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db/config: reader_concurrency_semaphore_cpu_concurrency: bump default to 2

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This config item controls how many CPU-bound reads are allowed to run in
parallel. The effective concurrency of a single CPU core is 1, so
allowing more than one CPU-bound reads to run concurrently will just
result in time-sharing and both reads having higher latency.
However, restricting concurrency to 1 means that a CPU bound read that
takes a lot of time to complete can block other quick reads while it is
running. Increase this default setting to 2 as a compromise between not
over-using time-sharing, while not allowing such slow reads to block the
queue behind them.

Fixes: #22450

Closes scylladb/scylladb#22679
This commit is contained in:
Botond Dénes
2025-02-04 04:50:33 -05:00
committed by Avi Kivity
parent e22e3b21b1
commit 3d12451d1f
1 changed files with 1 additions and 1 deletions
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2
db/config.cc
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@@ -1209,7 +1209,7 @@ db::config::config(std::shared_ptr<db::extensions> exts)
"Start serializing reads after their collective memory consumption goes above $normal_limit * $multiplier.")
, reader_concurrency_semaphore_kill_limit_multiplier(this, "reader_concurrency_semaphore_kill_limit_multiplier", liveness::LiveUpdate, value_status::Used, 4,
"Start killing reads after their collective memory consumption goes above $normal_limit * $multiplier.")
, reader_concurrency_semaphore_cpu_concurrency(this, "reader_concurrency_semaphore_cpu_concurrency", liveness::LiveUpdate, value_status::Used, 1,
, reader_concurrency_semaphore_cpu_concurrency(this, "reader_concurrency_semaphore_cpu_concurrency", liveness::LiveUpdate, value_status::Used, 2,
"Admit new reads while there are less than this number of requests that need CPU.")
, view_update_reader_concurrency_semaphore_serialize_limit_multiplier(this, "view_update_reader_concurrency_semaphore_serialize_limit_multiplier", liveness::LiveUpdate, value_status::Used, 2,
"Start serializing view update reads after their collective memory consumption goes above $normal_limit * $multiplier.")