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Force all deserialized objects to the oldest GC generation #19681

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Merged
merged 2 commits into from
Aug 19, 2025
Merged

Force all deserialized objects to the oldest GC generation #19681

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ef14a68
Force all deserialized objects to the oldest generation
ilevkivskyi Aug 18, 2025
b7051c3
Limit the hack to just one use
ilevkivskyi Aug 18, 2025
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25 changes: 25 additions & 0 deletions mypy/build.py
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Original file line number Diff line number Diff line change
Expand Up @@ -116,6 +116,8 @@
"abc",
}

# We are careful now, we can increase this in future if safe/useful.
MAX_GC_FREEZE_CYCLES = 1

Graph: _TypeAlias = dict[str, "State"]

Expand Down Expand Up @@ -707,6 +709,8 @@ def __init__(
# new file can be processed O(n**2) times. This cache
# avoids most of this redundant work.
self.ast_cache: dict[str, tuple[MypyFile, list[ErrorInfo]]] = {}
# Number of times we used GC optimization hack for fresh SCCs.
self.gc_freeze_cycles = 0

def dump_stats(self) -> None:
if self.options.dump_build_stats:
Expand Down Expand Up @@ -3326,8 +3330,29 @@ def process_graph(graph: Graph, manager: BuildManager) -> None:
#
# TODO: see if it's possible to determine if we need to process only a
# _subset_ of the past SCCs instead of having to process them all.
if (
platform.python_implementation() == "CPython"
and manager.gc_freeze_cycles < MAX_GC_FREEZE_CYCLES
):
# When deserializing cache we create huge amount of new objects, so even
# with our generous GC thresholds, GC is still doing a lot of pointless
# work searching for garbage. So, we temporarily disable it when
# processing fresh SCCs, and then move all the new objects to the oldest
# generation with the freeze()/unfreeze() trick below. This is arguably
# a hack, but it gives huge performance wins for large third-party
# libraries, like torch.
gc.collect()
gc.disable()
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Could we get here multiple times, if there are multiple dirty sub-DAGs? If yes, do you think it'll be a problem?

A quick workaround would be to do this only at most N times per run (possibly N=1).

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Yeah, I was thinking about this. FWIW, I don't think it will be a problem, since freeze/unfreeze are quite fast. Also, we may accidentally get some objects from the stale SCCs previously processed in the oldest generation, but it is probably not so bad. But also I think it is fine to start with just one pass per run and increase the limit as we get more data for this.

(With mypy -c 'import torch' we enter here only once)

for prev_scc in fresh_scc_queue:
process_fresh_modules(graph, prev_scc, manager)
if (
platform.python_implementation() == "CPython"
and manager.gc_freeze_cycles < MAX_GC_FREEZE_CYCLES
):
manager.gc_freeze_cycles += 1
gc.freeze()
gc.unfreeze()
gc.enable()
fresh_scc_queue = []
size = len(scc)
if size == 1:
Expand Down
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