improve-histogram-performance: remove valueIndexMap_ (#8625)

Summary:
`valueIndexMap_` in histogram is redundant and search in `valueIndexMap_` is slower than search in `bucketValues_`.

this PR delete `valueIndexMap_` and search in `bucketValues_` by `std::lower_bound`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8625

Reviewed By: zhichao-cao

Differential Revision: D31613386

Pulled By: ajkr

fbshipit-source-id: d7415d724f5c8f41f80cbe82afd7467cfad6f009

monitoring/histogram.cc

@@ -10,6 +10,8 @@
 #include "monitoring/histogram.h"
 
 #include <stdio.h>
+
+#include <algorithm>
 #include <cassert>
 #include <cinttypes>
 #include <cmath>
@@ -23,7 +25,6 @@ HistogramBucketMapper::HistogramBucketMapper() {
   // If you change this, you also need to change
   // size of array buckets_ in HistogramImpl
   bucketValues_ = {1, 2};
-  valueIndexMap_ = {{1, 0}, {2, 1}};
   double bucket_val = static_cast<double>(bucketValues_.back());
   while ((bucket_val = 1.5 * bucket_val) <= static_cast<double>(port::kMaxUint64)) {
     bucketValues_.push_back(static_cast<uint64_t>(bucket_val));
@@ -35,26 +36,18 @@ HistogramBucketMapper::HistogramBucketMapper() {
       pow_of_ten *= 10;
     }
     bucketValues_.back() *= pow_of_ten;
-    valueIndexMap_[bucketValues_.back()] = bucketValues_.size() - 1;
   }
   maxBucketValue_ = bucketValues_.back();
   minBucketValue_ = bucketValues_.front();
 }
 
 size_t HistogramBucketMapper::IndexForValue(const uint64_t value) const {
-  if (value >= maxBucketValue_) {
-    return bucketValues_.size() - 1;
-  } else if ( value >= minBucketValue_ ) {
-    std::map<uint64_t, uint64_t>::const_iterator lowerBound =
-      valueIndexMap_.lower_bound(value);
-    if (lowerBound != valueIndexMap_.end()) {
-      return static_cast<size_t>(lowerBound->second);
-    } else {
-      return 0;
-    }
-  } else {
-    return 0;
-  }
+  auto beg = bucketValues_.begin();
+  auto end = bucketValues_.end();
+  if (value >= maxBucketValue_)
+    return end - beg - 1;  // bucketValues_.size() - 1
+  else
+    return std::lower_bound(beg, end, value) - beg;
 }
 
 namespace {

monitoring/histogram.h

@@ -48,7 +48,6 @@ class HistogramBucketMapper {
   std::vector<uint64_t> bucketValues_;
   uint64_t maxBucketValue_;
   uint64_t minBucketValue_;
-  std::map<uint64_t, uint64_t> valueIndexMap_;
 };
 
 struct HistogramStat {