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Summary: ReadOptions is copied frequently on read paths, and its only non-trivially-copyable member, table_filter, is a std::function that is almost always empty yet still gets deep-copied on every ReadOptions copy (and can heap-allocate). This changes table_filter to a pointer to a caller-owned std::function instead of storing one by value: when non-nullptr, the caller is responsible for keeping the pointed-to function alive for the duration of the read (like the other pointer fields such as iterate_upper_bound). With this change every ReadOptions member is trivially copyable, so copying ReadOptions no longer touches a std::function at all. That trivial-copyability is locked in with a static_assert in options_test.cc (kept out of the public header). This is inspired by D113109635, which targeted the same ReadOptions-copy hotspot by relocating table_filter to the end of the struct so the trivial members memcpy in one contiguous block. This is intended to be a superior solution: rather than only defragmenting the copy, it removes the std::function copy entirely. This is a breaking API change, tolerated because table_filter is a relatively obscure option. A nullptr, or a pointer to an empty std::function, means no filtering (preserving the previous empty-function behavior). A shared internal helper HasTableFilter(const ReadOptions&) in options_helper.h centralizes the nullptr + empty-function check used on the read paths. The C API and Java bindings now own the std::function and hand ReadOptions a pointer to it; their public signatures are unchanged, and the C API rocksdb_readoptions_t clears the owned function when the callback is null. Making ReadOptions trivially copyable also let -Werror=unused-but-set-variable fire on a pre-existing dead local in db_bench's MultiScan: a ReadOptions that set async_io/readahead_size but was never passed to NewMultiScan (the warning had been suppressed while ReadOptions had a non-trivial member). That local was in fact inert for multiscan -- BlockBasedTableIterator::Prepare overrides ReadOptions::async_io with MultiScanArgs::use_async_io for the coalesced prefetch, so multiscan async I/O is driven by --multiscan_use_async_io, not ReadOptions::async_io. Removed the dead local; MultiScan now uses read_options_ directly, consistent with MultiScanRandom. Pull Request resolved: https://github.com/facebook/rocksdb/pull/15005 Test Plan: Existing table_filter coverage was adapted to the new pointer API and still exercises the behavior end to end: db_iterator_test.TableFilter (callback invoked once per table; returning false hides a table) and its range-conversion rejection case; db_bloom_filter_test SstQueryFilter/FixedWidthSegments range queries; and trie_index_db_test's range-query iterator (which returns iterators outliving the filter, exercising the caller-owned lifetime). db/c_test covers the C API set/clear/has_table_filter path. options_test asserts std::is_trivially_copyable_v<ReadOptions>. Manually verified the db_bench MultiScan async behavior with a ~200MB DB and --use_direct_reads: with ReadOptions::async_io held true, async reads tracked only --multiscan_use_async_io (rocksdb.read.async.micros count 329511 with use_async_io=true vs 0 with false), confirming ReadOptions::async_io does not drive multiscan async and the removed local had no effect. Reviewed By: anand1976 Differential Revision: D113613525 Pulled By: pdillinger fbshipit-source-id: 38d73b607500a5c7ca2b89dd8f4187ef43940aa4
RocksDB: A Persistent Key-Value Store for Flash and RAM Storage
RocksDB is developed and maintained by Facebook Database Engineering Team. It is built on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)
This code is a library that forms the core building block for a fast key-value server, especially suited for storing data on flash drives. It has a Log-Structured-Merge-Database (LSM) design with flexible tradeoffs between Write-Amplification-Factor (WAF), Read-Amplification-Factor (RAF) and Space-Amplification-Factor (SAF). It has multi-threaded compactions, making it especially suitable for storing multiple terabytes of data in a single database.
Start with example usage here: https://github.com/facebook/rocksdb/tree/main/examples
See the github wiki for more explanation.
The public interface is in include/. Callers should not include or
rely on the details of any other header files in this package. Those
internal APIs may be changed without warning.
Questions and discussions are welcome on the RocksDB Developers Public Facebook group and email list on Google Groups.
License
RocksDB is dual-licensed under both the GPLv2 (found in the COPYING file in the root directory) and Apache 2.0 License (found in the LICENSE.Apache file in the root directory). You may select, at your option, one of the above-listed licenses.