title: process spend too much time blocking at select.go with no explicit corresponding operation
body: <p>I'm trying to figure out the bottleneck of my program. Accroding to the benchmark result, the process is blocked at line 485 of runtime/select.go for a considerably long time which is caused by http.(*persistConn).writeLoop, but I have no idea of the root cause.</p>
  
  <p>I've tried to analyse the http operation in my program. but accroding to the flamegraph, my url fetch operations executed by several goroutines parallelly didn't become a significant burden.  I have not use "select" in my program, either </p>
  
  <p>Here is the pprof output in the "source" view:</p>
  
  <pre><code>runtime.selectgo
  /usr/lib/golang/src/runtime/select.go
  Total:    2.75mins   2.75mins (flat, cum) 79.42%
  
  480            .          .             } 
  481            .          .             goto retc 
  482            .          .            
  483            .          .           retc: 
  484            .          .             if cas.releasetime &gt; 0 { 
  485     2.75mins   2.75mins                 blockevent(cas.releasetime-t0, 1) 
  486            .          .             } 
  487            .          .             return casi, recvOK 
  488            .          .            
  489            .          .           sclose: 
  490            .          .             // send on closed channel 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/uuGbq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uuGbq.png" alt="flamegraph of blocking"></a></p>
  
  <p>I'd like to know the possible cause of this cost or the way to analyze the reason of this cost.</p>