cloud_storage_clients: rewrite pick_two_random_shards for O(1) sampling #29099
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Replace the shuffle-based algorithm with rejection-free direct sampling. Improvements: - O(1) instead of O(n) shuffle per call - Reuses thread-local RNG instead of creating new std::random_device and std::mt19937 on every invocation - Exactly one random draw per selection, no retries - No heap allocation (old version used std::vector) 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <[email protected]>
nvartolomei
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Lazin,
bashtanov and
oleiman
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CI test resultstest results on build#78308
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nvartolomei
commented
Dec 24, 2025
| [[nodiscard]] std::pair<ss::shard_id, ss::shard_id> pick_two_random_shards() { | ||
| using dist_t = std::uniform_int_distribution<ss::shard_id>; | ||
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| static thread_local std::mt19937 gen{std::random_device{}()}; |
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gemini
Random Number Generation: std::random_device is often implemented as a read from /dev/urandom (syscall), which is too slow for a "Zero-Cost" abstraction, and std::mt19937 is large (2.5KB state).
Fix: Use a lightweight PRNG (like PCG or Xorshift) or simply a thread_local std::default_random_engine seeded once (not on every jitter call).
bashtanov
approved these changes
Jan 2, 2026
dotnwat
requested changes
Jan 2, 2026
| [[nodiscard]] std::pair<ss::shard_id, ss::shard_id> pick_two_random_shards() { | ||
| using dist_t = std::uniform_int_distribution<ss::shard_id>; | ||
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| static thread_local std::mt19937 gen{std::random_device{}()}; |
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Can we use our random generator library? Travis updated it a few months ago to have different seeding policies to improve reproducibility, along with other improvements.
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Replace the shuffle-based algorithm with rejection-free direct sampling.
Improvements:
🤖 Generated with Claude Code
Backports Required
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