a045941bd2
Previous limits were chosen when locking primitives had spurious lock accesses. Flipping the starting point to 1 (or rather 2 as the first call shifts it) provides a modest win when mild contention is seen while not hurting worse cases. Tested on a bunch of one, two and four socket old and new systems (Westmere, Skylake, Threadreaper and others) by doing concurrent page faults, buildkernel/buildworld and other stuff (although not all systems got all the tests). Another thing is the upper limit. It is semi-arbitrarily chosen as it was getting out of hand for slightly less small systems (e.g. a 128-thread one). Note that backoff is fundamentally a speculative bandaid and this change just makes it fit a little bit better. It remains completely oblivious to the hardware topology or the contention pattern. This is being experimented with.
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