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

[R. Matt Barnett]

My test is a little bogus anyway because I realized on the drive home  
I did an invalid test/set on max seen.  But I think the conclusion  
still stands because we only see 4 printfs.

I'm not very experienced with nio, but the way I assumed Undertow  
worked, at a high level, was as follows:

1.) Each incoming socket connection generated a channel.
2.) Each channel created by step 1.) was associated with a singleton selector.
3.) All IO threads polled the singleton selector waiting for requests  
to process.

Sort of a multi-producer/multi-consumer with a singleton queue model.

Is this not the case? Is it the case that only one thread can poll  
from a selector?

-- Matt

Quoting Stuart Douglas <sdouglas at redhat.com>:

> There is no guarantee that connections will be evenly distributed between
> IO threads. Once a client has connected the connection is tied to that IO
> thread, so it may be that you are just ending up with 2 connections on 4
> threads.
>
> Stuart
>
>
>
>
> On Wed, Jul 25, 2018 at 10:02 AM R. Matt Barnett <barnett at rice.edu> 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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