net: Introduce round-robin connection balancing policy#3371
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Useful for some protocols which assume this policy, like shard-aware CQL protocol, which keeps opening connections until it hits the desired shard. With the current default load balancing strategy, which picks the least-loaded shard, that is not guaranteed to converge and can live-lock if there is connection imbalance.
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Needed by ScyllaDB: scylladb/scylladb#29687 |
nyh
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Apr 30, 2026
| port, | ||
| // This algorithm distributes all new connections to listen_options::fixed_cpu shard only. | ||
| fixed, | ||
| // This algorithm distributes new connections to shards in a round-robin fashion, |
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nitpick: typo in the commit message title ("balaning").
| class conntrack { | ||
| class load_balancer { | ||
| std::vector<unsigned> _cpu_load; | ||
| unsigned _rr_counter = 0; |
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nitpick: If you're familiar with this code, you can guess that the acronym "rr" refers to round-robin, but it's not obvious. I would add a comment that this variable is used by load_balancing_algorithm::round_robin to choose the next shard.
| return cpu; | ||
| } | ||
| shard_id next_cpu_rr() { | ||
| auto cpu = shard_id(_rr_counter % smp::count); |
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Note that in #3355 @avikivity deprecates smp::count, so this call will need to be fixed (in the same way Avi fixed the lines above).
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Useful for some protocols which assume this policy, like shard-aware CQL protocol, which keeps opening connections until it hits the desired shard. With the current default load balancing strategy, which picks the least-loaded shard, that is not guaranteed to converge and can live-lock if there is connection imbalance.