Consensus: Tuning runtime behaviour with environment variables.

[Andreas Jaeger]

Carlos, thanks for starting this and documenting everything.

My general feeling is that we should have sane defaults so that the 
library does the right thing without any further interaction - and the 
need for tuneables implies that we have not reached that goal.

Tuneable environment variables might make debugging harder. For bug 
reporting we need to ask next time not only in some cases the CPU (to 
check which ifunc is used) but also ask for all relevant environment 
variables.

Now looking at some of the concrete proposals, like Andi Kleen's 
patches, I see the benefit of adding environment variables.

Seeing what you noticed, I'm fine with adding tuneable environment 
variables and like to add a large caution: These should be the exception 
and each new one should be discussed properly - and then also documented.

Andreas
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