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Task started out as a self-organizing parallel-make algorithm, but ended up becoming a half-broken wait-die algorithm. When a contended object is already locked, Tasks enter a FIFO queue to restart and acquire the lock. This is the "die" part of wait-die (all locks on an Exclusion are non-blocking, so no Task ever does "wait"). The lock queue is FIFO wrt _lock acquisition order_, not _Task age_ as required by the wait-die algorithm. Make it a 25%-broken wait-die algorithm by sorting the Tasks on lock queues in order of Task ID, i.e. oldest-first, or FIFO wrt Task age. This ensures the oldest Task waiting for an object is the one to get it when it becomes available, as expected from the wait-die algorithm. This should reduce the amount of time Tasks spend on the execution queue, and reduce memory usage by avoiding the accumulation of Tasks that cannot make forward progress. Note that turning `TaskQueue` into an ordered container would have undesirable side-effects: * `std::list` has some useful properties wrt stability of object location and cost of splicing. Other containers may not have these, and `std::list` does have a `sort` method. * Some Task objects are created at the beginning and reused continually, but we really do want those Tasks to be executed in FIFO order wrt submission, not Task ID. We can exclude these tasks by only doing the sorting when a Task is queued for an Exclusin object. Signed-off-by: Zygo Blaxell <bees@furryterror.org>
BEES
Best-Effort Extent-Same, a btrfs deduplication agent.
About bees
bees is a block-oriented userspace deduplication agent designed to scale up to large btrfs filesystems. It is an offline dedupe combined with an incremental data scan capability to minimize time data spends on disk from write to dedupe.
Strengths
- Space-efficient hash table - can use as little as 1 GB hash table per 10 TB unique data (0.1GB/TB)
- Daemon mode - incrementally dedupes new data as it appears
- Largest extents first - recover more free space during fixed maintenance windows
- Works with btrfs compression - dedupe any combination of compressed and uncompressed files
- Whole-filesystem dedupe - scans data only once, even with snapshots and reflinks
- Persistent hash table for rapid restart after shutdown
- Constant hash table size - no increased RAM usage if data set becomes larger
- Works on live data - no scheduled downtime required
- Automatic self-throttling - reduces system load
- btrfs support - recovers more free space from btrfs than naive dedupers
Weaknesses
- Whole-filesystem dedupe - has no include/exclude filters, does not accept file lists
- Requires root privilege (
CAP_SYS_ADMINplus the usual filesystem read/modify caps) - First run may increase metadata space usage if many snapshots exist
- Constant hash table size - no decreased RAM usage if data set becomes smaller
- btrfs only
Installation and Usage
Recommended Reading
- bees Gotchas
- btrfs kernel bugs - especially DATA CORRUPTION WARNING for old kernels
- bees vs. other btrfs features
- What to do when something goes wrong
More Information
Bug Reports and Contributions
Email bug reports and patches to Zygo Blaxell bees@furryterror.org.
You can also use Github:
https://github.com/Zygo/bees
Copyright & License
Copyright 2015-2025 Zygo Blaxell bees@furryterror.org.
GPL (version 3 or later).