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Lustre is mostly used under cluster environments. We should control the RPC rate from single client to guarantee it is fair for all clients as much as possible. Otherwise a powerful and busy client maybe occupies most of services on server(s), and then causes other client(s) starvation, that is unfair.
Currently, Lustre has some RPC rate control, like inflight lookup/getattr RPC count is limited by the 'cl_max_rpcs_in_flight'. But the existing RPC rate control mechanism is not complete, for example, the inflight OST side lock enqueue RPC count is unlimited.
We want to implement more general RPC rate control framework, and try to cover operations as much as possible (if need rate control), including the inflight OST side lock enqueue RPC.
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