I've been able to observe 1...8 on Red Hat by adding the following statements to my handler (and setting the worker thread pool size to 8):

@Override
public void handleRequest(HttpServerExchange httpServerExchange) throws Exception
{
    if (httpServerExchange.isInIoThread()) {
        httpServerExchange.dispatch(this);
        return;
    }
...
}

I have a few questions about this technique though:

1.) How are dispatch actions mapped onto worker threads? New connections were not mapped to available idle IO threads, so is it possible dispatches also won't be mapped to available idle worker threads but instead queued for currently
busy threads?

2.) The Undertow documentation states that HttpServerExchange is not thread-safe. However the documentation states that dispatch(...) has happens-before semantics with respect to the worker thread accessing httpServerExchange.
That would seem to make it ok to read from httpServerExchange in the worker thread.  Assuming that an IO thread will be responsible for writing the http response back to the client, what steps do I need to take in the body
of handleRequest to ensure that my writes to httpServerExchange in the worker thread are observed by the IO thread responsible for transmitting the response to the client? Is invoking httpServerExchange.endExchange(); in the 
worker thread as the final statement sufficient?  

Corrected test to resolve test/set race.


https://gist.github.com/rmbarnett-rice/1179c4ad1d3344bb247c8b8daed3e4fa


I've also discovered this morning that I *can* see 1-8 printed on Red 
Hat when I generate load using ab from Windows, but only 1-4 when 
running ab on Red Hat (both locally and from a remote server).  I'm 
wondering if perhaps there is some sort of connection reuse shenanigans 
going on.  My assumption of the use of the -c 8 parameter was "make 8 
sockets" but maybe not.  I'll dig in and report back.


-- Matt


On 7/24/2018 6:56 PM, R. Matt Barnett wrote:

Hello,

I'm experiencing an Undertow performance issue I fail to understand.  I
am able to reproduce the issue with the code linked bellow. The problem
is that on Red Hat (and not Windows) I'm unable to concurrently process
more than 4 overlapping requests even with 8 configured IO Threads.
For example, if I run the following program (1 file, 55 lines):

https://gist.github.com/rmbarnett-rice/668db6b4e9f8f8da7093a3659b6ae2b5

... on Red Hat and then send requests to the server using Apache
Benchmark...

      > ab -n 1000 -c 8 localhost:8080/

I see the following output from the Undertow process:

      Server started on port 8080

      1
      2
      3
      4

I believe this demonstrates that only 4 requests are ever processed in
parallel.  I would expect 8.  In fact, when I run the same experiment on
Windows I see the expected output of

      Server started on port 8080
      1
      2
      3
      4
      5
      6
      7
      8

Any thoughts as to what might explain this behavior?

Best,

Matt

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