chenBright commented on code in PR #3561:
URL: https://github.com/apache/brpc/pull/3561#discussion_r4073801679
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src/bvar/histogram.h:
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@@ -219,6 +259,101 @@ namespace detail {
template <>
struct HasPlottableSeries<Histogram::Value> : butil::false_type {};
+#if WITH_BABYLON_COUNTER
+
+// One thread's slice of a Histogram.
+//
+// Only the thread owning the slot writes it, so the counters are updated with
+// a relaxed load plus a relaxed store rather than an atomic read-modify-write.
+// They are atomic all the same because the sampling thread reads them while
+// they are being written, which the seqlock allows but does not by itself make
+// race free. The seqlock is what keeps the buckets, the sum and the count of
one
+// slot mutually consistent.
+class HistogramSlot {
+public:
+ HistogramSlot() {
+ for (size_t i = 0; i < MAX_HISTOGRAM_BUCKETS; ++i) {
+ _counts[i].store(0, butil::memory_order_relaxed);
+ }
+ }
+
+ DISALLOW_COPY_AND_ASSIGN(HistogramSlot);
+
+ void add(size_t bucket_index, double value) {
+ _seqlock.store([&] {
+ relaxed_add(&_counts[bucket_index], (uint64_t)1);
+ relaxed_add(&_sum, value);
+ relaxed_add(&_num, (int64_t)1);
+ });
+ }
+
+ Histogram::Value load(size_t num_buckets) const {
+ return _seqlock.load([&] {
+ Histogram::Value v(num_buckets);
+ for (size_t i = 0; i < num_buckets; ++i) {
+ v.counts[i] = _counts[i].load(butil::memory_order_relaxed);
+ }
+ v.sum = _sum.load(butil::memory_order_relaxed);
+ v.num = _num.load(butil::memory_order_relaxed);
+ return v;
+ });
+ }
+
+private:
+ template <typename T, typename U>
+ static void relaxed_add(butil::atomic<T>* target, U delta) {
Review Comment:
`load + store` can achieve better performance than `fetch_add`.
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