zstd: Fix incorrect buffer size in dictionary encodes

[klauspost]

Fix incorrect dLongTableShardSize leading to inefficient zeroing of

Also make shards 128 bytes to reduce memory use somewhat.

Summary by CodeRabbit

• Chores
  • Optimized internal compression settings to improve dictionary management.
  • Revised shard calculation methods to potentially enhance memory utilization and processing efficiency.

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📝 Walkthrough

Walkthrough

The pull request modifies constants in the ZSTD encoder module to adjust dictionary sharding and long table shard size calculations. In zstd/enc_base.go, the constant dictShardBits is increased from 6 to 7, which may affect the number of dictionary shards during encoding. In zstd/enc_dfast.go, the formula for computing dLongTableShardSize is updated by replacing the divisor tableShardCnt with dLongTableShardCnt (derived from bit constants), altering how shard sizes are determined.

Changes

File(s)	Change Summary
zstd/enc_base.go	Updated constant dictShardBits from 6 to 7, impacting dictionary sharding during encoding.
zstd/enc_dfast.go	Modified calculation of dLongTableShardSize by replacing tableShardCnt with dLongTableShardCnt (computed as 1 << (dFastLongTableBits - dictShardBits)).

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📥 Commits

Reviewing files that changed from the base of the PR and between 517288e and 387aaca.

📒 Files selected for processing (2)

• zstd/enc_base.go (1 hunks)
• zstd/enc_dfast.go (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms (17)

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🔇 Additional comments (2)

zstd/enc_base.go (1)

11-11: Logical change to increase dictionary shard bits

Increasing dictShardBits from 6 to 7 doubles the number of dictionary shards (from 64 to 128), which should result in more efficient dictionary encoding by providing smaller individual shards.

zstd/enc_dfast.go (1)

16-16: Fixing the incorrect buffer size calculation

This change correctly updates the formula to use dLongTableShardCnt instead of tableShardCnt for calculating dLongTableShardSize. Since dLongTableShardCnt is directly derived from dictShardBits (which was increased in the other file), this ensures that the shard size calculation is properly aligned with the dictionary sharding strategy.

This fix addresses the issue mentioned in the PR description about incorrect buffer size in dictionary encodes, which would lead to inefficient zeroing processes. Using the correct divisor ensures that operations on lines 1114-1115 (copying specific shards between tables) will work correctly with properly sized memory chunks.

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[klauspost]

Reported by @cfif1982 - fuzz testing before merge.

[klauspost]

fuzz: elapsed: 18h39m23s, execs: 905973345 (31266/sec), new interesting: 21 (total: 7898)

Seems stable. Merging.