The reader concurrency semaphore restricts the concurrency of reads that require CPU (intention: they read from the cache) to 1, meaning that if there is even a single active read which declares that it needs just CPU to proceed, no new read is admitted. This is meant to keep the concurrency of reads in the cache at 1. The idea is that concurrency in the cache is not useful: it just leads to the reactor rotating between these reads, all of the finishing later then they could if they were the only active read in the cache.
This was observed to backfire in the case where there reads from a single table are mostly very fast, but on some keys are very slow (hint: collection full of tombstones). In this case the slow read keeps up the fast reads in the queue, increasing the 99th percentile latencies significantly.
This series proposes to fix this, by making the CPU concurrency configurable. We don't like tunables like this and this is not a proper fix, but a workaround. The proper fix would be to allow to cut any page early, but we cannot cut a page in the middle of a row. We could maybe have a way of detecting slow reads and excluding them from the CPU concurrency. This would be a heuristic and it would be hard to get right. So in this series a robust and simple configurable is offered, which can be used on those few clusters which do suffer from the too strict concurrency limit. We have seen it in very few cases so far, so this doesn't seem to be wide-spread.
Fixes: https://github.com/scylladb/scylladb/issues/19017
This fixes a regression introduced in 5.0, so we have to backport to all currently supported releases
Closesscylladb/scylladb#19018
* github.com:scylladb/scylladb:
test/boost/reader_concurrency_semaphore_test: add test for live-configurable cpu concurrenc Please enter the commit message for your changes. Lines starting
test/boost/reader_concurrency_semaphore_test: hoist require_can_admit
reader_concurrency_semaphore: wire in the configurable cpu concurrency
reader_concurrency_semaphore: add cpu_concurrency constructor parameter
db/config: introduce reader_concurrency_semahore_cpu_concurrency
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