add a cheap unique ID generator

Summary:
Add a very cheap unique ID allocator designed for extremely
low-latency uses like D10384071.

Differential Revision: D10501712

fbshipit-source-id: 15a0b7a2d344c0f6100082850dd967c585f3e2da

eden/fs/utils/IDGen.cpp

@@ -0,0 +1,58 @@
+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+#include "eden/fs/utils/IDGen.h"
+#include <folly/CachelinePadded.h>
+#include <folly/ThreadLocal.h>
+#include <atomic>
+
+namespace {
+/**
+ * Allocating one unique ID per nanosecond would wrap around in over 500 years.
+ *
+ * CachelinePadded may be excessive here.
+ */
+folly::CachelinePadded<std::atomic<uint64_t>> globalCounter;
+
+struct LocalRange {
+  uint64_t begin{0};
+  uint64_t end{0};
+};
+
+folly::ThreadLocal<LocalRange> localRange;
+
+/**
+ * Number of unique IDs to hand out to a thread at a time. This avoids cache
+ * line contention on globalCounter. kRangeSize should be large enough to reduce
+ * contention but small enough that the pathological case of threads being
+ * spawned in a tight loop, each allocating one unique ID, does not rapidly
+ * exhaust the 64-bit counter space.
+ *
+ * I haven't measured, but I'd be surprised if a thread could be created in
+ * 2000 nanoseconds.
+ */
+constexpr uint64_t kRangeSize = 2000;
+} // namespace
+
+namespace facebook {
+namespace eden {
+
+uint64_t generateUniqueID() {
+  auto range = localRange.get();
+  if (UNLIKELY(range->begin == range->end)) {
+    auto begin =
+        globalCounter->fetch_add(kRangeSize, std::memory_order_relaxed);
+    range->begin = begin;
+    range->end = begin + kRangeSize;
+  }
+  return range->begin++;
+}
+
+} // namespace eden
+} // namespace facebook

eden/fs/utils/IDGen.h

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+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+#pragma once
+
+#include <stdint.h>
+
+namespace facebook {
+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.
+ */
+uint64_t generateUniqueID();
+
+} // namespace eden
+} // namespace facebook

eden/fs/utils/test/IDGenTest.cpp

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+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#include "eden/fs/utils/IDGen.h"
+#include <gtest/gtest.h>
+
+using namespace facebook::eden;
+
+TEST(IDGenTest, producesUniqueIDs) {
+  auto id1 = generateUniqueID();
+  auto id2 = generateUniqueID();
+  auto id3 = generateUniqueID();
+  EXPECT_NE(id1, id2);
+  EXPECT_NE(id2, id3);
+  EXPECT_NE(id2, id3);
+}