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GeezerVison is showing github.com — gh-118392: Update genrand_uint32 to use atomic operation by corona10 · Pull Request #118393 · python/cpython

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@corona10

@corona10 corona10 commented Apr 29, 2024 •
edited by bedevere-app Bot
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  • Issue: Dramatic slowdown with random.random on free-threading build. #118392

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@corona10
pythongh-118392: Update genrand_uint32 to use atomic operation
d1cf0bf
@bedevere-app bedevere-app Bot mentioned this pull request Apr 29, 2024
Dramatic slowdown with random.random on free-threading build. #118392
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@corona10 corona10 added skip news DO-NOT-MERGE and removed DO-NOT-MERGE labels Apr 29, 2024
@corona10
corona10 requested review from colesbury and pitrou April 29, 2024 14:52
@corona10
corona10 marked this pull request as ready for review April 29, 2024 14:53
@corona10
corona10 requested a review from rhettinger as a code owner April 29, 2024 14:53
@bedevere-app bedevere-app Bot added the awaiting core review label Apr 29, 2024
@corona10

corona10 commented Apr 29, 2024 •
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@pitrou @colesbury cc @rhettinger
I am not sure if this change will be safe(or even acceptable) or if it needs some changes for a better, safe random algorithm.
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colesbury
colesbury reviewed Apr 29, 2024
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Comment thread Modules/_randommodule.c
Comment on lines +143 to +144
int index = FT_ATOMIC_LOAD_INT(self->index);
if (index >= N) { /* generate N words at one time */

@colesbury colesbury Apr 29, 2024

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I don't think this will work well. This can end up returning the same generated numbers to different threads, which we don't want.

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@corona10 corona10 Apr 29, 2024 •
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Hmm in that case, should we change the algorithm(thread-safe one) or fine-grained locking for this use case?

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@colesbury colesbury Apr 29, 2024

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I don't think there's a simple fix to the random module.

I commented on the issue. It'd be helpful to document the behavior and provide alternatives. For example, Java's java.util.Random includes the following comment:

Instances of java.util.Random are threadsafe. However, the concurrent use of the same java.util.Random instance across threads may encounter contention and consequent poor performance. Consider instead using ThreadLocalRandom in multithreaded designs.

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@corona10 corona10 Apr 29, 2024

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I commented on the issue. It'd be helpful to document the behavior and provide alternatives. For example, Java's java.util.Random includes the following comment:

Okay sounds acceptable :)

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@corona10
corona10 force-pushed the gh-118392 branch from ac857d0 to d1cf0bf Compare April 29, 2024 15:21
@corona10 corona10 added the DO-NOT-MERGE label Apr 29, 2024
@corona10 corona10 closed this Apr 29, 2024
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@colesbury colesbury colesbury left review comments
@pitrou pitrou Awaiting requested review from pitrou
@rhettinger rhettinger Awaiting requested review from rhettinger rhettinger is a code owner

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@corona10 @colesbury
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