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020a9c931b
When base write triggers mv write and it needs to be send to another shard it used the same service group and we could end up with a deadlock. This fix affects also alternator's secondary indexes. Testing was done using (yet) not committed framework for easy alternator performance testing: https://github.com/scylladb/scylladb/pull/13121. I've changed hardcoded max_nonlocal_requests config in scylla from 5000 to 500 and then ran: ./build/release/scylla perf-alternator-workloads --workdir /tmp/scylla-workdir/ --smp 2 \ --developer-mode 1 --alternator-port 8000 --alternator-write-isolation forbid --workload write_gsi \ --duration 60 --ring-delay-ms 0 --skip-wait-for-gossip-to-settle 0 --continue-after-error true --concurrency 2000 Without the patch when scylla is overloaded (i.e. number of scheduled futures being close to max_nonlocal_requests) after couple seconds scylla hangs, cpu usage drops to zero, no progress is made. We can confirm we're hitting this issue by seeing under gdb: p seastar::get_smp_service_groups_semaphore(2,0)._count $1 = 0 With the patch I wasn't able to observe the problem, even with 2x concurrency. I was able to make the process hang with 10x concurrency but I think it's hitting different limit as there wasn't any depleted smp service group semaphore and it was happening also on non mv loads. Fixes https://github.com/scylladb/scylladb/issues/15844 Closes scylladb/scylladb#15845