Thread-specific ticker statistics
Summary: The global atomics we previously used for tickers had poor cache performance since they were typically updated from different threads, causing frequent invalidations. In this diff, - recordTick() updates a local ticker value specific to the thread in which it was called - When a thread exits, its local ticker value is added into merged_sum - getTickerCount() returns the sum of all threads' local ticker values and the merged_sum - setTickerCount() resets all threads' local ticker values and sets merged_sum to the value provided by the caller. In a next diff I will make a similar change for histogram stats. Test Plan: before: $ TEST_TMPDIR=/dev/shm/ perf record -g ./db_bench --benchmarks=readwhilewriting --statistics --num=1000000 --use_existing_db --threads=64 --cache_size=250000000 --compression_type=lz4 $ perf report -g --stdio | grep recordTick 7.59% db_bench db_bench [.] rocksdb::StatisticsImpl::recordTick ... after: $ TEST_TMPDIR=/dev/shm/ perf record -g ./db_bench --benchmarks=readwhilewriting --statistics --num=1000000 --use_existing_db --threads=64 --cache_size=250000000 --compression_type=lz4 $ perf report -g --stdio | grep recordTick 1.46% db_bench db_bench [.] rocksdb::StatisticsImpl::recordTick ... Reviewers: kradhakrishnan, MarkCallaghan, IslamAbdelRahman, sdong Reviewed By: sdong Subscribers: yiwu, andrewkr, dhruba, leveldb Differential Revision: https://reviews.facebook.net/D62337
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