Short-circuit root-ish check for many deps#5113
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While looking into dask#5083 I happened to notice that the dashboard felt very sluggish. I profiled with py-spy and discovered that the scheduler was spending 20% of runtime calculaing `sum(map(len, group._dependencies)) < 5`! A quick print statement showed some task groups depended on 25,728 other groups (each of size 1). We can easily skip those. I originally had this conditional in dask#4967 but we removed it for simplicity: dask#4967 (comment); turns out it was relevant after all!
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While looking into #5083 I happened to notice that the dashboard felt very sluggish. I profiled with py-spy and discovered that the scheduler was spending 20% of runtime calculaing
sum(map(len, group._dependencies)) < 5! A quick print statement showed some task groups depended on 25,728 other groups (each of size 1). We can easily skip those.I originally had this conditional in the PR but we removed it for simplicity: #4967 (comment); turns out it was relevant after all!
FYI @d-v-b this solves your performance regression from #5083.
cc @mrocklin @jrbourbeau
black distributed/flake8 distributed/isort distributed