Commits on Feb 11, 2021

1. coop: allow disabling cooperative scheduling …

   With deeply nested FuturesUnordered where most leaf futures are ready,
   yields forced by the cooperative scheduling mechanism result in a lot of
   useless polls when Tokio attempts to force a return to the runtime.
   
   Indeed, it takes 32 useless polls to exit a single layer of
   FuturesUnordered, so with e.g. 5 layers, it takes 32^5 polls (it doesn't
   matter how many futures are in the FuturesUnordered instance since it'll
   poll 32 times, even if it's the same future being polled 32 times). This
   has to be done # total futures / 128 times, which can be a prohibitively
   high amount of polling.
   
   In our services, we've seen this result in ~50% of our CPU time being
   spent in `wake_by_ref()` called by `poll_proceed`. Conversely, we've not
   actually seen benefits from using cooperative scheduling, so turning it
   off seems like a natural solution.
   
   This commit adds support for doing this in the runtime builder. It
   targets the 2.x branch since that is the one we're currently using, but
   I'm happy to port this to 1.x.

   

   krallin committed Feb 11, 2021
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