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Fix waiter-table hashing #16

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Merged
vadimskipin merged 1 commit into from
May 7, 2026
+15 −3
Merged

Fix waiter-table hashing #16

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11 changes: 11 additions & 0 deletions include/silk/util/platform.h
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Original file line number Diff line number Diff line change
Expand Up @@ -118,6 +118,17 @@ static inline uint64_t getTimeNanoseconds() noexcept
return static_cast<uint64_t>(ts.tv_sec) * 1'000'000'000 + static_cast<uint64_t>(ts.tv_nsec);
}

/** 64-bit integer hash (Murmur3 fmix64 finalizer). */
static constexpr uint64_t intHash(uint64_t key) noexcept
{
key ^= key >> 33;
key *= 0xff51afd7ed558ccdULL;
key ^= key >> 33;
key *= 0xc4ceb9fe1a85ec53ULL;
key ^= key >> 33;
return key;
}

/**
* Initialize librseq. Must be called before any rseq critical sections run.
* Idempotent -- safe to call multiple times.
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7 changes: 4 additions & 3 deletions src/fibers/fiber.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,6 @@
#include <cerrno>
#include <cstdint>
#include <format>
#include <functional>
#include <memory>
#include <mutex>
#include <random>
Expand Down Expand Up @@ -58,6 +57,8 @@ static constexpr uint64_t CQE_TAG_CANCEL = 0;
static constexpr uint64_t CQE_TAG_TIMEOUT = 1;
static constexpr uint64_t CQE_TAG_DOORBELL = 2;

static_assert((WAITER_TABLE_SIZE & (WAITER_TABLE_SIZE - 1)) == 0);

// clang-format off
#define FIBER_SIMPLE_COUNTERS(x) \
x(FIBER_STARTED, "FiberStarted") \
Expand Down Expand Up @@ -1192,7 +1193,7 @@ void FiberScheduler::suspend(SuspendCallback * callback, void * context) noexcep

void FiberScheduler::enqueueWaiter(uint64_t key, Fiber * fiber) noexcept
{
uint64_t index = std::hash<uint64_t>{}(key) % WAITER_TABLE_SIZE;
uint64_t index = intHash(key) & (WAITER_TABLE_SIZE - 1);
scheduler->waiterTable[index].push(fiber);

// seq_cst fence pairs with the one in releaseWaiters to prevent the
Expand All @@ -1207,7 +1208,7 @@ void FiberScheduler::releaseWaiters(uint64_t key) noexcept
// seq_cst fence pairs with the one in enqueueWaiter.
std::atomic_thread_fence(std::memory_order_seq_cst);

uint64_t index = std::hash<uint64_t>{}(key) % WAITER_TABLE_SIZE;
uint64_t index = intHash(key) & (WAITER_TABLE_SIZE - 1);
Fiber * fiber = scheduler->waiterTable[index].popAll();
while (fiber)
{
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