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NAPI ID based worker thread selection -N <num_napi_ids> | --napi_ids=<num_napi_ids> By default memcached assigns connections to worker threads in a round-robin manner. NAPI ID based worker thread selection enables worker threads to be selected based on the NIC HW RX queue on which the incoming request is received. This is enabled using SO_INCOMING_NAPI_ID socket option that is supported in linux kernels 4.12 or higher. This socket option returns a system level unique ID called NAPI ID that is associated with a RX queue on which the last packet associated with that socket is received. This allows memcached to split the incoming flows among threads based on the RX queue on which they are received. Each worker thread is associated with a NIC HW receive queue and services all the connection requests received on a specific RX queue. This mapping between a memcached thread and a HW NIC queue streamlines the flow of data from the NIC to the application. In addition, an optimal path with reduced context switches is possible, if epoll based busy polling (sysctl -w net.core.busy_poll = <non-zero value>) is also enabled. This feature is enabled via a new command line parameter -N <num> or "--napi_ids=<num>", where <num> is the number of available/assigned NIC hardware RX queues through which requests associated with a connection are received. The number of napi_ids specified cannot be greater than the number of worker threads specified using -t/--threads option. If the option is not specified, or the conditions not met, the code defaults to round robin thread selection. During a normal run, each worker thread gets associated with a napi_id and this will establish a 1:1 mapping between the thread and queues. If a new napi_id is received after each thread is associated with its own napi_id (this can happen if num_napi_ids argument doesn't exactly match with the number of RX queues OR if the NIC driver goes through a reload), a stats error counter called 'unexpected_napi_ids' is incremented and all the napi_id's associated with the threads are reset. If -N option is used, but the connection requests are received from a virtual interface like loopback, napi_id returned can be 0. This condition is tracked via a stats counter called 'round_robin_fallback'.