[638][df] Revert choice to change default Snapshot TTree compression settings#22001
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vepadulano wants to merge 1 commit intoroot-project:v6-38-00-patchesfrom
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[638][df] Revert choice to change default Snapshot TTree compression settings#22001vepadulano wants to merge 1 commit intoroot-project:v6-38-00-patchesfrom
vepadulano wants to merge 1 commit intoroot-project:v6-38-00-patchesfrom
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root-project@61088a3 made the deliberate choice to change the default compression settings when calling Snapshot with TTree output format from 101 to 505. This choice was the result of internal discussion within the team, based on the empirical evidence available up to that point that showed ZSTD outperforming ZLIB on all metrics for the TTree datasets (as well as for RNTuple datasets). This commit proposes to revert that choice based on new evidence, summarised at https://github.com/vepadulano/ttree-lossless-compression-studies. The main takeaway message from that study is that TTree datasets with branches of type ROOT::RVec where many (if not all) of the collections are empty are compressed better with ZLIB than with ZSTD. Being this case actually quite relevant, as most datasets are made of branches with collection types and as the result of analysis steps these collections may be skimmed quite drastically, there is enough motivation to move the default compression settings for TTree back to 101. This commit changes the default RSnapshotOptions values for compression settings respectively to 'kUndefined' and '0' for the compression algorithm and the compression level. When the 'kUndefined' compression algorithm is used, Snapshot will behave differently depending on the output format: the settings will be 101 for TTree and 505 for RNTuple. Add one test per respective output format to check the default values are respected. The message shown to users of ROOT 6.38 has been updated to take the new findings into account.
dpiparo
changed the title
Test Results 19 files 19 suites 2d 23h 17m 52s ⏱️ Results for commit f85cec8. |
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Backport of #21753. Unfortunately it can't be made retroactive, but I believe future patch releases of 6.38 should also include the revert.