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Summary: There was an overflow bug when computing the variance in the HistogramStat class. This manifests, for instance, when running cache_bench with default arguments. This executes 32M lookups/inserts/deletes in a block cache, and then computes (among other things) the variance of the latencies. The variance is computed as ``variance = (cur_sum_squares * cur_num - cur_sum * cur_sum) / (cur_num * cur_num)``, where ``cum_sum_squares`` is the sum of the squares of the samples, ``cur_num`` is the number of samples, and ``cur_sum`` is the sum of the samples. Because the median latency in a typical run is around 3800 nanoseconds, both the ``cur_sum_squares * cur_num`` and ``cur_sum * cur_sum`` terms overflow as uint64_t. Pull Request resolved: https://github.com/facebook/rocksdb/pull/10100 Test Plan: Added a unit test. Run ``make -j24 histogram_test && ./histogram_test``. Reviewed By: pdillinger Differential Revision: D36942738 Pulled By: guidotag fbshipit-source-id: 0af5fb9e2a297a284e8e74c24e604d302906006e |
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| .. | ||
| file_read_sample.h | ||
| histogram.cc | ||
| histogram.h | ||
| histogram_test.cc | ||
| histogram_windowing.cc | ||
| histogram_windowing.h | ||
| in_memory_stats_history.cc | ||
| in_memory_stats_history.h | ||
| instrumented_mutex.cc | ||
| instrumented_mutex.h | ||
| iostats_context.cc | ||
| iostats_context_imp.h | ||
| iostats_context_test.cc | ||
| perf_context.cc | ||
| perf_context_imp.h | ||
| perf_level.cc | ||
| perf_level_imp.h | ||
| perf_step_timer.h | ||
| persistent_stats_history.cc | ||
| persistent_stats_history.h | ||
| statistics.cc | ||
| statistics.h | ||
| statistics_test.cc | ||
| stats_history_test.cc | ||
| thread_status_impl.cc | ||
| thread_status_updater.cc | ||
| thread_status_updater.h | ||
| thread_status_updater_debug.cc | ||
| thread_status_util.cc | ||
| thread_status_util.h | ||
| thread_status_util_debug.cc | ||