fork of https://github.com/oxigraph/rocksdb and https://github.com/facebook/rocksdb for nextgraph and oxigraph

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Mike Kolupaev 236d4c67e9 Less linear search in DBIter::Seek() when keys are overwritten a lot ... Summary: In one deployment we saw high latencies (presumably from slow iterator operations) and a lot of CPU time reported by perf with this stack: ``` rocksdb::MergingIterator::Next rocksdb::DBIter::FindNextUserEntryInternal rocksdb::DBIter::Seek ``` I think what's happening is: 1. we create a snapshot iterator, 2. we do lots of Put()s for the same key x; this creates lots of entries in memtable, 3. we seek the iterator to a key slightly smaller than x, 4. the seek walks over lots of entries in memtable for key x, skipping them because of high sequence numbers. CC IslamAbdelRahman Closes https://github.com/facebook/rocksdb/pull/1413 Differential Revision: D4083879 Pulled By: IslamAbdelRahman fbshipit-source-id: a83ddae		8 years ago
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rocksdb	Less linear search in DBIter::Seek() when keys are overwritten a lot	8 years ago