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approximate histogram stats to save cpu 

Summary:
sounds like we're willing to tradeoff minor inaccuracy in stats for speed. start with histogram stats. ticker stats will be harder (and, IMO, we shouldn't change them in this manner) as many test cases rely on them being exactly correct.
Closes https://github.com/facebook/rocksdb/pull/2720

Differential Revision: D5607884

Pulled By: ajkr

fbshipit-source-id: 1b754cda35ea6b252d1fdd5aa3cfb58866506372
This commit is contained in:
Andrew Kryczka 2017-08-11 13:09:38 -07:00 committed by Facebook Github Bot
parent 3f5888430a
commit 5de98f2d50
1 changed files with 15 additions and 6 deletions
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21
monitoring/histogram.cc
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@ -105,17 +105,26 @@ void HistogramStat::Add(uint64_t value) {
// by concurrent threads is tolerable.
const size_t index = bucketMapper.IndexForValue(value);
assert(index < num_buckets_);
buckets_[index].fetch_add(1, std::memory_order_relaxed);
buckets_[index].store(buckets_[index].load(std::memory_order_relaxed) + 1,
std::memory_order_relaxed);
uint64_t old_min = min();
while (value < old_min && !min_.compare_exchange_weak(old_min, value)) {}
if (value < old_min) {
min_.store(value, std::memory_order_relaxed);
}
uint64_t old_max = max();
while (value > old_max && !max_.compare_exchange_weak(old_max, value)) {}
if (value > old_max) {
max_.store(value, std::memory_order_relaxed);
}
num_.fetch_add(1, std::memory_order_relaxed);
sum_.fetch_add(value, std::memory_order_relaxed);
sum_squares_.fetch_add(value * value, std::memory_order_relaxed);
num_.store(num_.load(std::memory_order_relaxed) + 1,
std::memory_order_relaxed);
sum_.store(sum_.load(std::memory_order_relaxed) + value,
std::memory_order_relaxed);
sum_squares_.store(
sum_squares_.load(std::memory_order_relaxed) + value * value,
std::memory_order_relaxed);
}
void HistogramStat::Merge(const HistogramStat& other) {