diff --git a/lualib/skynet.lua b/lualib/skynet.lua
index 9ef95974..e29da6c4 100644
--- a/lualib/skynet.lua
+++ b/lualib/skynet.lua
@@ -131,15 +131,13 @@ function skynet.sleep(ti)
 end
 
 function skynet.yield()
-	local session = c.command("TIMEOUT","0")
-	assert(session)
-	coroutine_yield("SLEEP", tonumber(session))
-	sleep_session[coroutine.running()] = nil
+	return skynet.sleep("0")
 end
 
 function skynet.wait()
 	local session = c.genid()
 	coroutine_yield("SLEEP", session)
+	c.trace_switch(trace_handle, session)
 	local co = coroutine.running()
 	sleep_session[co] = nil
 	session_id_coroutine[session] = nil
@@ -294,7 +292,6 @@ local function dispatch_message(prototype, msg, sz, session, source, ...)
 		if co == "BREAK" then
 			session_id_coroutine[session] = nil
 		elseif co == nil then
-			c.trace_switch(trace_handle, session)
 			unknown_response(session, source, msg, sz)
 		else
 			c.trace_switch(trace_handle, session)
diff --git a/service/testtrace.lua b/service/testtrace.lua
new file mode 100644
index 00000000..adedc634
--- /dev/null
+++ b/service/testtrace.lua
@@ -0,0 +1,28 @@
+local skynet = require "skynet"
+
+local trace_cache = {}
+
+skynet.trace_callback(function(handle, ti)
+	-- skynet will call this function when the thread which create handle(traceid) end, and pass the time (ti)
+	print("TRACE",trace_cache[handle],ti)
+	trace_cache[handle] = nil
+end)
+
+local function f (i)
+	-- create a trace object
+	local traceid = skynet.trace()
+	-- name traceid in trace_cache[]
+	trace_cache[traceid] = "thread " .. i
+
+	for i=1,i do
+		skynet.sleep(i)
+	end
+end
+
+skynet.start(function()
+	for i = 1 , 100 do
+		skynet.fork(f, i)
+	end
+end)
+
+