Re: [PATCH 3/3] index-pack: adjust default threading cap

> So what's a good default value? It's clear that the current cap of 3 is
> too low; our default values are 42% and 57% slower than the best times
> on each machine. The results on the 40-core machine imply that 20
> threads is an actual barrier regardless of the number of cores, so we'll
> take that as a maximum. We get the best results on these machines at
> half of the online-cpus value. That's presumably a result of the
> hyperthreading. That's common on multi-core Intel processors, but not
> necessarily elsewhere. But if we take it as an assumption, we can
> perform optimally on hyperthreaded machines and still do much better
> than the status quo on other machines, as long as we never half below
> the current value of 3.