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vendor/github.com/cockroachdb/pebble/internal/arenaskl/README.md

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+# arenaskl
+
+Fast, lock-free, arena-based Skiplist implementation in Go that supports iteration
+in both directions.
+
+## Advantages
+
+Arenaskl offers several advantages over other skiplist implementations:
+
+* High performance that linearly scales with the number of cores. This is
+  achieved by allocating from a fixed-size arena and by avoiding locks.
+* Iterators that can be allocated on the stack and easily cloned by value.
+* Simple-to-use and low overhead model for detecting and handling race conditions
+  with other threads.
+* Support for iterating in reverse (i.e. previous links). 
+
+## Limitations
+
+The advantages come at a cost that prevents arenaskl from being a general-purpose
+skiplist implementation:
+
+* The size of the arena sets a hard upper bound on the combined size of skiplist
+  nodes, keys, and values. This limit includes even the size of deleted nodes,
+  keys, and values.
+* Deletion is not supported. Instead, higher-level code is expected to
+  add deletion tombstones and needs to process those tombstones
+  appropriately.
+
+## Pedigree
+
+This code is based on Andy Kimball's arenaskl code:
+
+https://github.com/andy-kimball/arenaskl
+
+The arenaskl code is based on the skiplist found in Badger, a Go-based
+KV store:
+
+https://github.com/dgraph-io/badger/tree/master/skl
+
+The skiplist in Badger is itself based on a C++ skiplist built for
+Facebook's RocksDB:
+
+https://github.com/facebook/rocksdb/tree/master/memtable
+
+## Benchmarks
+
+The benchmarks consist of a mix of reads and writes executed in parallel. The
+fraction of reads is indicated in the run name: "frac_X" indicates a run where
+X percent of the operations are reads.
+
+The results are much better than `skiplist` and `slist`.
+
+```
+name                  time/op
+ReadWrite/frac_0-8     470ns ±11%
+ReadWrite/frac_10-8    462ns ± 3%
+ReadWrite/frac_20-8    436ns ± 2%
+ReadWrite/frac_30-8    410ns ± 2%
+ReadWrite/frac_40-8    385ns ± 2%
+ReadWrite/frac_50-8    360ns ± 4%
+ReadWrite/frac_60-8    386ns ± 1%
+ReadWrite/frac_70-8    352ns ± 2%
+ReadWrite/frac_80-8    306ns ± 3%
+ReadWrite/frac_90-8    253ns ± 4%
+ReadWrite/frac_100-8  28.1ns ± 2%
+```
+
+Note that the above numbers are for concurrent operations using 8x
+parallelism. The same benchmarks without concurrency (use these
+numbers when comparing vs batchskl):
+
+```
+name                time/op
+ReadWrite/frac_0    1.53µs ± 1%
+ReadWrite/frac_10   1.46µs ± 2%
+ReadWrite/frac_20   1.39µs ± 3%
+ReadWrite/frac_30   1.28µs ± 3%
+ReadWrite/frac_40   1.21µs ± 2%
+ReadWrite/frac_50   1.11µs ± 3%
+ReadWrite/frac_60   1.23µs ±17%
+ReadWrite/frac_70   1.16µs ± 4%
+ReadWrite/frac_80    959ns ± 3%
+ReadWrite/frac_90    738ns ± 5%
+ReadWrite/frac_100  81.9ns ± 2%
+```
+
+Forward and backward iteration are also fast:
+
+```
+name                time/op
+IterNext            3.97ns ± 5%
+IterPrev            3.88ns ± 3%
+```