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A 25Gbps NIC tops out near 4Gbps while one core sits pinned in softirq. What is happening and how do you fix it?

One CPU doing all the packet work is a topology failure, not a tuning mystery. RSS queues, MSI-X vectors, local_cpulist and irqbalance: the whole chain from symptom to pin.

Updated Sep 2026 · Grounded in researched DevOps, SRE and platform engineering interview loops, written to a senior-engineer editorial bar, and never padded to hit a word count.

One CPU doing all the packet work is a topology failure, not a tuning mystery. RSS queues, MSI-X vectors, local_cpulist and irqbalance: the whole chain from symptom to pin.

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