Reopen after triage from pydata/xarray#5165

import dask.array as da
d = da.random.random((15000, 250000), chunks=(1, -1)).sum()
d.compute(optimize_graph=False)

Peak RAM usage:
Before #6322 (2021.3.1): 200 MiB
After #6322 (2021.4.0): 28 GiB

This regression impacts the threads and sync schedulers. processes and distributed are unaffected.

CC @jakirkham