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Before this patch, when a base table has many materialized views, each write to this table can start up to 128 view updates in parallel. With high client write concurrency, the actual concurrency of writes executed on the node may grow unexpectedly, which can lead to higher latency and higher memory usage compared to a sequential approach. In this patch we add a per-shard, per-service-level semaphore which limits the number of concurrent view updates processed on the shard in this service level to a constant value. We take one unit from the semaphore for each local view update write, and releasing it when it finishes. The remote view updates do not take units from the semaphore because they don't consume nearly as much processing power and they are limited by another semaphore based on their memory usage. Fixes https://github.com/scylladb/scylladb/issues/25341 Closes scylladb/scylladb#25456 * github.com:scylladb/scylladb: mv: limit concurrent view updates from all sources database: rename _view_update_concurrency_sem to _view_update_memory_sem