I missed this post before. 

I'd like to add another point. Akka Http hasn't been performance tested on 
a 40 core machine. The high idle CPU percecntage means that either Akka / 
Akka Http is not configured correctly for this amount of cores or that 
there are actual contention issues at these levels of scale. It would be 
definitely interesting to know what the problem is to offer a better 
default experience for running Akka Http on this kind of hardware.

If you are still listening in, Jakub, it would be nice if you could set 
parallelism-max to the number of cores on your machine and/or set 
`parallelism-factor = 1` as Patrik suggested. One reason for bad 
performance could be that the default parallelism-factor of 3 would lead to 
120 threads battling for resources, starving each other off CPU time maybe 
even while keeping some resource. If this alone doesn't increase 
performance, a few stack dumps from the server process during steady state 
would help because that would likely point out places with high contention.

For anyone else listening in here, I also wanted to stress, that you need 
to put any kind of performance numbers into perspective. We cannot test 
everything in every environment and details usually matter in benchmarks. 
High CPU idle times like in this case mean that something currently just 
doesn't work correctly in this setting. For best performance, you need to 
benchmark for yourself on your own hardware and then be prepared to dig 
into issues.

Johannes

>

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