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make generateUniqueID noexcept

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Summary:
In a later diff, I needed generateUniqueID to be
noexcept. folly::ThreadLocal does not guarantee that (and it allocates
the first time a thread calls get()), so use C++ thread_local
instead. Bonus: it's about half a nanosecond faster now.

Reviewed By: strager

Differential Revision: D12914625

fbshipit-source-id: 9ddbe65d0ba1d317907f821c03dea5a207a73a68
This commit is contained in:
Chad Austin 2018-11-10 11:10:59 -08:00 committed by Facebook Github Bot
parent f77d0963bf
commit 46441b3765
2 changed files with 15 additions and 13 deletions
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19
eden/fs/utils/IDGen.cpp
View File

@ -9,13 +9,13 @@
*/
#include "eden/fs/utils/IDGen.h"
#include <folly/CachelinePadded.h>
#include <folly/ThreadLocal.h>
#include <folly/Likely.h>
#include <atomic>
namespace {
/**
* Allocating one unique ID per nanosecond would wrap around in over 500 years.
* Initialized to 1 so that consumers can assume 0 is not a valid id
* Initialized to 1 so that consumers can assume 0 is not a valid id.
*
* CachelinePadded may be excessive here.
*/
@ -25,8 +25,7 @@ struct LocalRange {
uint64_t begin{0};
uint64_t end{0};
};
folly::ThreadLocal<LocalRange> localRange;
thread_local LocalRange localRange;
/**
* Number of unique IDs to hand out to a thread at a time. This avoids cache
@ -44,15 +43,15 @@ constexpr uint64_t kRangeSize = 2000;
namespace facebook {
namespace eden {
uint64_t generateUniqueID() {
auto range = localRange.get();
if (UNLIKELY(range->begin == range->end)) {
uint64_t generateUniqueID() noexcept {
auto& range = localRange;
if (UNLIKELY(range.begin == range.end)) {
auto begin =
globalCounter->fetch_add(kRangeSize, std::memory_order_relaxed);
range->begin = begin;
range->end = begin + kRangeSize;
range.begin = begin;
range.end = begin + kRangeSize;
}
return range->begin++;
return range.begin++;
}
} // namespace eden

9
eden/fs/utils/IDGen.h
View File

@ -18,10 +18,13 @@ namespace eden {
* Very efficiently returns a new uint64_t unique to this process. Amortizes
* the cost of synchronizing threads across many ID allocations.
*
* TODO: It could be beneficial to add a parameter to request more than one
* unique ID at a time.
* All returned IDs are nonzero.
*
* TODO: It might be beneficial to add a parameter to request more than one
* unique ID at a time, though such an API would make it possible to exhaust
* the range of a 64-bit integer.
*/
uint64_t generateUniqueID();
uint64_t generateUniqueID() noexcept;
} // namespace eden
} // namespace facebook