What are conditions to spread traffic across different NIC queues?

[Lev Serebryakov]

 I have "router" with two em (82574L) NICs. It has dual-core CPU and
each NIC creates two receive and two transmit queues:

$ sysctl dev.em | grep queue | grep _irq
dev.em.1.queue_rx_1.rx_irq: 0
dev.em.1.queue_rx_0.rx_irq: 0
dev.em.1.queue_tx_1.tx_irq: 0
dev.em.1.queue_tx_0.tx_irq: 0
dev.em.0.queue_rx_1.rx_irq: 0
dev.em.0.queue_rx_0.rx_irq: 0
dev.em.0.queue_tx_1.tx_irq: 0
dev.em.0.queue_tx_0.tx_irq: 0
$

 When I pass, say, 64 UDP streams through this router, only one core is
100% loaded and other is 100% idle. These streams are for same IP pair,
but port pairs are all different.

 What are right conditions to spread such traffic per queues and cores?

-- 
// Lev Serebryakov

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