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Adjust transfer costs in worker_objective #5326

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Adjust transfer costs in worker_objective #5326

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4c67b0b
10ms penalty per transfer
gjoseph92 Sep 17, 2021
b4ebbee
Amortize transfer cost
gjoseph92 Sep 17, 2021
cbc145a
Add fixed transfer cost in `get_comm_cost` as well
gjoseph92 Sep 17, 2021
7f32a1a
Merge branch 'main' into worker_objective/adjust-transfer-cost
gjoseph92 May 4, 2022
a6c2315
Update distributed/scheduler.py
crusaderky May 4, 2022
5f18151
Update distributed/scheduler.py
crusaderky May 4, 2022
01a0e8c
Update distributed/scheduler.py
crusaderky May 4, 2022
e3d62f6
Update distributed/scheduler.py
crusaderky May 4, 2022
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12 changes: 8 additions & 4 deletions distributed/scheduler.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3304,7 +3304,8 @@ def get_comm_cost(self, ts: TaskState, ws: WorkerState) -> double:
nbytes: Py_ssize_t = 0
for dts in deps:
nbytes += dts._nbytes
return nbytes / self._bandwidth
# Add a fixed 10ms penalty per transfer. See distributed#5324
return nbytes / self._bandwidth + 0.01 * len(deps)
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@ccall
def get_task_duration(self, ts: TaskState, default: double = -1) -> double:
Expand Down Expand Up @@ -3415,14 +3416,17 @@ def worker_objective(self, ts: TaskState, ws: WorkerState) -> tuple:
"""
dts: TaskState
nbytes: Py_ssize_t
comm_bytes: Py_ssize_t = 0
comm_bytes: double = 0
xfers: Py_ssize_t = 0
for dts in ts._dependencies:
if ws not in dts._who_has:
nbytes = dts.get_nbytes()
comm_bytes += nbytes
# amortize transfer cost over all waiters
comm_bytes += nbytes / len(dts._waiters)
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Can you add an in-code comment explaining how this division amortizes cost? I assume this is again a "local topology" argument related to the fan-out tasks (#5325 (comment)) where we try to "ignore" tasks which will likely end up everywhere anyhow?

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Will do. It's related to that, but actually a simpler idea. Basically, if we transfer to this worker now, that opens up the potential for N other tasks to run on this worker without transferring the data. So you could look at as, rather than this task paying the whole cost up front and others getting the benefit for free, all the sibling tasks split the cost of the transferring evenly between them. (That's an analogy of course—once transferred, the other tasks don't actually pay anything!)

xfers += 1

stack_time: double = ws._occupancy / ws._nthreads
start_time: double = stack_time + comm_bytes / self._bandwidth
start_time: double = stack_time + comm_bytes / self._bandwidth + xfers * 0.01
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if ts._actor:
return (len(ws._actors), start_time, ws._nbytes)
Expand Down