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Summary: Auto-skip compression (stop attempting compression on data that isn't compressing well, to save CPU) was previously an experimental CompressionManager wrapper (CreateAutoSkipCompressionManager) whose predictor lived in each per-thread working area. Besides being fragmented across parallel workers and reset on a tumbling window, it decided based on how OFTEN blocks failed to compress, ignoring how MUCH they compress -- so on mixed data it could give up substantial compression to save a little CPU (see performance results). This makes auto-skip a first-class, optional, ratio-aware feature of the block-based table builder, controlled by two new CompressionOptions fields: - auto_skip (default false): the on/off switch. There is no separate threshold knob; the decision reuses max_compressed_bytes_per_kb (the existing per-block "minimum compression worth keeping" bar) as the bar for whether attempting compression on the stream is worthwhile. - auto_skip_min_sample_every: sampling interval that bounds the local impact around a transition back to compressible data. 0 selects an internal default (currently 64 skipped data blocks between forced samples). The emit thread makes the skip/attempt decision once per data block (serial and parallel paths alike; data blocks only), so a skipped block is never dispatched to a compression worker. It is driven by a continuously updated, shared estimate q_hat of the aggregate stored/uncompressed ratio the builder would achieve by attempting -- each block counted at its achieved ratio, or 1.0 when stored raw: attempt while q_hat is at or below the bar, otherwise skip. Because the estimate is payoff-weighted, a stream whose blocks each save little -- or where only a few blocks compress well -- does not clear the bar and is skipped, while a stream that compresses substantially is attempted. The compression given up per KB is bounded by (1024 - max_compressed_bytes_per_kb). q_hat uses an asymmetric exponentially weighted moving average (EWMA), slightly faster toward better compression than toward worse; the fast side is deliberately gentle so a single marginally-compressible sample does not flip a whole file back to attempting. It is kept on its own cache line to avoid false sharing. Under parallel compression q_hat is updated with relaxed atomics, and the emit thread can decide from a value that lags the learning it drives by the in-flight depth -- tolerable concessions of accuracy and responsiveness for a coarse heuristic. The first data block of each file is always attempted (the sample countdown starts at 0), and while skipping the builder keeps sampling (one block after a randomized gap) so a return to compressible data is detected. q_hat is carried across the files a flush/compaction thread emits via thread_local state keyed by preferred compression type, so a sustained region does not re-learn on every file. The old CompressionManager-based AutoSkip is removed (CostAware is retained); db_stress, db_bench, and db_crashtest are migrated to the new option. db_bench is extended so it can generate the relevant regimes: --compressible_data_fraction and --compressible_data_run_length produce a mix of compressible/incompressible blocks -- i.i.d. per value (run length 1) or in alternating regions (large run length). The default (fraction 1.0) reproduces the historical single-pool behavior. Pull Request resolved: https://github.com/facebook/rocksdb/pull/15079 Test Plan: New/updated unit tests in `util/compression_test.cc` (`DBAutoSkipTest`): - skips sustained incompressible data; - never skips compressible data; - disabled by default never bypasses a block; - 4-thread parallel compression round-trips all data across alternating compressible/incompressible regions. **Performance** (`db_bench fillseq`, release, zstd level 3; 100k values of 16 KiB each, roughly one value per data block; WAL and auto-compaction off; median of 5 runs on a quiet host). Auto-skip trades one cost for another, so each cell reports two costs, both lower-is-better: CPU cost as process CPU time (user+sys) per GB of uncompressed input (s/GB; ~1.64 GB uncompressed here), and storage cost as the resulting DB size. Auto-skip does not appear to change parallelizability, so wall-clock s/GB tracks CPU s/GB (the two agree here); CPU s/GB is reported. Percentages are relative to the no-auto-skip baseline: negative CPU = cheaper, positive size = more storage (shown only where storage grows). | Workload | baseline (off) | (a) old wrapper | (b) draft | (c) this version | |---|---|---|---|---| | Incompressible | 2.33 s/GB, 1568 MB | 0.96 (-59%), 1568 MB | 0.82 (-65%), 1568 MB | 0.82 (-65%), 1568 MB | | Compressible | 3.55 s/GB, 710 MB | 3.54 (~0%), 710 MB | 3.56 (~0%), 710 MB | 3.55 (~0%), 710 MB | | Marginal-payoff mix | 3.80 s/GB, 1376 MB | 1.54 (-59%), 1523 MB (+11%) | 3.81 (~0%), 1376 MB | 1.43 (-62%), 1531 MB (+11%) | | High-payoff mix | 2.20 s/GB, 929 MB | 1.14 (-48%), 1417 MB (+53%) | 2.16 (~0%), 929 MB | 2.17 (~0%), 929 MB | A strategy wins by cutting CPU cost without paying much storage cost. On incompressible data all three cut CPU ~60% at no storage cost; on compressible data none skip (within run-to-run noise). The two mixes (50% incompressible, 50% compressible per block) separate the strategies: - Marginal-payoff mix: the compressible half barely clears the per-block bar, so the stream is not worth compressing -- skipping is right. (c) and (a) cut CPU ~60% for ~11% more storage. That extra storage is compression (c) intentionally declines because its aggregate benefit is below the configured worthwhile threshold (max_compressed_bytes_per_kb, ~12.5% per KB); it stays within the (1024 - max_compressed_bytes_per_kb) bound. The ratio-unaware draft (b) cannot see the low payoff, keeps compressing, and saves no CPU. - High-payoff mix: the compressible half compresses hard, so compressing is right. (c) and (b) keep full compression at baseline storage; the old wrapper (a) cuts CPU 48% but inflates storage +53% -- far beyond that bound -- giving up worthwhile compression. Only (c) makes the right call on both mixes: it never gives up worthwhile compression (unlike (a)) and does not waste CPU on low-payoff data (unlike (b)). Reviewed By: xingbowang Differential Revision: D115444567 Pulled By: pdillinger fbshipit-source-id: 05e122d25de95ddcd0edeb0dd07c9d25bead7293