Reduce prune memory usage #4354
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This was referenced Jun 5, 2023
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Compared to restic 0.16.4, this PR + #4812 drastically reduce the prune memory usage (using numbers from gcvis, which are slightly lower than those shown by htop, but still appear to be reasonable): Before (restic 0.16.4): After (this PR + #4812): (The repository has 400GB with 8 million blobs. The repack phase differs as the previous prune run already removed a bit of data) This PR optimizes the "search used blobs"-phase, whereas #4812 optimizes the "rebuild index"-phase. |
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use the same index size for compressed and uncompressed indexes. Otherwise, decoding the index of a compressed repository requires significantly more memory.
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What does this PR change? What problem does it solve?
The main memory usage of prune (besides the repository index) is a huge CountedBlobSet. The blob ids stored in this set are already stored in the repository index. This PR contains a proof of concept for a data structure that no longer duplicates these ids, but rather reuses them.
This PR fundamentally depends on #4352, which stores all indexEntries in a large array. Thus an indexEntry and its contained blob id becomes identifiable by its array index. As an indexMap can only add but not remove entries, this array index is guaranteed to be stable. Thus the AssociatedData structure in the PR can allocate an array that only consists of the values that were stored in a CountedBlobSet and use the array index to look up the matching blob id in the indexMap.
Using this PR,
prunerequires up to 50% less memory while determining which blobs to keep.Was the change previously discussed in an issue or on the forum?
Alternative to the minimal perfect hashes discussed in #3328
Checklist
[ ] I have added documentation for relevant changes (in the manual).changelog/unreleased/that describes the changes for our users (see template).gofmton the code in all commits.