串联起来，线程池从actor中取出相等数量的消息进行执行，实现公平调度。

但是，actor消息队列长度可能不一致，会出现部分actor "饿死"现象，skynet通过对线程池的工作线程赋予不同权重来规避这个问题。

二、调度流程源码分析

actor=隔离的运行环境+回调函数+消息队列。

2.1、thread_worker()

actor是由线程调度，所以从线程入口函数thread_worker开始。线程作为消费者，会不断循环从消息队列中取消息，如果没有消息就进入等待（pthread_cond_wait）。skynet_context_message_dispatch()用来取出消息和消费消息。

/skynet-src/skynet_start.c

static void *
thread_worker(void *p) {struct worker_parm *wp = p;int id = wp->id;int weight = wp->weight;struct monitor *m = wp->m;struct skynet_monitor *sm = m->m[id];skynet_initthread(THREAD_WORKER);struct message_queue * q = NULL;while (!m->quit) {q = skynet_context_message_dispatch(sm, q, weight);if (q == NULL) {if (pthread_mutex_lock(&m->mutex) == 0) {++ m->sleep;// "spurious wakeup" is harmless,// because skynet_context_message_dispatch() can be call at any time.if (!m->quit)pthread_cond_wait(&m->cond, &m->mutex);-- m->sleep;if (pthread_mutex_unlock(&m->mutex)) {fprintf(stderr, "unlock mutex error");exit(1);}}}}return NULL;
}

2.2、struct skynet_context

struct skynet_context保存的是actor的上下文信息。

1. instance是lua虚拟机。
2. cb是自己的回调函数。
3. queue是消息队列，回调函数会从消息队列中取出消息作为参数传给回调函数，从而驱动actor运行；这个消息队列是专属于该actor的消息队列。

/skynet-src/skynet_server.c

struct skynet_context {void * instance;struct skynet_module * mod;void * cb_ud;skynet_cb cb;struct message_queue *queue;ATOM_POINTER logfile;uint64_t cpu_cost;      // in microsecuint64_t cpu_start;     // in microsecchar result[32];uint32_t handle;int session_id;ATOM_INT ref;int message_count;bool init;bool endless;bool profile;CHECKCALLING_DECL
};

/skynet-src/skynet_mq.c

struct message_queue {struct spinlock lock;uint32_t handle;int cap;int head;int tail;int release;int in_global;int overload;int overload_threshold;struct skynet_message *queue;struct message_queue *next;
};

2.3、skynet_context_message_dispatch()

1. 通过skynet_globalmq_pop()从全局的消息队列中获取活跃消息队列（活跃actor的队列）；
2. 然后skynet_mq_handle()获取消息队列的句柄，即actor的唯一标识ID；
3. skynet_handle_grab()获取actor的上下文。
4. for循环开始执行调度，注意变量n的初始值是1，它代表着权重，因为skynet默认前面几个线程只消费一个消息。
5. 通过skynet_mq_pop()先从actor的专属消息队列中取出消息，然后调用dispatch_message()分发消息。
6. 权重变化 n >>= weight。

/skynet-src/skynet_server.c

struct message_queue *
skynet_context_message_dispatch(struct skynet_monitor *sm, struct message_queue *q, int weight) {if (q == NULL) {q = skynet_globalmq_pop();if (q==NULL)return NULL;}uint32_t handle = skynet_mq_handle(q);struct skynet_context * ctx = skynet_handle_grab(handle);if (ctx == NULL) {struct drop_t d = { handle };skynet_mq_release(q, drop_message, &d);return skynet_globalmq_pop();}int i,n=1;struct skynet_message msg;for (i=0;i<n;i++) {if (skynet_mq_pop(q,&msg)) {skynet_context_release(ctx);return skynet_globalmq_pop();} else if (i==0 && weight >= 0) {n = skynet_mq_length(q);n >>= weight;}int overload = skynet_mq_overload(q);if (overload) {skynet_error(ctx, "May overload, message queue length = %d", overload);}skynet_monitor_trigger(sm, msg.source , handle);if (ctx->cb == NULL) {skynet_free(msg.data);} else {dispatch_message(ctx, &msg);}skynet_monitor_trigger(sm, 0,0);}assert(q == ctx->queue);struct message_queue *nq = skynet_globalmq_pop();if (nq) {// If global mq is not empty , push q back, and return next queue (nq)// Else (global mq is empty or block, don't push q back, and return q again (for next dispatch)skynet_globalmq_push(q);q = nq;}skynet_context_release(ctx);return q;
}

2.4、dispatch_message()

dispatch_message()本质上调用回调函数来处理消息，消息内容作为参数；这里就是真正的运行actor了。

/skynet-src/skynet_server.c

static void
dispatch_message(struct skynet_context *ctx, struct skynet_message *msg) {assert(ctx->init);CHECKCALLING_BEGIN(ctx)pthread_setspecific(G_NODE.handle_key, (void *)(uintptr_t)(ctx->handle));int type = msg->sz >> MESSAGE_TYPE_SHIFT;size_t sz = msg->sz & MESSAGE_TYPE_MASK;FILE *f = (FILE *)ATOM_LOAD(&ctx->logfile);if (f) {skynet_log_output(f, msg->source, type, msg->session, msg->data, sz);}++ctx->message_count;int reserve_msg;if (ctx->profile) {ctx->cpu_start = skynet_thread_time();reserve_msg = ctx->cb(ctx, ctx->cb_ud, type, msg->session, msg->source, msg->data, sz);uint64_t cost_time = skynet_thread_time() - ctx->cpu_start;ctx->cpu_cost += cost_time;} else {reserve_msg = ctx->cb(ctx, ctx->cb_ud, type, msg->session, msg->source, msg->data, sz);}if (!reserve_msg) {skynet_free(msg->data);}CHECKCALLING_END(ctx)
}

三、c语言到lua的调用过程分析

了解完调度流程，那么如果在c语言的callback函数跳到lua层执行actor呢？
lua可以调用c语言，c语言需要导入一个方法给lua使用，skynet中lcallback()就是c语言导出给lua使用的方法。

在lcallback设置回调函数：skynet_callback()。

/lualib-src/lua-skynet.c

static int
lcallback(lua_State *L) {struct skynet_context * context = lua_touserdata(L, lua_upvalueindex(1));int forward = lua_toboolean(L, 2);luaL_checktype(L,1,LUA_TFUNCTION);lua_settop(L,1);struct callback_context *cb_ctx = (struct callback_context *)lua_newuserdata(L, sizeof(*cb_ctx));cb_ctx->L = lua_newthread(L);lua_pushcfunction(cb_ctx->L, traceback);lua_setuservalue(L, -2);lua_setfield(L, LUA_REGISTRYINDEX, "callback_context");lua_xmove(L, cb_ctx->L, 1);if (forward) {skynet_callback(context, cb_ctx, forward_cb);} else {skynet_callback(context, cb_ctx, _cb);}return 0;
}// ...LUAMOD_API int
luaopen_skynet_core(lua_State *L) {luaL_checkversion(L);luaL_Reg l[] = {{ "send" , lsend },{ "genid", lgenid },{ "redirect", lredirect },{ "command" , lcommand },{ "intcommand", lintcommand },{ "addresscommand", laddresscommand },{ "error", lerror },{ "harbor", lharbor },{ "callback", lcallback },{ "trace", ltrace },{ NULL, NULL },};// functions without skynet_contextluaL_Reg l2[] = {{ "tostring", ltostring },{ "pack", luaseri_pack },{ "unpack", luaseri_unpack },{ "packstring", lpackstring },{ "trash" , ltrash },{ "now", lnow },{ "hpc", lhpc },        // getHPCounter{ NULL, NULL },};lua_createtable(L, 0, sizeof(l)/sizeof(l[0]) + sizeof(l2)/sizeof(l2[0]) -2);lua_getfield(L, LUA_REGISTRYINDEX, "skynet_context");struct skynet_context *ctx = lua_touserdata(L,-1);if (ctx == NULL) {return luaL_error(L, "Init skynet context first");}luaL_setfuncs(L,l,1);luaL_setfuncs(L,l2,0);return 1;
}

当执行lua的skynet.start时会调用c.callback()设置回调函数skynet.dispatch_message，skynet.dispatch_message是一个lua方法；每次消息到来就会调用lua的skynet.dispatch_message，通过它分发消息。

/lualib/skynet.lua

function skynet.start(start_func)c.callback(skynet.dispatch_message)init_thread = skynet.timeout(0, function()skynet.init_service(start_func)init_thread = nilend)
end

总结

后言

本专栏知识点是通过<零声教育>的系统学习，进行梳理总结写下文章，对c/c++linux系统提升感兴趣的读者，可以点击链接查看详细的服务：C/C++服务器开发 。