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fbdb46f5cb
Reviewed By: chadaustin Differential Revision: D17872966 fbshipit-source-id: cd60a364a2146f0dadbeca693b1d4a5d7c97ff63
57 lines
1.5 KiB
C++
57 lines
1.5 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This software may be used and distributed according to the terms of the
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* GNU General Public License version 2.
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*/
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#include "eden/fs/utils/IDGen.h"
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#include <folly/Likely.h>
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#include <folly/lang/Align.h>
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#include <atomic>
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namespace {
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/**
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* Allocating one unique ID per nanosecond would wrap around in over 500 years.
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*
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* Allocated to its own cache line.
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*/
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struct alignas(folly::hardware_destructive_interference_size) {
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std::atomic<uint64_t> counter{0};
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} global;
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thread_local uint64_t localCounter{0};
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/**
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* Number of unique IDs to hand out to a thread at a time. This avoids cache
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* line contention on globalCounter. kRangeSize should be large enough to reduce
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* contention but small enough that the pathological case of threads being
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* spawned in a tight loop, each allocating one unique ID, does not rapidly
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* exhaust the 64-bit counter space.
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*
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* I haven't measured, but I'd be surprised if a thread could be created in
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* 2000 nanoseconds.
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*/
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constexpr uint64_t kRangeSize = 2048;
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static_assert(
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(kRangeSize & (kRangeSize - 1)) == 0,
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"kRangeSize must be a power of two");
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} // namespace
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namespace facebook {
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namespace eden {
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uint64_t generateUniqueID() noexcept {
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uint64_t current = localCounter;
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if (UNLIKELY(current % kRangeSize == 0)) {
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current = global.counter.fetch_add(kRangeSize, std::memory_order_relaxed);
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}
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++current;
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localCounter = current;
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return current;
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}
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} // namespace eden
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} // namespace facebook
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