Mark HyperClockCache as production-ready (#10963)

Summary:
After a couple minor bug fixes and successful productions roll-outs in a few places, I think we can mark this as production-ready. It has a clear value proposition for many workloads, even if we don't have clear advice for every workload yet.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10963

Test Plan: existing tests, comment changes only

Reviewed By: siying

Differential Revision: D41384083

Pulled By: pdillinger

fbshipit-source-id: 56359f01a57bb28de8697666b342382fac72ce6d

HISTORY.md

@@ -15,6 +15,7 @@
 * Add basic support for user-defined timestamp to Merge (#10819).
 * Add stats for ReadAsync time spent and async read errors.
 * Basic support for the wide-column data model is now available. Wide-column entities can be stored using the `PutEntity` API, and retrieved using `GetEntity` and the new `columns` API of iterator. For compatibility, the classic APIs `Get` and `MultiGet`, as well as iterator's `value` API return the value of the anonymous default column of wide-column entities; also, `GetEntity` and iterator's `columns` return any plain key-values in the form of an entity which only has the anonymous default column. `Merge` (and `GetMergeOperands`) currently also apply to the default column; any other columns of entities are unaffected by `Merge` operations. Note that some features like compaction filters, transactions, user-defined timestamps, and the SST file writer do not yet support wide-column entities; also, there is currently no `MultiGet`-like API to retrieve multiple entities at once. We plan to gradually close the above gaps and also implement new features like column-level operations (e.g. updating or querying only certain columns of an entity).
+* Marked HyperClockCache as a production-ready alternative to LRUCache for the block cache. HyperClockCache greatly improves hot-path CPU efficiency under high parallel load or high contention, with some documented caveats and limitations. As much as 4.5x higher ops/sec vs. LRUCache has been seen in db_bench under high parallel load.
 
 ### Public API Changes
 * Marked `block_cache_compressed` as a deprecated feature. Use SecondaryCache instead.

cache/clock_cache.h

@@ -32,7 +32,8 @@ namespace clock_cache {
 // Forward declaration of friend class.
 class ClockCacheTest;
 
-// HyperClockCache is an experimental alternative to LRUCache.
+// HyperClockCache is an alternative to LRUCache specifically tailored for
+// use as BlockBasedTableOptions::block_cache
 //
 // Benefits
 // --------

include/rocksdb/cache.h

@@ -287,11 +287,21 @@ extern std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
 extern std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
     const CompressedSecondaryCacheOptions& opts);
 
-// HyperClockCache - EXPERIMENTAL
-//
-// A lock-free Cache alternative for RocksDB block cache that offers much
-// improved CPU efficiency under high parallel load or high contention, with
-// some caveats.
+// HyperClockCache - A lock-free Cache alternative for RocksDB block cache
+// that offers much improved CPU efficiency vs. LRUCache under high parallel
+// load or high contention, with some caveats:
+// * Not a general Cache implementation: can only be used for
+// BlockBasedTableOptions::block_cache, which RocksDB uses in a way that is
+// compatible with HyperClockCache.
+// * Requires an extra tuning parameter: see estimated_entry_charge below.
+// Similarly, substantially changing the capacity with SetCapacity could
+// harm efficiency.
+// * SecondaryCache is not yet supported.
+// * Cache priorities are less aggressively enforced, which could cause
+// cache dilution from long range scans (unless they use fill_cache=false).
+// * Can be worse for small caches, because if almost all of a cache shard is
+// pinned (more likely with non-partitioned filters), then CLOCK eviction
+// becomes very CPU intensive.
 //
 // See internal cache/clock_cache.h for full description.
 struct HyperClockCacheOptions : public ShardedCacheOptions {