[undertow-dev] Unable to concurrently use all available IO Threads under load on Red Hat

[R. Matt Barnett]

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